Air quality standards for the concentration of particulate matter 2.5, global descriptive analysis

Abstract Objective To compare ambient air quality standards for the mass concentration of aerosol particles smaller than approximately 2.5 μm (PM2.5) and exposure to these particles in national and regional jurisdictions worldwide. Methods We did a review of government documents and literature on air quality standards. We extracted and summarized the PM2.5 concentration limits effective before July 2020, noting whether standards were enforced, voluntary or target. We compared averaging methods and permitted periods of time that standards may be exceeded. We made a descriptive analysis of PM2.5 standards by population, total area and population density of jurisdictions. We also compared data on actual PM2.5 air quality against the standards. Findings We obtained data on standards from 62 jurisdictions worldwide, including 58 countries. Of the world’s 136.06 million km2 land under national jurisdictions, 71.70 million km2 (52.7%) lack an official PM2.5 air quality standard, and 3.17 billion people live in areas without a standard. The existing standards ranged from 8 to 75 µg/m3, mostly higher than the World Health Organization guideline annual limit of < 10 µg/m3. The weakest PM2.5 standards were often exceeded, while the more stringent standards were often met. Several jurisdictions with the highest population density demonstrated compliance with relatively stringent standards. Conclusion The metrics used in PM2.5 ambient air quality standards should be harmonized worldwide to facilitate accurate assessment of risks associated with PM2.5 exposure. Population density alone does not preclude stringent PM2.5 standards. Modernization of standards can also include short-term standards to unmask PM2.5 fluctuations in high-pollution areas.


Introduction
Millions of people die prematurely every year due to cardiovascular disease, pulmonary disease and cancer caused by air pollution. 1 For the premature deaths due to cancer, air pollution is a leading environmental cause. 2 Pollutants in the air exist as gases, and solid and liquid airborne particles also called aerosols. Aerosols occur in wide-ranging sizes. Among the different metrics describing particle size, the most common is aerodynamic diameter (diameter of the spherical particle with a density of 1 g/m 3 that has the same settling velocity as the given particle). 3 Three particle size ranges with the upper limits of 10 μm, 2.5 μm and 1 μm are named PM 10 , PM 2.5 and PM 1 , respectively. They are used to define fractions of aerosols for regulatory purposes. Only PM 10 and PM 2.5 are currently regulated in the form of ambient air quality standards. Of these two, we focus on PM 2.5 due to its stronger association with adverse health effects. 1 The PM 2.5 component of air pollution was responsible for an estimated 4.2 million annual premature deaths globally in 2015. 4 In 2010, China had 1.3 million premature deaths due to exposure to PM 2.5 , India had 575 000 and Pakistan had 105 000 deaths per year. 5 The 28 European Union (EU) countries had 173 000 and the United States of America (USA) 52 000 annual premature deaths. 5 Therefore, tightening and enforcing PM 2.5 ambient air quality standards could reduce the burden of disease and premature mortality.
Here, we review PM 2.5 standards worldwide and compare standards across different jurisdictions.

Methods
We carried out a review of PM 2.5 air quality standards worldwide, following the applicable guidelines of Preferred Report-ing Items for Systematic Review and Meta-Analysis Protocols (data repository). 6 Research Ambient air quality standards worldwide Yevgen Nazarenko et al.
We obtained the data on population and area of jurisdictions from the World Population Review 7 , and the data on country estimates for mean PM 2.5 ambient concentrations for 2016 from the World Health Organization (WHO). 8 These WHO data are synthesized from the data routinely measured at selected stationary monitoring stations in urban areas, satellite remote sensing, topography and population estimates.
The data on the standards were initially compiled by one author in 2018 and 2019 and were independently verified and updated in September 2019 against the sources by another author to ensure accuracy, except for Egypt, interpreted by a colleague and native speaker. We later updated and reanalysed the standards effective in July 2020.
We converted the Minguo calendar dates in China, Taiwan's regulations to the Roman calendar.

Data analysis
We made a descriptive analysis of how the metrics of the standards compared across different jurisdictions. We analysed the standards against the total population of jurisdictions, population density and geographical area of jurisdictions. We also compared the standards against the levels of actual urban PM 2.5 air pollution in different jurisdictions to determine where the standards were met and where they were exceeded.
We categorized the PM 2.5 air quality standards as: (i) enforced, when a penalty, enforcement, compliance or a similar term was mentioned in the source; (ii) voluntary, when stated so in the source; or (iii) target, when a policy statement existed regarding a level of PM 2.5 that various stakeholders agreed to work to achieve. We provide this classification to illustrate the approximate relative occurrence of the three different regulatory approaches. This classification should be interpreted with caution because stakeholders in each jurisdiction may by law or in reality apply differing interpretations of regulatory statements regarding enforcement or lack thereof.

Results
We identified the existence of PM 2.5 ambient air quality standards in 62 subnational, national and supranational jurisdictions worldwide, including 58 countries. The analysed national and regional PM 2.5 ambient air quality standards are listed in Table 1 (available at: http:// www .who .int/ bulletin/ volumes/ 99/ 2/ 19 -245704). We obtained data on actual PM 2.5 ambient air pollution for 175 national jurisdictions. Out of these, we used the data on actual PM 2.5 ambient air pollution for 57 jurisdictions for the analyses of PM 2.5 ambient air quality standards versus ambient PM 2.5 air pollution.

Averaging periods for measurements
Different jurisdictions set different intervals over which they average the measured PM 2.5 concentrations, such as 20 minutes, 24 hours, annual and 3 years. Most jurisdictions used the 98th or 99th percentile, and some used the arithmetic mean of all PM 2.5 measurements over a prescribed period. For example, in the USA, the an-  The standards had to be published in government documents, on government websites, in government-commissioned reports, reports of nationally or globally recognized organizations, or in peer-reviewed publications. Documents in any official language were acceptable. Eligible standards had to specifically mention PM 2.5 or its equivalent in the language of the document or define the regulated fraction of ambient particulate air pollution as particles or aerosols smaller than approximately 2.5 µm. Conditions constituting a part of, or the full, ISO definition of PM 2.5 were allowed. Only annual and 24-hour standards were considered for the summary analysis. Standards with other averaging periods were included in the summary table only. Multinational, national and regional jurisdictions were included. Self-determination by jurisdictions was sufficient. Standards must have been in force at the time of the summary analysis. Standards scheduled to come into force on a future date were included in the summary table only. The level of enforceability of the standards or lack thereof was not considered as a criterion for inclusion in the summary analysis.   (Fig. 2). Therefore, most annual standards exceeded both the level at which no detected health effects are expected according to WHO (3-5 µg/m 3 ) and the guideline annual PM 2.5 pollution limits set by WHO. These guidelines are 10 µg/m 3 (annual) and 25 µg/m 3 (24-hour). 9 The real ambient air pollution also exceeded WHO guidelines in most of the world (Fig. 4). Fewer jurisdictions had PM 2.5 24hour standards than annual standards. Notably, only the Russian Federation had a 24-hour standard in the European Region. The Russian Federation had a 20-minute PM 2.5 standard along with the 24-hour and annual standards, while most other countries of the former Soviet Union did not have any PM 2.5 standards.

Stringency of air quality standards
In the USA, there were primary and secondary standards. This primary standard allows for an adequate safety margin to protect public health, considering the uncertainties of available technical and scientific information. The secondary standard has no attainment deadline and is based on known or anticipated adverse effects on public welfare, including ecosystems, buildings and monuments. 18 In the EU countries, additional PM 2.5 objectives targeted population exposure to fine particles. These objectives are set at the national level and based on the average exposure indicator, which is a 3-year running annual mean PM 2.5 concentration averaged over selected monitoring stations in urban areas (Table 1). 44 Ukraine, which has an association agreement with the EU, adopted the EU's PM 2.5 standard to take effect in 2018. The EU supported the creation of the air quality monitoring infrastructure and implementation of the standard in Ukraine since 2015, yet progress has been slow, and the monitoring network has not been completed as of 2020. 45 In the Eastern Mediterranean Region, with known high levels of PM 2.5 air pollution due to desert dust, fuel-burning emissions and oil refining, only Egypt, Pakistan and Saudi Arabia had PM 2.5 air quality standards. 46,47 South Africa was the only country in the African Region with a PM 2.5 standard. The current annual standard of 20 µg/m 3 and the 24-hour standard of 40 µg/m 3 will be lowered to 15 µg/m 3 and 25 µg/m 3 , respectively, on 1 January 2030. 10 China used different PM 2.5 standards for the first-class (residential) and the second-class (commercial) zones. Both the annual and the 24-hour standards differed substantially for the two zones: 15 µg/m 3 annual and 35 µg/m 3 24-hour for the first-class zones and 35 µg/m 3 annual and 75 µg/m 3 24-hour for the second-class zones.

Air quality standards by population density
Of the world's total area of jurisdictions in the WHO World Population Review (136.06 million km 2 ), just under half (64.36 million km 2 ; 47.3%) was part of national jurisdictions with any PM 2.5 annual ambient air quality standard (Fig. 5). The medium-stringency annual standards ≤ 25 µg/m 3 covered 52.52 million km 2 or 38.6% of the world's total area of national jurisdictions, including 28.98 million km 2 or 21.3% protected by the strictest official annual PM 2.5 ambient air quality standards ≤ 15 µg/   7 Areas where no PM 2.5 ambient air quality standard was in effect are home to 3.17 billion people.
We compared the total population and area of jurisdictions by annual PM 2.5 standard and population density (Fig. 6). The areas of low population density (< 100 inhabitants per km 2 ) applied only the strictest (≤ 15 µg/m 3 ) or medium (20-25 µg/m 3 ) annual PM 2.5 standards. In the areas of high population density of 100-1000 inhabitants per km 2 , most people and land were covered by the least stringent annual PM 2.5 standards (> 25 µg/ m 3 ). However, in areas with the highest population density (> 1000 inhabitants per km 2 ) with a PM 2.5 ambient air quality standard, most population and land were covered by the strictest standards (≤ 15 µg/ m 3 ). Therefore, high population density alone cannot be a barrier to achieving compliance with stringent standards. Many densely populated cities within sparsely populated jurisdictions were covered by and often met the strictest standards set by those jurisdictions.
We plotted annual PM 2.5 standards in individual jurisdictions listed in Table 1 versus the population density (logarithmic scale), including individual EU's national jurisdictions (Fig. 7). Several notable clusters of jurisdictions stood out. Australia and Canada had a combination of very strict annual PM 2.5 ambient air quality standards (8 and 8.8 µg/m 3 , respectively) and low population density (3.3 and 3.7 inhabitants per km 2 , respectively), but contained several densely populated cities. Singapore had one of the highest population densities (8265 inhabitants per km 2 ) yet one of the lowest annual PM 2.5 ambient air quality standards (12 µg/m 3 ). Hong Kong SAR also had one of the highest population densities (6785 inhabitants per km 2 ), but, unlike Singapore, one of the least stringent annual PM 2.5 standards (35 µg/m 3 ). Both China and India had one of the least stringent annual PM 2.5 standards in the world (35 and 40 µg/ m 3 , respectively) combined with high but different population densities (146 and 416 inhabitants per km 2 ). Norway and Paraguay stood out with their stricter annual PM 2.5 standards (15 µg/m 3 in both) and low population densities (16.7 and 17.2 inhabitants per km 2 ) relative to those in their respective regions. The EU's annual PM 2.5 ambient air quality standard was relatively lax among the prosperous jurisdictions, notably higher than in Australia, Canada, Japan, Singapore, South Africa and the USA. Several densely populated jurisdictions could maintain relatively strict annual PM 2.5 ambient air quality standards: Dominican Republic, El Salvador, Japan, Singapore, China (Taiwan only) and Trinidad and Tobago.  Research Ambient air quality standards worldwide Yevgen Nazarenko et al.

Comparison of air quality to standards
The annual PM 2.5 ambient air quality standards were often exceeded in the jurisdictions with the highest PM 2.5 ambient air pollution ( Fig. 8; available at: http:// www .who .int/ bulletin/ volumes/ 99/ 2/ 19 -245704). Singapore stood out by its relatively strict annual PM 2.5 standard despite PM 2.5 air pollution that considerably exceeded the standard. Where the EU's standard was in effect, the PM 2.5 air pollution was highly variable, ranging from 20.8 µg/m 3 in Bulgaria to 5.9 µg/ m 3 in Iceland.
We excluded many jurisdictions where PM 2.5 pollution exceeded 30 µg/ m 3 (Fig. 9) from the analysis because they lacked an annual PM 2.5 ambient air quality standard. These jurisdictions need urgent PM 2.5 air pollution reduction measures. These excluded jurisdictions included Armenia, Mongolia, Nepal, North Macedonia, Tajikistan and Turkey and many countries in the African and Eastern Mediterranean Regions.

Discussion
In many jurisdictions, air quality regulations defined PM 2.5 as all particles smaller than 2.5 μm. This definition does not match the definition published by the International Organization for Standardization (ISO). 3 Many regulatory documents referred simply to particle diameter rather than aerodynamic diameter, even though the definition of particle diameter as aerodynamic diameter is critical to the ISO definition of PM 2.5 . Various metrics exist for particle diameter besides aerodynamic diameter (detailed list in data repository). 6,48,49 Therefore, regulations referring only to particle diameter without defining it introduce ambiguity. Jurisdictions can solve the problem by updating regulations with references to aerodynamic diameter specifically.
Some jurisdictions used a two-tier system of standards, such as different standards for commercial versus residential areas. One example of such a two-tier system is China, where a laxer standard was used in commercial zones where air pollution levels are generally higher, even though many people live next to China's factories. Geographically uniform standards are more useful for protecting occupational and public health. However, China's current zone-based system may better protect vulnerable populations, such as children and the elderly in the residential zones, in a time of transition towards a geographically uniform standard.
Jurisdictions within nations may set subnational standards that are weaker than national standards. Canada is one example. The federal PM 2.5 air quality standard was 8.8 µg/m 3 (annual) and 27 µg/m 3 (24-hour). Quebec and Ontario had their own 24-hour PM 2.5 standards of 30 µg/m 3 , which prevailed over the federal standard. Quebec did not sign on to the federal annual PM 2.5 standard. However, because air quality standards in Canada are voluntary and overwhelmingly met, no conflict exists.
Short-term standards, such as the 20-minute 160 µg/m 3 PM 2.5 standard in the Russian Federation, could be used in parallel with the annual and the 24-hour standards to reveal acute short-term spikes of PM 2.5 concentrations. The use of such a short-term averaging period, but only when combined with an adequately strict PM 2.5 concentration limit, can be useful in light of the current knowledge from controlled-exposure research on healthy adults that short-term exposures to high PM 2.5 concentrations can cause adverse health effects. 50,51 The PM 2.5 fraction contributes the most to the total burden of disease from particulate air pollution exposure. 4 In the past, jurisdictions with high ambient PM 2.5 air pollution saw health and environmental benefits from the implementation of PM 2.5 ambient air quality standards and measures to reduce PM 2.5 exposure. 52,53 However, many jurisdictions still do not regulate PM 2.5 air pollution or still have standards that are far from the safer levels based on the evidence from epidemiological studies. 9 Mechanistic studies found that the chemical composition of inhaled particles influences the biological effects these particles cause upon inhalation. 54 However, health studies conducted to date have predominantly assessed the impact of the total mass of inhaled PM 2.5 particles over time, irrespective of PM 2.5 aerosol composition. 55 Nevertheless, investing efforts into the total PM 2.5 air pollution reduction may be more beneficial than regulating different PM 2.5 air pollution components separately. An exception to this approach might be made in areas with strong natural dust sources, such as the Middle East, where monitoring and controlling anthropogenic source emissions could be more effective.  Research Ambient air quality standards worldwide Yevgen Nazarenko et al.
Standards and air quality monitoring data cannot be accurately compared between different jurisdictions when data collection and processing methods differ (different PM 2.5 definitions, averaging periods, exceedances, percentiles). The differences in these metrics result in potential discrepancies between PM 2.5 ambient air pollution levels and the values recorded and used to determine compliance with the standards. Currently, there is no universal set of metrics used in PM 2.5 ambient air quality standards that would ensure comparability of monitoring data globally. Without a universal metric, the same absolute PM 2.5 mass concentration limit can permit different levels of PM 2.5 pollution. The temporal and spatial distributions of the absolute recorded levels of PM 2.5 ambient air pollution are used in epidemiological studies and health risk assessment, where the differences in metrics can introduce errors. We suggest worldwide harmonization of the metrics of the PM 2.5 air quality standards to achieve the same averaging methods and exceedance allowances, or phasing out of exceedance allowances. This harmonization of the metrics of the PM 2.5 air quality standards may be achieved if the WHO guidelines specify a universal PM 2.5 definition based on aerodynamic diameter, and establish a common averaging and data recording method.
Enforced, target or voluntary standards were used in different jurisdictions. The goal to achieve the target standards is generally political, where accountability between responsible government branches exists. There is no universal enforcement mechanism and no definition of enforcement in the case of target standards. Enforced standards function through the possibility that at least one responsible party will bear potential financial, administrative or other costs resulting from non-compliance. Unless standards are explicitly defined as voluntary, various types of costs of non-compliance are possible. Canada is a notable exception where PM 2.5 ambient air quality standards were defined as voluntary. The voluntary PM 2.5 air quality standards in Canada are uniquely associated with a robust, extensive network of air quality monitoring stations registering only rare local exceedances. Outside of this context, voluntary air quality standards may not be justified.
The success of strict ambient air quality standards in several densely populated jurisdictions demonstrates that high population density should not discourage the implementation of PM 2.5 ambient air pollution reduction measures, including stricter PM 2.5 ambient air quality standards.
The current 24-hour standards mask sharp PM 2.5 concentration spikes over short periods of minutes to hours. Jurisdictions with a high temporal variability of PM 2.5 concentration, such as in India and China, should consider short-term averaging (such as over 20 minutes or 1 hour) along with high percentiles (such as the 98th or 99th) of 1-hour arithmetic means to monitor and reduce short-term PM 2.5 spikes.
Our study has some limitations. We could not confirm the existence of PM 2.5 regulations in certain countries with high PM 2.5 pollution and associated mortality, including Indonesia, Iraq, Myanmar, Nigeria, Sudan, Thailand and Turkey, even though PM 10 or other standards may be in place and some jurisdictions without an identified standard might be using WHO guidelines. The Islamic Republic of Iran is an example of such a situation. The Iranian government's environment department stated on their website that they are guided by the PM 2.5 standards of the United States Environmental Protection Agency (the department could not be reached for comment). We also found recommendations in the government documents of some of these countries regarding the reduction of particulate emissions. Iranian authorities, for example, have recommendations for numerous interventions to reduce emissions, including limits on vehicle emissions, industry, open burning, cooking fuels and enforcement mechanisms. Also some jurisdictions might have had regulations that included PM 2.5 that were not included in the analysis because they were not defined as PM 2.5 or were not accessible to the authors due to the language barrier or other difficulties with access to information. Inaccessibility, along with our specific inclusion and exclusion criteria, and our data reflecting the standards in 2020, could have caused slight differences between our results and the WHO maps on air quality standards. 56 In conclusion, to protect people's health from harmful PM 2.5 air pollution, we suggest that regulatory agencies and governments adopt and regularly tighten PM 2.5 ambient air quality standards. Where PM 2.5 air quality often exceeds WHO guidelines, these standards should be enforced with clearly defined enforcement mechanisms. The standards must be stringent enough for each local level of PM 2.5 ambient air pollution to drive meaningful air pollution reduction actions that are adequate and meaningful considering the level of PM 2.5 ambient air pollution in a given jurisdiction. Governments and agencies must avoid using the arithmetic mean metric, which tends to conceal high-pollution episodes reducing governments' ability to identify and remediate sources of PM 2.5 . We suggest that high percentiles should be used instead of the arithmetic mean. ■