Energy efficient ventilation and indoor air quality in the context of COVID-19 - A systematic review

New COVID-19 ventilation guidelines have resulted in higher energy consumption to maintain indoor air quality (IAQ), and energy efficiency has become a secondary concern. Despite the significance of the studies conducted on COVID-19 ventilation requirements, a comprehensive investigation of the associated energy challenges has not been discussed. This study aims to present a critical systematic review of the Coronavirus viral spreading risk mitigation through ventilation systems (VS) and its relation to energy use. COVID-19 heating, ventilation and air conditioning (HVAC)-related countermeasures proposed by industry professionals have been reviewed and their influence on operating VS and energy consumption have also been discussed. A critical review analysis was then conducted on publications from 2020 to 2022. Four research questions (RQs) have been selected for this review concerning i) maturity of the existing literature, ii) building types and occupancy profile, iii) ventilation types and effective control strategies and iv) challenges and related causes. The results reveal that employing HVAC auxiliary equipment is mostly effective and increased fresh air supply is the most significant challenge associated with increased energy consumption due to maintaining IAQ. Future studies should focus on novel approaches toward solving the apparently conflicting objectives of minimizing energy consumption and maximizing IAQ. Also, effective ventilation control strategies should be assessed in various buildings with different occupancy densities. The implications of this study can be useful for future development of this topic not only to enhance the energy efficiency of the VS but also to enable more resiliency and health in buildings.


Introduction
Since late 2019, the world has dealt with the consequences of the universal spread of the serve acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) [1].The virus is recognized to be extremely infectious and primarily spreads by droplet, contact, and airborne modes [2].Droplets exceeding 8 to 4 μm in diameter vanish after 20-90 min, respectively.Smaller droplets (probably ≤3 μm and not ≤5 μm) stay airborne within greater amount of time [3].Studies in both indoor and outdoor settings have emphasized the significance of airborne spread through droplets and, especially, aerosols [4].Aerosols have a longer lifespan and greater airborne mobility than droplets [5] and can be transmitted through inhalation of virally contaminated air [6].Therefore, indoor air quality (IAQ) has become a priority in building design [7].It has brought global attention to the utilization of efficient engineering methods to preserve environmental quality [8], and human health and to prevent shutdowns in busy interior areas [9].
Ventilation is usually regarded as an effective engineering control strategy to prevent airborne transmission since it increases the availability of clean outside air [6], via a simple application [10].There is an undeniable relationship between ventilation and the spread of viral diseases [11] through removing contaminants [12].Ventilation is also accomplished by lowering the concentration of the airborne particles produced during expiration [6,13].However, the risk of infection increases if the ventilation is not efficient [14] due to blocked airflow passage [15], low ventilation rate shared with a high number of occupants [13] in compact zones [16], lack of proper filters [17] or improper operation management [14].
Addressing the COVID-19 outbreak, ventilation countermeasures proposed through several organizations, institutions, and industry professionals, seem to be developed without energy efficiency concerns [18].These guidelines have resulted in higher energy consumption to achieve comfort levels.In essence, during the Coronavirus outbreak, health has taken precedence [19], pushing other issues like energy use or weather crisis, as a secondary concern [1].In light of the current health and environmental concerns, if we want to create healthy and energy-efficient indoor settings, correct ventilation systems (VS) design should not be limited to the evaluations of comfort and IAQ but is also required to consider energy, economy, and carbon emissions [18].Fig. 1 illustrates a link among IAQ control, energy use and the VS operation.According to this figure, the airborne transmission of the virus caused by human respiration can happen indoors.The top diagram represents the details of the viability and mobility of the virus and the variations in recommended physical distance ranges from 1 to 2 m.All of these distances provide a reduction in the risk of infection transmission.Factors such as time, droplet velocity, wind movement, the role of masks, and ventilation impact this gradient of risk.Ventilation can dilute the viral load by increasing the amount of fresh air either naturally or by force [20].However, there is a conflict between the desire to improve IAQ by reducing the pollutant and aerosol concentration and the desire to minimize the ventilation rate, as shown in the bottom right diagram of Fig. 1 [21].This will also increase energy consumption to purify and heat the excessive fresh outdoor air [22].Therefore, an optimal ventilation control strategy is required to maintain a balance between the two conflicting objectives of minimizing energy use and maximizing IAQ [21].
Several studies investigated the impact of COVID-19's new ventilation guidelines on IAQ and energy consumption in different settings.Through a bibliometric analysis, Moghadam et al. [23] highlighted the need for comprehensive scientific studies on the energy challenges of COVID-19 ventilation.Zheng et al. [22] investigated the effect of the Coronavirus outbreak on VS energy use through identifying the indoor transmission modes of the virus, comparing the VS operational guidelines and their quantitative impact on energy use.Franco et al. [21] proposed an optimized operation of Heating, ventilation, and air conditioning (HVAC) equipment operation to balance IAQ standards and energy use.Settimo and Avino [24] represented their concern regarding the significance of increased energy use related to new COVID-19 HAVC operational control strategies in public and private settings to maintain IAQ.Cortiços and Duarte [1] compared the energy use and costs and the CO 2 emissions of US office buildings due to applying new COVID-19 guidelines before and after the pandemic.Sha et al. [25] measured a VS operation of a high-rise building during the outbreak.The research proposed an optimal operation of these systems to reduce the transmission risk and energy use through dilution ventilation and ventilative cooling.Qin et al. [26] explored the application of impinging jet ventilation system to reduce virus airborne transmission and energy use in high occupancy density indoor environments.Through a systematic review study, Zaniboni and Albatici [27] suggested the application of both natural and mechanical ventilation to maintain a balance between energy and IAQ, considering the increased energy use after the COVID-19 pandemic regulations.Using machine learning models, Jiang et al. [28] suggested an occupancy-based predictive method to operate VS, reducing energy use and airborne transmission risk.
Despite the significance of the studies conducted on ventilation requirements and COVID-19 prevention, there has yet to be any extensive study on the scientific literature relating to ventilation, IAQ, COVID-19, and energy efficiency published as a result of COVID-19 HVAC-related industry guidelines [23].Although the reviewed studies examined the application of the new Coronavirus HAVC settings for energy consumption and transmission risk reduction, a comprehensive investigation of the VS energy challenges in response to the COVID-19 outbreak has not been discussed in the previous literature.To bridge this gap, this review analyses the literature on excessive energy consumption due to the new building ventilation paradigm to ensure IAQ during and after the pandemic.This has been done through a review analysis of existing literature and led to determining the major research gaps.To the best of the authors' knowledge, this is the first systematic review of its kind, which reviews the state-of-the-art energy penalties of VS operation from the start of the COVID-19 pandemic to the present.To complete this systematic review, this research has examined published papers on building ventilation, IAQ, and energy consumption considering the Coronavirus outbreak from January 2020 to July 2022.This review expands the existing knowledge through analysing recent information on the energy constraints of operating VS and IAQ provision during and after the pandemic and demonstrating the existing knowledge gaps and future directions.The practical implications of the review offer tested energy-efficient strategies to increase the sustainability of current and future buildings, improve IAQ, lower fossil fuels, and the energy costs anticipated to persist even after the COVID-19 pandemic.In essence, the increased amount of fossil fuels used by the building sector has increased CO 2 emissions and, ultimately, significant weather changes [29].Also, today's businesses avidly explore creative innovations compatible with environmentally friendly manufacturing due to the fast rise in environmental preservation requirements [30] and building operational conditions.This targets industry and policymakers regarding regulations and standards considering cost and emission reductions of the correct design and operation of the VS to achieve sustainability.The proper operation of the VS will also bring cost and energy efficiency benefits for occupants and governments.
The results of this critical review analysis contribute to the development of the building industry and services.From an industrial perspective, the energy-efficiency consideration of VS contributes to developing intelligent technologies for future predictions and monitoring in the building service sector [22] and reliable and quick load forecast frameworks for energy supply and demand management [31].HVAC system accounts for almost 10% of global energy consumption [32].Also, nearly 50% of the world's greenhouse gas emissions are caused by power generation, heating and other sectors [33], and changes in fuel cost and energy prices in different operating conditions [34].Thus, the industrial benefits of energy-efficient VS operation fit well with adequate cost savings, compliance with regulations, and enhanced productivity of these systems [35] during severe pandemic periods.

COVID-19 HAVC guidelines proposed by industry professionals and institutions
Since the start of the COVID-19 outbreak, several worldwide organizations and institutions have published ventilation guidelines, as shown in the timeline of Fig. 3. Since the pandemic's start, the number of published guidelines by different associations has decreased monthly, given that the topic was developed through publications, experiments, and medical advancements [36].Also, among all the organizations, ASHRAE continuously released more guidelines with broader topics, from infectious aerosols management [37] to operation and maintenance in different settings [38].The guideline topics released by each of the organizations are provided in Fig. 3.
After institutions published the COVID-19 ventilation countermeasures, the scientific publication trend increased through different However, before reviewing the scientific literature on IAQ, ventilation, energy efficiency, and COVID-19 published following the COVID-19 industry countermeasures, it is essential to review these recommendations based on several key factors.These recommendations are developed without energy efficiency concerns, particularly with a substantial increase of fresh outside air [6] at its nominal speed [39], either naturally or mechanically [40].As shown in Fig. 1, the recommended increased ventilation rates to mitigate the transmission probability can lead to higher energy use to achieve comfort levels [6], particularly from a long-term environmental perspective [41].Additionally, most mechanical ventilation (MV) systems now in use, which were created to operate efficiently under normal circumstances, cannot handle the additional fresh air supply [6].During an outbreak, this extra usage could be acceptable; nevertheless, balancing the exchange between energy and transmission reduction is essential for sustainability and ongoing public health [41].Moreover, no single recommendation may be used in every situation since, in reality, indoor air conditions differ in terms of climate, building type, and use [41].
Table 1 compares and contrasts the COVID-19 HVAC-related countermeasures proposed by industry professionals based on six key factors.Overall, these guidelines mainly include outdoor air and airflow pattern, indoor air and pressure differentials, operating HVAC systems and

Table 1
COVID-19 HVAC-related countermeasures proposed by industry professionals.Summary of [1,22,25,39,42,43].auxiliary equipment, filtration and air purification and the temperature (T) and relative humidity (RH) setpoints.The institutions commonly agree on these critical factors as the leading ventilation solutions to significantly reduce the danger of the virus spread [42].Although most of these recommendations seem to be consistent, several conflicting details arise from the uncertainties of the virus characteristics and transmission mechanism in buildings [42].
The comparisons in Table 1 demonstrate that providing excessive outdoor air supply and creating efficient circulation patterns is a crucial tactic to lower the danger of virus airborne spread.Also, the following common countermeasure is the increased running time of HVAC systems, 2 h before and after occupancies.It is also essential to keep regular filter maintenance and possibly disable the air recirculation for air conditioning devices and maintain negative pressure to avoid airflow routes running from polluted areas to clean area.However, these measures vary at some points, such as T and RH setpoints and the operation of heat recovery equipment, given that more research is yet required to investigate whether and in what condition setpoints changes impact virus transmission [42].

Pre and post-COVID-19 HVAC operation settings
Fig. 4 classifies HVAC operation settings, comparing ASHRAE 62.1 standard and the ASHRAE COVID-19 mitigation guidelines.These parameters show variations in ventilation outdoor air volume, in which there is a substantial increase of fresh and outdoor air provision, as well as cancelation of the 20% re-entrainment air for energy efficiency.The additional fresh outdoor air supply is due to decreasing the virus transmission by removing and diluting indoor air contaminants [40].The cancelation of 20% re-entrainment air also corresponds to maintaining standard IAQ by reducing the exhaust air and improving more fresh air supply to reduce the risk of transmission through the dilution of airborne contaminants.If air recirculation is needed, VS requires to include an exhaust air filtration and increase the proportion of fresh outside air by more than 40% [42].Due to the inefficiency of the pre-pandemic settings, the exhaust air transfer rate changed from less than 10% to less than 3% with the same principle.Air distribution based on the use of MV or MV and natural ventilation (NV) also varies under favorable climate assessment.Filters are upgraded from MERV 8 to MERV 13, and additional HEPA filters and ultraviolet germicidal irradiation (UVGI) devices were introduced for hyalinization of air.While the T setpoints remained the same as previous ones, the RH has changed to a broader range, reducing 20% and 10% on the minimum and maximum margins, respectively [1].

Comparison on VS energy use prior to and during the coronavirus outbreak
Concerning the rise in fresh airflow rate as well as the load, Zheng et al. [22] compared VS energy use prior to and during the Coronavirus outbreak.The comparison process based on the energy equations is presented in Table 2.As seen in this table, the increased energy use has been calculated based on the additional fresh air supply recommended by the guidelines as the influencing factor of increased energy consumption.The extra consumption is also correlated to the mass flow and enthalpy of the external airflow as well as the enthalpy of air for pre-heating or moistening [22].Based on the analysis conducted in Tsinghua University Building Energy Research Centre, it has been proved that in comparison to pre-pandemic time, VS energy use increased by 128% during the COVID-19 outbreak, that calls attention to energy conservation [22].
Having the guidelines reviewed and the energy impact of the HVAC operation compared before and after the pandemic period, the study represents the methodology of a review analysis of the scientific literature published as a result of the guidelines.

Review methodology
According to PRISMA (preferred reporting items for systematic review and meta-analysis) criteria, a systematic review was presented [44].The aim was to create a representative compilation of current literature concerning the published COVID-19 HVAC-related industry guidelines, clarify gaps, and guide future directions.The comprehensive review framework includes three steps of planning, conducting, and reporting [45], in which each step's output becomes the following step's input [46].
The first step of the review process was to clearly define RQs, each of which targets a different aspect of the topic for the comprehensive evaluation of the subject.Based on the main research problem and determination of the research scope, which is analysing recent information on the energy constraints of operating VS and IAQ provision during and after the pandemic and highlighting the gaps and future directions, four main RQs have been identified in this review.The motivations behind defining each of the questions are presented in Table 3.
A comprehensive repository of metadata of the relevant literature from reliable databases was initially created to secure a representative compilation of current literature related to COVID-19 energy challenges of operating VS and IAQ provision for the review [47].Other reliable methods, such as (semi)automation of data extraction, can also be employed to decrease the workload for data information aggregation [48].However, to reduce any risks of ambiguity, incomplete or unseen data aggregation, it was decided to create the paper repository for this review study manually.Out of the top search engines to obtain academic literature's richest metadata, four peer-reviewed databases including, IEEE Xplore, Science Direct, Scopus, and Google Scholar, were chosen as the sources of data extraction in the initial search [47].Other databases, such as Dimensions and Web of Science, were also used; however, due to overlapping in publications between Elsevier and several unavailabilities of full access, these databases were finally removed from the list of databases.The study looked for the keywords within all fields of materials, including article titles, abstracts, keywords, and source titles.A timeframe from January 2020 to July 2022 has been taken into consideration while creating the repository to ensure the most recent relevant data.The initial search result showed 1681 articles numbered down using a chosen eligibility criteria.The selected papers were then inserted to the Mendeley reference manager.Table 4 lists each database's search terms, area, and results.
The eligibility criteria for selecting relevant literature include several inclusion and exclusion factors to refine search results and reduce the papers to the most relevant ones aligned with the scope of this research [46].Table 5 lists the eligibility criteria used in this systematic review.
The initial search results excluded duplicate papers based on PRISMA guidelines, numbering 658 articles.Following the eligibility criteria, 560 papers were excluded, and the remaining 98 papers were considered for full-text review.Based on clear association with the scope of the review and exclusion of any non-qualified publications based on eligibility criteria, 61 publications have been found eligible for further analysis in this study.The details of each RQs were investigated in all 61 publications.Using Microsoft Excel, the final selected publications were compiled into spreadsheets to efficiently manage the data related to the answer of each RQs.Fig. 5 illustrates the PRISMA diagram of this review analysis.

Results and discussion
By comprehensively reviewing the current literature on IAQ, ventilation, energy efficiency, and COVID-19, it was possible to evaluate the level of research interest in this area and outline essential research sources.Most of the publications on the COVID-19 epidemic were written quickly due to the high demand for more information to advance research and help humanity, speedy online publishing following approval and expedited review, and the journals' special supplements [49].Thus, it was crucial to investigate the research area's nature and its main driving elements.This section details the results and provides a discussion over the review analysis and provides answers to the RQs discussed recently based on the findings of this process.

RQ1: what is the publication landscape relating to IAQ, ventilation, energy efficiency, and COVID-19?
• Industrial guidelines and scientific publication trends The Coronavirus disease was deemed a Public Health Emergency of International Concern on January 30, 2020, and a pandemic on 11 March 2020 by the World Health Organization (WHO) [50].The

Table 2
Calculation of VS energy use prior to and during the Coronavirus outbreak.Summary of [22].escalating outbreak has prompted a flurry of research activity on the coronavirus.Therefore, through examining the publication date of both scientific and industrial publications, it was possible to determine how the subject has changed over time.Fig. 6 illustrates the month-on-month number of publications by guidelines and scientific papers in this area, from the first guideline document appearing in January 2020 to the single recorded paper published when this review was conducted in June 2022.Overall, while the number of guidelines is decreasing, the overall global interest in this topic is growing every year, considering the rise in 8 papers published in 2020 compared to 36 documents in 2021.After institutions published the COVID-19 ventilation countermeasures, particularly in April and August 2020, the scientific publication trend started to increase from       suggesting that this area began to gain interest.Fig. 7 illustrates the top-performing institutions in the top 8 countries regarding the number of publications on this topic.Based on this figure, the widespread distribution of publications represents an interest in this area globally, corresponding to the hypothesis that it is a significant evolving area.However, Italy appears as a top-publishing country on this topic, with 11 papers making up 18.0% of all publications.In essence, Italy was the first European nation that experienced the COVID-19 outbreak, and the repercussions on the populace were profound [51].According to Ref. [24], in Italy, there was a slow implementation of the appropriate precautionary measures, health and IAQ enhancement of traditional structures [24].Thus, Italy has increased its research contribution to the Covid-19 pandemic due to its initial engagement in this area [51].After Italy, USA and China have significant contributions in publications with 8 papers.It should be noted that countries and institutions with less than 2 published papers have yet to be illustrated in this figure .Out of 6 published papers from the most contributing institution in Italy, 3 have mainly focused on optimizing HVAC operations for balancing energy use and IAQ improvements.Franco et al. [21] have examined the optimal HVAC system control aiming to obtain improved level of health and energy efficiency.Up to 30% energy-saving and 25% increased comfort level were obtained through the proposed multi-objective optimization.In another research, Franco et al. [52] suggested a method for broadening the energy perspective to IAQ using occupant-centric control strategies and HVAC supervisory control strategies.Using experimental analysis and real-time knowledge of occupation for maintaining IAQ levels and energy efficiency, Franco and Leccese [53] analysed the link between C CO2 and the status of occupancy in different classrooms of the University of Pisa.The rest of the 3 papers have mainly concentrated on applying information and communication technologies (ICT) and internet of things (IoT) technologies to increase the efficiency of the HVAC systems and IAQ in the COVID-19 context.For instance, Anastasi et al. [54] investigated IAQ and energy use of smart buildings considering the occupancy profile and building management system.With the aim of addressing the conflicting objectives of optimizing energy use and IAQ, Franco [55] investigated application of ICT for HVAC system operation.Testi et al. [9] proposed a novel building simulation methodology using dynamic modelling, aggregating   consumption metrics, pre-management of energy system operation, reducing non-renewables, and a post-management of avoiding predicting mistakes through VS flowrate modifications.

• Type of results
Comparison of papers presenting qualitative or quantitative results is another interesting metric for the systematic review of energy efficient ventilation and IAQ in COVID-19.Fig. 8 illustrates the number of publications by type of results.Based on this figure, there is an apparent sway toward quantitative outputs using first-hand data, by 67.2% of papers.
Less focus has been placed on meanings and experiences related to Coronavirus transmission mitigation for various groups of people [56].Since quality standards of qualitative papers are highly correlated with how the research questions, proposed methodology, and primary objectives have been defined, conducting high-quality qualitative research has been more challenging [57], especially with the swift evolution of the Coronavirus.While quantitative research is well suited to compile numerical data to test causal relationships among different variables and to develop a statistical picture of energy-related issues of ventilation and IAQ, in-depth lessons of qualitative studies inform the local, experiential knowledge as complementary to the existing information [58].The reason behind less qualitative research in the COVID-19 context is due to two main constraints of "time" and "physical distancing," according to Tremblay et al. [59].Also, the quality of qualitative research is influenced by the research framework, research plan, and the topic of interest, which can significantly affect the proportion of this research type [59].Qualitative research's role in exploring the policies and practices changes during the pandemic, their adaptation, and implementation in new circumstances is also inevitable during and after the pandemic [60].Table 6 illustrates the breakdown of Fig. 16.Classification of papers by their proposed ventilation control strategies, building type, ventilation type, occupancy density and location.

Table 8
Classification of papers by their proposed ventilation control strategies and their energy saving potentials.

HVAC auxiliary equipment
Application of a pre-fabricated system "window machine" linked to soffit-integrated decentralized regenerative VSs Up to 77% reduction of heating energy [66] Coupling thermal recovery through a heat exchanger plus thermodynamic recovery using a heat pump 60% and 72% reduction of energy use [124] Application of an autonomous high-efficiency AHU 31% and 46% energy savings using highefficiency AHU and a heat recuperator, respectively [13] Coupling novel membrane-assisted radiant cooling systems with NV 10-45% reduction of energy use [72] Application of mechanical ventilation with heat recovery systems and NV strategies with onesided openings 40% reduction of heating demand [65] Application of an automatic environmental-controlled fan Transitions to a mixed (home and office) working habits and activity-based office settings as well as smart indoor services operations Up to 50% reduction of energy use [71] Engaging occupants through ICT + application of DCV flowrate linked to the occupancy density 15-30% reduction of energy use [55] Application of advanced ICT and IoT technologies for monitoring environmental conditions indoors ND [54] DCV Application of a new DCV inspired by the concept of intermittent ventilation Up to 88% energy efficiency [80] Application of a DCV based on occupancy, and provision of improvements for heat pumps, chiller supply water T, and heat recovery control 44% energy saving (33% of which is by the application of DCV) [52] Application of DCV with absolute air filtration + Application of more window airing and, consequently, NV solutions 10-40% energy saving [79] Application of a DCV based on coupling real-time C CO2 and occupancy data ND [53]

Data fusion
Application of dynamic setpoints, combining IEQ and IAQ, as well as application of irregular occupancy control Up to 30% energy saving [21] Indoor dynamic modelling, aggregating consumption metrics, pre-management of energy system operation, reducing non-renewables, and a post-management of avoidingpredictingmistakes through VS flowrate modifications ND [9] Application of data fusion combined with providing high-resolution data for occupancy and IEQ ND [74] Application of a set of dynamical models connected with weather predictions and HVAC operation setpoints as well as physics-based material balances with ventilation phenomenological airflow controls ND [87] Pollutant concentration monitoring

Stay time modification
Modifying teachings periods from winter to summer seasons using the Monte Carlo approach and the Weibull method for identifying energy usage ND [81] Covering face with a mask and reducing the staying duration to half ND [89] a ND= No data.

T.T. Moghadam et al.
the number of papers by results type per building type.It is essential to mention that more than one-third of the quantitative papers have mainly focused on educational buildings as a case study.This is due to the extended stay of students and teachers, a rethought for a new "in-presence" living [14], and the high demand for quick response to the COVID-19 pandemic in such a high crowding setting [9].This is also further discussed in section 4.2, where building types and occupancy profiles of different case studies are investigated.According to Wieringa et al. [103], engineering papers can be classified by type of research based on the categorizations and evaluation criteria presented in Fig. 9.This classification is an indicator of the maturity of the research.
Fig. 10 shows a comparison of the document type by research type.Looking at the results in more depth, there is a mixture of document types among the publications.However, almost three-quarters of publications are sourced from journal articles constituting the most common document type.This implies that research development has been mainly made on in-depth journal articles rather than short research papers for conferences.In essence, to quickly publish studies connected to the pandemic [104], funding organizations created additional grant sources, and journals sped up the review procedures [105].Also, numerous face-to-face activities, such as seminars and conferences, were postponed, held online, or put on hold during the outbreak [106].Thus, more contributions from other document types are needed in IAQ, COVID-19 ventilation, and energy efficiency.This classification includes evaluation, solution, and philosophical papers; however, most papers in this category are evaluation-type research, implementing a technique already in practice to address management of IAQ and VS energy use under the COVID-19 context.This proportion is supported by recording both causal properties studied empirically by case studies, field studies, field experiments, and survey publications and logical properties studied by conceptual means, such as mathematics or logic.Review and conference publications represent the same research type variation, including the evaluation and philosophical research.Also, articles contribute more evaluation research with 33 publications, and solution research type as the second most common type of research with 9 published papers.Philosophical research is the third most common type of research supported by the lower number of qualitative-type publications compared to quantitative ones.The number of published philosophical research in review papers is 5, whereas there are 3 philosophical journal articles.There is only 1 evaluation type research published within the review category.Conferences contribute with 3 evaluation and 2 philosophical research type papers.Solution was the only type of research that originated from the theses.Editorial and Note contribute 2 and 1 opinion type research, respectively.Books concerning IAQ, COVID-19 ventilation, and energy efficiency have yet to be published.This shortage implies that this area of research has not been evolved by books, authors' opinions, or any lessons learned by authors' personal experiences.This section's primary focus is to compare each publication's main contribution in response to the barriers of reducing energy consumption and improving IAQ during pandemic periods.This analysis represents the maturity of the topic through classification of the publication outputs and outlining the influential existing themes and interest [46].
As a qualitative approach, "keywording" was used to cluster the contribution types [107].According to this method, the main keywords representing the research contribution were chosen from the abstracts, introduction, and conclusions.Through aggregation of the keywords, the results were simplified to create better visibility of trends and promote a comprehensive understanding of the research of this particular topic area [50].Fig. 11 summarizes different research contributions.
Fig. 12 depicts the distribution of publications by type of research and type of contribution.Overall, the most common output in this area is the methodology, and with 32 publications, the role of evaluation-type research is inevitable in maturing and evolving research output in this contribution type.Several research examples also propose new solutions with their intended use, which have contributed to research Methodology outputs.The rest of the evaluation type research belongs to Model, Platform, and Architecture types of contributions with 2 and 1 publications, respectively.Solution is the following most common type of research that is significantly published in Methodology-based contributions with 9 papers and Model and Platform-based contributions with only one publication.Philosophical is the subsequent common research type with the overall 10 published papers, mainly resulting in 8 and 2 Framework and Architecture-based types of contribution, respectively.The Opinion type research papers contributed only Architecture and Framework with 2 and 1 published papers, respectively.It should be noted that due to the lack of publications with Experience and Validation research types, no contributions have been made within this research type.Therefore, the future research direction could be more validation and experience-based research being developed to implement the already-in-hand theories into Model, Platform, Tool, Process, and Theory-based research.

RQ2: what building types and occupancy profile have been studied in the context of IAQ, ventilation, energy efficiency, and COVID-19?
The answer to this question contributes to gaining more accurate information regarding different building occupancy densities and profiles.
Fig. 13 lists the building types used as a case study in the publications.Within this general classification, educational buildings include    classrooms in schools or university buildings.Residential buildings account for single houses, terraced houses, apartments, and public social dwellings.Public buildings include public transportation (train stations and airport terminal), temples (churches and mosques), gyms, and restaurants.Any typical commercial building, shopping mall, and office building fall into the office and commercial building categorization.There are also a few research which protects laboratory condition.It is also important to note that some papers presented comparison research between various types of buildings and have not merely focused on one specific environment, which has created the category of various building types.
Based on Fig. 13, the COVID-19 energy challenges of operating the VS have been significantly investigated in educational buildings by nearly one-third (29.5%) of the publications.Approximately one-fifth (21.3%) of the publications have not mentioned or used any specific building type in their research.Early in the emergency, practically all educational facilities were closed due to the pandemic danger brought on by the high population of asymptomatic young people.Schools sometimes have high crowding levels, given the extended stay of students and teachers.Due to increased hygienic-sanitary regulations, the outdoor fresh air supply will become more energy-intensive [9].In order to prevent a long-term adverse effect on pupils, schools and educational settings should be quickly reopened.Additionally, additional work has to be done to fulfil the criteria for air quality while using less energy [9].After the COVID-19 epidemic, educational facilities also needed rethought for a new "in-presence" living [14].This problem necessitates innovative energy management to address serious health and weather hazards [9].
Office and commercial building types and residential buildings provide quite the same number of publications constituting 13.1% and 11.5% of all papers, respectively.By definition, office buildings run on centralized, enclosed temperature control and VS with high occupancy [1].Airborne contamination has become a severe issue due to the increasing use of MV and the high occupancy patterns in offices [75].Considering the introduction of technologies like WELL and Fitwel, targeting enhancing the inhabitants' well-being, airborne transmission has become more significant [108].Scientists, environmentalists, and authorities have raised concerns regarding exceeding energy use due to the new pandemic HVAC operational countermeasures being restricted to office buildings/telework and third-party logistics [109].
The COVID-19 outbreak has also led to novel house occupancy patterns, the effects of which on IAQ and energy usage are not fully understood [65].Actions typically carried out in schools [110], offices [111], or outdoors [112], were focused within the home during the lockdown since households were continually using common space [64].This has resulted in the deterioration of IAQ due to working from home during imposed lockdowns.Changes in occupancy can also directly affect building energy use, making them a useful tool for predicting of energy consumption in the future [113].Due to the mentioned issues, numerous articles have examined the effects of home-office lifestyles on the power consumption in residential structures.This becomes more significant considering the fact that even if the COVID-19 health crisis is resolved, energy expenditures for households related to changes in working habits will be predicted to continue rising [114].
There are only 2 papers that have focused on hospitals, constituting 3.3% of all papers.From the remaining building types, 9.8% and 8.2% of the papers conducted comparison research between various types of buildings and public buildings, respectively.Finally, at an equal percentage of 1.6%, high-rise buildings and laboratory conditions fall into the least frequent building type within the repository.
The energy used for building air conditioning may be significantly influenced by occupancy schedules and density.Accurate occupancy data is of major significance to measuring the active influence of building occupants over energy consumption.However, the challenge is to identify every sources of energy consumption and its corresponding component considering how ambiguous the behaviour of the occupants is.The interior environment is impacted by this uncertain behaviour, as a significant parameter within the wide fluctuations in energy use.Additionally, occupant behaviour is affected by interior circumstances, affecting the overall energy consumption.By providing the ASHRAE 62.1 defined airflow rate depending on correct occupancy, considerable consumption reductions related to VS may be possible.A similar approach, however, cannot be applicable at the moment due to that the bare minimum of airflow is not adequate to stop the Coronavirus transmission in enclosed places.Thus, to implement a demand-based operational strategy, it is crucial to connect the building HVAC consumption pattern to the occupants' consumption habits.However, if precise data on this interaction is poor or non-existent, this cannot be accomplished [101].
Fig. 14 classifies different building types studied by the papers based on their maximum number of occupants (#) or occupancy density (person/m 2 ).According to this classification, high-rise, office and commercial, public, hospital, and educational buildings have been mainly categorized in very high-density (VHD) and high-density (HD) indoor environments where the maximum number of occupants are between 3500 to 500 and 500 to 50, respectively; mid-density (MD)  where between 50 and 10, and low-density (LD) in residential buildings where the maximum number of occupants are between 10 and 0.
The dynamic relationship among occupants and building systems needs to be accurately reflected by an oversimplified understanding of occupancy.However, the majority of the occupancy-related variable air volume control has been restricted to energy-saving evaluation, and enough evidence does not exist to support its effects on indoor environmental quality (IEQ), particularly using air handling unit (AHU) for T and RH management.The related savings can be attributable to providing bare minimal airflow [101].Therefore, accurate building occupancy density and schedule information is significant in building air conditioning energy usage.

RQ3: what ventilation types and effective control strategies have been discussed in the context of IAQ, ventilation, energy efficiency, and COVID-19?
Overall, three general ventilation modes of natural, mechanical, and hybrid (natural and mechanical) are utilized to ventilate a building [115].The answer to this question allows a comparison of papers using NV or MV systems or hybrid ventilation (HV) in their research.Fig. 15 presents the number of publications by type of ventilation.Based on this figure, there is an apparent sway towards MV systems in research by the 65.6% of papers.The remaining 34.4% of publications offer HV and NV, with 19.7% and 14.8% of published papers, respectively.
The results of this analysis allow comparison of papers using each of the three ventilation modes in their research.While NV is lowmaintenance and energy-free, it increases outside pollutants concentration inside the building and results in occupants' discomfort, especially during the winter time [116].Occupants may also need to take into account noise pollution and low air quality outside the buildings along with the security considerations especially in cities and crowded areas [117].Filters are frequently included in MV systems, which may remove or dilute particulates from external air [118].Table 7 illustrates the comparison of the type of building by type of ventilation in the assessed papers within the repository.
Overall, all three ventilation types have been studied in educational, office and commercial, and residential categories, and MV is dominantly studied by these three building types.There are 4 papers concentrating on hospitals, high-rise buildings, and laboratory conditions that have merely investigated MV.The utilization of such systems has also been more dominant in very high-occupancy indoor environments such as educational buildings, public buildings, and office and commercial buildings where the number of occupants is high, and occupants are often together for more extended periods, resulting in increased infection risk probability.According to Ref. [117], the educational building sector in developed countries necessitates installing MV, considering that the provision of IAQ and IEQ rely mainly on outdoor climatic conditions when using NV.This highlights the high prevalence of indoor pollutant sources when using NV and the fact that NV is not adequate to provide fresh air supply to meet COVID-19 ventilation criteria in high-occupancy indoor environments.
NV is not studied in public and various buildings and by the group of papers that have not specified any building types in their research.While there are 12 papers focusing on MV by the not specified group, only 1 published paper targets HV.This could be due to some challenges related to the operation and control of HV.According to Ledo Gomis et al. [120], HV systems are significantly associated with outdoor weather conditions and air quality to serve adequate airflow to the building.These systems also require effectively harmonizing NV with MV to maintain a standard level of IAQ and reduce energy consumption through correct window opening schedules and MV setpoint changes [120].Within various and public building types, the splits between the studied VS are quite the same, except for various building types representing 2 publications focusing on HV.While MV is more commonly studied in educational, office, and commercial buildings with 11 and 4 published papers, respectively, NV and HV modes are more prevalent in residential buildings with an overall 6 published papers.The even number of papers investigating NV and HV proves the significance of considering both two potential ventilation modes in residential buildings [68].Studies has shown that homes, in which individuals spend a significant period, are mostly exposed to aerosol contaminants [121].Considering that householders sometimes lack the funds to equip buildings with MV systems, particularly in older properties, housing IAQ enhancements through MV systems are frequently given less consideration [122].
The COVID-19 pandemic mitigation measures encourage retrofitting existing structures and switching to innovative approaches that prioritize well-being above consumption.This can impose excessive consumptions due to higher ventilation rates and energy costs [1], particularly in large-scale environments such as high-rise buildings.Besides, due to the increased infection probability, the Coronavirus outbreak led to the abandonment of high-rise structures with high occupancy density [25].To reduce HVAC-related energy use and infection risk, papers that have considered high-rise public and various building types have deeply investigated the operation of the MV systems.
Investigating the airflow pattern by controlling an MV system is one of the main factors in effectively reducing the airborne particles in hospitals and isolation rooms [123].This investigation enables the development of novel ventilation strategies for use in real world applications through environmentally-intelligent controller.Also, investigating the early implementation of COVID-19 infection control interventions through MV systems in hospitals lead to financial savings by decreased clinical-related infections [62].
Fig. 16 demonstrates the classification of papers by their proposed ventilation control strategies, building type, ventilation type, occupancy density, and location and Table 8 shows the breakdown of these ventilation control strategies and their energy-saving potentials.According to this classification.
• The most frequent ventilation control strategies mentioned by the papers relate to the application, improvement, and efficiency optimization of HVAC auxiliary equipment.According to Zheng et al. [18], HVAC auxiliary equipment are mostly efficient strategies once there is no possibility of boosting fresh air supply, and the indoor transmission risk is more probable.Therefore, the correct implementation of such measures is effective both from IAQ and energy-efficiency perspectives [18].From this category, the utilization of HEPA filters, as well as the UVGI air purification technology have been suggested by several publications.• Source control occupant-focused design of the VS based on occupant density, intermittent occupancy schedule, and behaviour-based, infection risk-energy consumption modelling is the second most prominent ventilation solution mentioned by the papers.• Improved NV strategies and the application of novel ventilation operation strategies using ICT, IoT, and artificial intelligence (AI) technologies for monitoring IAQ and IEQ conditions have also been highlighted by the papers with the same significance level.• Application of mobile air purifiers and personalized ventilation, DCV an example of occupant-cantered control techniques and data fusion, synthesizing high-resolution IEQ and occupant tracking data have been commonly mentioned by several publications.
The rest of the papers have focused on pollutant concentration monitoring, indoor stay time modification, and other strategies, as shown in Table 8.

RQ4: what are the challenges associated with increased energy consumption as a results of maintaining IAQ during and after COVID-19?
The answer to this question identifies the primary negative aspects of IAQ, ventilation, energy efficiency, and COVID-19, highlighting existing solutions' limitations.Fig. 17 depicts the distribution of challenges associated with this subject area.Analysing this figure, it is clear that "Increased fresh air supply" is the most significant obstacle found by 20 papers within the repository.Given that it is necessary to purify, warm up or chill, dry, or moisten the fresh outside air before flushing it into the space, the HVAC heating and cooling load is significantly raised.Also, the increased electricity consumption to run ventilation fans for more fresh air supply substantially affects the final energy use [18].
The following most common difficulties, in 14 publications, is the "Low IAQ & IEQ." "Infectious diseases transmission & increased occupants' health risk" and "Occupancy density & schedule limitation" are the subsequent prominent limitations with almost the same occurrences of 9 and 7 in publications, respectively.With the same split, 5 publications outlined the challenges of "HVAC design, modelling, installation, operation & maintenance issues, and cost" and "Limited ventilation capacity."Similarly, "Seasonal efficiency variation," "Increased carbon emissions," and "Occupants' behaviour" were discussed by 4 publications within the repository.Other significant challenges that can increase the conflicts between reducing the energy consumption of VS and improving IAQ during the COVID-19 pandemic are "Regulation, design & standard changes," "Low advanced HVAC auxiliary equipment," "Unknown efficient methodologies & Uncontrollable characteristics of the aerosols."Each has occurred 3 times by 9 publications."Geometrical, physical, urban & building characteristics" and "Energy performance gap between operational & initial costs" are the least frequent obstacles mentioned only by one paper.
The causes of each challenge can be classified into 8 main categories, as shown in Fig. 18.However, some of the reasons overlap the others in terms of their group categories.According to this classification, most of the limitations arise from "Insufficient ventilation relating to low IAQ and IEQ levels."A shift from NV to MV for additional ventilation rate and the wrong mode of ventilation are the most significant causes mentioned by this category."New occupancy patterns" is the second most common group of causes associated with frequent limitations.From this group, changes in home occupancy and remote work during the COVID-19 outbreak have been discussed by several papers within the repository.Relevant examples of some frequent causes in this category are the performance shift between predicted energy performance and the real system operation and the lack of supervisory control on HVAC systems operation.Reasons relating to "Indoor airborne viral transmission" play essential roles in creating energy efficiency limitations of the VS.From this category, lack of effective strategies to assess the airborne virus concentration and distribution is one of the prominent causes mentioned by different publications.Other reasons, such as air leakages relating to the wrong "Building design," have also been discussed in papers."Weather condition," "New regulations and guidelines," and "Financial issues" have also been elaborated per each of the limitations.
This study investigated the publication landscape on the energy challenges of providing standard limits of IAQ via ventilation during the Coronavirus outbreak.A critical systematic review of the publications concentrating on ventilation paradigm as well as excessive energy consumption to ensure IAQ during and after the pandemic has been conducted to define the key results, gaps, barriers, and future direction of this topic.According to these gaps, future research should consider not only increased energy usage due to recommended higher airflow rate supply but also different ventilation operation methods related to source control measures as well as occupancy density in other building types.The maturity of the research should also be developed by different research types with various contributions, as shown in Fig. 19.
To sum up, the excessive energy use by VS during the pandemic sounds the alarm on energy conservation.This trend assumes to be continued after the pandemic period, as proved by previous studies, including Cortiços and Duarte [1], who calculated a 21.72% rise in HVAC energy consumption of US high-rise office buildings in mixed-humid locations, and Cortiços and Duarte [39] who estimated increased HVAC energy use and CO2 emissions as a result of implementing the COVID-19 guidelines in Europe's top five economies.Therefore, Innovative design and operation methods for building VS that improve energy efficiency and enable greater resiliency, health, and safety for occupants, particularly during pandemic occurrences, need to be addressed.This calls for considering more resilient solutions that can make it more sustainable and profitable.
It is also important to note that, within this systematic review, the provision of relevant data has been limited to a period from January 2020 to July 2022, and the papers beyond this period have yet to be considered.This timeframe was also limited due to the allocation of time to conduct the review analysis.Thus, the limitations are related to the period, the research trend, and the eligibility criteria for selecting relevant data that ultimately determined the categories and quantities of the literature.
Additionally, due to the topical nature of COVID-19 studies, this might affect the comprehensiveness and recency of the review and how the research on the energy efficiency of the VS has been developed recently.However, this review analysis's outcomes reveal opportunities for future studies on the same topic that have addressed the limited timeframe and publications.This includes research studies that consider more selection criteria and the inclusion of grey literature sources, industrial reports, and white papers to complement the findings of this study.

Conclusion
This study critically reviewed the challenges related to COVID-19 transmission risk mitigation through VS and its associated energy use.A systematic review analysis of the existing literature was conducted to define this subject area's research gaps and future direction.The method provides a comprehensive standardized framework ensuring a reliable and organized review process with minimized bias.
The findings of this review are of great significance in identifying the energy constraints of operating VS and IAQ provisions through critiques and comparisons during and after the pandemic.The review results also contribute to disseminating issues and innovative VS advancements that will aid in the sustainability and resiliency of the buildings and greenhouse gas reductions through energy-efficient strategies during unprecedented periods.With today's energy crisis, if the increased share of fossil fuel used by building services and technologies and the associated CO 2 emissions are not considered, there will be significant negative impacts on the environment, public health, and the economy.These impacts will become more severe during unforeseen situations such as pandemic periods.Therefore, learning from the past will help adopt tested energy-efficient approaches in buildings and the provision of resiliency for the design and operation of the VS during and after the pandemic.This targets industry and policymakers regarding regulation, standards, and applications considering cost, energy, and emission reductions of the correct design and operation of the VS to achieve sustainable development goals.In essence, building industries are encouraged to invest in developing intelligent technologies for future predictions and monitoring, considering the energy efficiency of the VS.
With the Covid-19 ventilation countermeasures published by industry professionals, the review findings highlight that further research is necessary to validate the proposed approaches, particularly in highlycrowded interior settings.
Additionally, new home occupancy patterns have increased the importance of residential structures' energy usage to estimate future potential energy efficiencies.Since correct information on occupancy and occupant behaviour may result in significant energy savings for airconditioning, occupancy schedules, density, and the proactive impact of inhabitants on energy consumption should be prioritized.Also, addressing low IAQ and IEQ due to insufficient ventilation increases energy use to maintain IAQ.Thus, application and efficiency optimization of HVAC auxiliary equipment, such as HEPA filters and UVGI technologies, have been commonly recommended to meet COVID-19 ventilation criteria in high-occupancy indoor environments effectively.
According to the findings of this review, future research should be conducted based on different types of research and contributions with novel methodologies and approaches toward addressing the mentioned challenges even after the resolution of the Coronavirus outbreak.This will enable building managers, industry professionals, and academics to learn from the past and prepare for any future disasters to guarantee the resiliency of the buildings through tested and informed decisions.Also, effective ventilation control strategies should be assessed in various indoor settings with different occupancy profiles.A paradigm shift from space-based design to the occupant-based design of the HVAC systems is required to balance between the conflicting goals of minimizing energy use and maximizing IAQ standards through the effective operation of the VS.Lastly, due to some limitations related to the publication timeline and the eligibility criteria, the outcome of this review provides opportunities for more comprehensive, up-to-date future studies on the energy efficiency of the VS and IAQ provision during the extreme conditions.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 2 .
According to this figure, section 1 of the paper reviews the state-of-the-art.Section 2 mainly compares HVAC-related guidelines provided by different associations based on some key operational factors.Section 3 of the paper provides the details of the methodology and the RQs.The review results and a detailed discussion of the outcomes of the RQs are presented in section 4. Finally, the conclusions and the future work orientations are given in section 5.

Fig. 1 .
Fig. 1.Energy efficient ventilation and IAQ in the context of COVID-19 (The bottom right diagram has been redrawn from Ref. [21]-The ventilation figures have been taken from Ref. [20]).
VS energy use before the COVID-19 outbreak Q total,n = Ʃ Q n VS energy use during the COVID-19 outbreak Q total,p = Ʃ Q p Q = hourly VS load by the equipment n = normal period Q = hourly VS load by the equipment n = pandemic period VS load before the COVID-19 outbreak Q n = Q o,n+Q r,n → Adding 100% fresh air supply → VS load during the COVID-19 outbreak Q p = Q o,p Q o,n = VS load of external airflow Q o,n = m o, n×| h o− h s| m o,n = mass flow of external airflow before the COVID-19 outbreak h o = enthalpy of external airflow h s = enthalpy of supply air Q r,n = VS load of return air Q r,n = m r, n×| h i− h s| m r,n = mass flow of return air h i = enthalpy of indoor air h s = enthalpy of supply air Q o,p = VS load of external airflow Q o,p = m o, p×| h o− h s| m o,p = mass flow of external airflow during the COVID-19 outbreak h o = enthalpy of external airflow h s = enthalpy of supply air ∝ = Q total,p − Q total,n Q total,n × 100% ∝ = VS load before and during the COVID-19 outbreak VS energy use during the COVID-19 outbreak Ep = En × (1 + ∝) Ep = VS energy use during the COVID-19 outbreak En = VS energy use before the COVID-19 outbreak

Fig. 5 .
Fig. 5.The PRISMA diagram for the systematic review analysis.

Fig. 6 .Fig. 7 .
Fig. 6.Research trend by guidelines and scientific publications per year.•Top-performing countries and institutions

Fig. 8 .
Fig. 8. Number of publications by type of results.

Fig. 10 .Fig. 11 .Fig. 12 .
Fig. 10.Comparison of type of document by research type.• Comparison of contribution type by research classification

Fig.
Fig. Number of publications by building type.

Fig.
Fig. of publications by ventilation type.• Comparison of type of building by type of ventilation

Fig. 17 .
Fig. 17.Number of publications by frequency of challenges.

Fig. 18 .
Fig. 18.Classification of challenges and the related causes mentioned by the papers.
The study is

Table 3
List of the RQs.What ventilation types and effective control strategies have been discussed in the context of IAQ, ventilation, energy efficiency, and COVID-19?
RQ1: What is the publication landscape relating to IAQ, ventilation, energy efficiency, and COVID-19?RQ2: What building types and occupancy profile have been studied in the context of IAQ, ventilation, energy efficiency, and COVID-19?

Table 4
Search terms in databases.

Table 5
Summary of the eligibility criteria.

Table 7
Comparison of type of buildings by type of ventilation.