In our present analysis of exposures to the chemical components of indoor PM2.5 in a COPD cohort, several associations with living environment, lifestyle and clinical parameters were identified. The concentration of Ti showed a significant association with small airway resistance and those of Br, K, Pb, and Zn showed correlations with COPD symptoms and/or the subjects’ quality of life. Higher concentrations of Al and Br had an association with more frequent acute exacerbations of COPD. In terms of the living environment, the higher concentrations of Cu were related to a shorter distance from the home to a road and a lower floor level of the residence. Several lifestyles such as ventilation times, duration of air purifier use, and frequency of cleaning the kitchen ventilator were also associated with lower exposure concentrations of Cu, Si, Zn, barium, Ti, Fe, and Mn. All these associations suggested that the impact of PM2.5 can be different depending on its components, and that lifestyle interventions to reduce the impact of PM2.5 exposure can be individualized in accordance with the patient’s environment and clinical status.
Most prior studies on the health effects of PM2.5 involved the total concentrations of this air pollutant even though it is a complex mixture of heterogeneous particles, and its composition can vary across locations depending on meteorological conditions and emission sources (Thangavel et al., 2022). Differences in PM2.5 compositions may also affect its overall toxicity (Korsiak et al., 2022; Thurston et al.). There is limited evidence however on which PM2.5 constituents are associated with the greatest risk of aggravating a respiratory disease and what the underlying mechanisms of this are. In our current study, several chemical constituents of PM2.5 correlated with poorer respiratory outcomes and some had associations across multiple outcomes (Br, Ca, Fe, Mn, Pb, Si, and Zn). Although exposure to metal dust or fumes can cause serious cardiopulmonary complications (Bergström, 1977; Merget et al., 2002; Nemery, 1990; Woolf and Shannon, 1999; Zhou et al., 2018), these chemicals as particulate matters may have different mechanisms involved in their hazardous effects. The adverse health effects of PM2.5 exposure are considered to be mediated by oxidative stress through the generation of reactive oxygen species, inflammation, and mitochondrial damage (Thangavel et al., 2022), with some PM2.5 constituents sharing similar mechanisms. In a previous study by Shang et al., the constituents of PM2.5 were measured before and after air quality control in Beijing and biological responses to PM2.5 and its components were analyzed in murine alveolar macrophages (Shang et al., 2013). Metals including Fe and Ba were found to be associated with increased granulocyte-macrophage colony-stimulating factor and interleukin-10 levels, whereas Zn showed a significant association with cell viability. Our present study results also support that PM2.5 may contribute differently to pro-inflammatory responses and cytotoxicity in accordance with its specific constituent levels.
The residential environments of our current study participants were also observed to be associated with certain PM2.5 constituents. Road traffic is a well-known factor affecting the profile of air pollutants that can even have mortality impacts (Beelen et al., 2008; Wang et al., 2021). In our present study, Mn was found to be associated with the road traffic levels, and Cu with both distance from the road to the home and the floor level of the residence, which can be closely correlated. The respiratory impacts of the living distance from major roads has been well-studied, but that of the floor level of the residence has not (Jung et al., 2011). Other than mining areas, a major source of Cu is the burning of fossil fuels including motor oils, in addition to ash, and other waste from industrial incinerators. A prior population-based study has reported that Cu as an air pollutant can increase cardiovascular mortality (Zhang et al., 2021a; Zhang et al., 2021b) and even alter neurologic function (Pujol et al., 2016). Our current study has found that the Cu concentration and airway resistance parameters are related with marginal significance. One of the interesting things we observed in our present analysis was the association between an increased ventilation time and a reduced Cu concentration. As a lifestyle parameter, the ventilation time was also observed to be associated with reduced concentrations of Si and Zn. Although outdoor air pollution is the main source of indoor PM2.5, indoor sources such as aerosols, fireworks, smoking, mixed dust are not negligible (Matthaios et al., 2021). These aforementioned data suggest that adequate ventilation times are no less important than controlling outdoor air pollution for respiratory disorders.
Some lifestyle parameters in our present analyses were found to be associated with lower concentrations of PM2.5 components. A longer duration of air purifier use on heavy dust days was significantly associated with lower indoor exposure concentrations of Ba and Ti. Ti was found to be associated with small airway resistance, suggesting that an air purifier could be an individualized intervention for patients with an airway disease. Indeed, this is a well-established intervention to reduce the indoor PM2.5 concentration and improve peak expiratory flow (Park et al., 2021). This consistent finding for airway resistance in relation to air purifier use suggests that some PM2.5 constituents such as Ti may be involved in this association (Zhan et al., 2018). Air purification is also effective in reducing the concentrations of PM2.5 components, as well as the overall PM2.5 levels (Zhan et al., 2018). We further observed that lower concentrations of Fe and Mn were noted if a kitchen ventilator was cleaned frequently. The Fe constituent of PM2.5 is considered to come mostly from anthropogenic sources such as brake wear and exhaust products, and Mn mainly from steel industries, but their indoor sources are not as well defined. Fe and Mn are utilized in a variety of kitchen tools, but it has not yet been studied whether these metals can be released into the air from those sources and affect air pollution. Mud floors and the burning of plastic can affect the indoor heavy metal concentrations (Nakora et al., 2020), but they are not common in households in Korea. Cooking activities are the major source of indoor PM2.5 in Korean residences with open kitchens (Kang et al., 2019; Kim et al., 2018), and can increase the levels of various PM2.5 components (Zhang et al., 2017; Zhang et al., 2016). Our present study findings suggest that effective ventilation in the kitchen can contribute to the control of these indoor sources of air pollutants.
There were some limitations of this study of note. First, this was a cross-sectional study, and the exact causal relationships could not be confirmed. However, previous studies on the chemical constituents of PM2.5 have not been conducted in patients with chronic respiratory diseases. Our current analyses, which included essential clinical parameters of COPD patients, thus provide an insight into possible individualized interventions for PM2.5 exposure in accordance with its constituents. Second, the major indoor sources of each PM2.5 constituent are not yet defined and the capacity to make environmental improvements to reduce these corresponding constituents is thus limited. Nonetheless, our current study presents some interesting findings that provide insights for future studies to identify the indoor sources of these constituents. For instance, the highly correlated nature of some constituents e.g., Si and Al suggests that they may be of similar or even the same origin. By contrast, PM2.5 constituents that did not show high correlations with others may be less-specific markers. Since our present study findings suggest that there may be indoor sources of chemical constituents related to clinical outcomes in COPD, more research into the indoor sources of these pollutants is required for meaningful and individualized interventions in the future.
In conclusion, PM2.5 comprises various chemical constituents that have different relationships with the clinical parameters of patients with COPD. Such constituents can be affected by environmental factors and lifestyles, and more sophisticated individualized interventions may well be possible upon the identification of their residential sources in future studies.