The smaller, the worse?
Authors: Haidong Kan
Journal: The Lancet Planetary Health (2017)

Ambient air pollution is a global public health challenge, especially for developing countries such as China and India.1Cohen AJ Brauer M Burnett R et al.Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015.Lancet. 2017; 389: 1907-1918Summary Full Text Full Text PDF PubMed Scopus (3224) Google Scholar Among various pollutants, particulate matter often shows the strongest evidence for adverse health effects.2Pope 3rd, CA Dockery DW Health effects of fine particulate air pollution: lines that connect.J Air Waste Manag Assoc. 2006; 56: 709-742Crossref PubMed Scopus (4030) Google Scholar Particulate matter consists of discrete particles that range in size over several orders of magnitude. In many countries, inhalable particulate matter (particulate matter with an aerodynamic diameter less than 10 μm, PM10) and fine particulate matter (particulate matter with an aerodynamic diameter less than 2·5 μm, PM2·5) have been regarded as criterial pollutants because several studies have documented the adverse health effects of PM10 and PM2·5. Size is an important determinant of health effects of particulate matter. Previous epidemiological studies suggested that PM2·5 has stronger effects on cardiorespiratory disorders than PM10.2Pope 3rd, CA Dockery DW Health effects of fine particulate air pollution: lines that connect.J Air Waste Manag Assoc. 2006; 56: 709-742Crossref PubMed Scopus (4030) Google Scholar However, there is very limited knowledge regarding which specific size fraction below 2·5 μm is mainly responsible for the observed health effects. Thus, it has been of great interest, both from a scientific viewpoint and from a regulatory perspective, to investigate which specific size fractions below PM2·5 are more toxic than others. Recently, PM1 (particulate matter with an aerodynamic diameter less than 1 μm), as a major part of PM2·5, has received more and more attention. In The Lancet Planetary Health, a nationwide study by Gungbo Chen and colleagues3Chen G Li S Zhang Y et al.Effects of ambient PM1 air pollution on daily emergency hospital visits in China: an epidemiological study.Lancet Planet Health. 2017; 1: e221-e229Summary Full Text Full Text PDF Scopus (129) Google Scholar provides new evidence about the relation between PM1 and increased risk for emergency hospital visits in 26 Chinese cities.3Chen G Li S Zhang Y et al.Effects of ambient PM1 air pollution on daily emergency hospital visits in China: an epidemiological study.Lancet Planet Health. 2017; 1: e221-e229Summary Full Text Full Text PDF Scopus (129) Google Scholar Thanks to a national monitoring network of PM1, the authors were able to conduct the first multi-city epidemiological study of PM1 in developing countries or even in the world. Interestingly, PM1 was found to have similar effects on emergency hospital visits as PM2·5 (relative risks [RRs] 1·011 [95% CI 1·006–1·017] and 1·010 [1·005–1·016], for a 10 μg/m3 increase in PM1 and PM2·5, respectively, at lags 0–2 days) and the attributable fraction of emergency hospital visits due to PM1 (4·47%) approached that due to PM2·5 (5·05%). Moreover, PM1–2·5 was not significantly associated with emergency hospital visits, suggesting that the health effects of PM2·5 were mostly from PM1. The findings are consistent with a previous study in Shanghai, China showing that, compared with PM1–2·5, PM1 had much stronger associations with circulating biomarkers of inflammation, coagulation, and vasoconstriction in a panel of young adults.4Chen R Zhao Z Sun Q et al.Size-fractionated particulate air pollution and circulating biomarkers of inflammation, coagulation and vasoconstriction in a panel of young adults.Epidemiology. 2015; 26: 328-336Crossref PubMed Google Scholar Together these findings suggest that PM1 has adverse effects on the health of the general population and strengthen the rationale for restricting levels of PM1 in outdoor air. The stronger effects of smaller particles—eg, PM1—is probably due to higher pulmonary deposition efficiency, easier vascular penetration, larger surface area, as well as more toxic components adhered. As the authors have pointed out, this new study has limitations that need to be addressed in future studies. The authors used the monitoring results from only one fixed-site monitoring station as a proxy for population exposures to PM1 and were not able to assess spatial variation of PM1. Several approaches, including statistical models of measurement error and use of personal exposure assessment, can be used to address this issue.5Zeger SL Thomas D Dominici F et al.Exposure measurement error in time-series studies of air pollution: concepts and consequences.Environ Health Perspect. 2000; 108: 419-426Crossref PubMed Scopus (885) Google Scholar The authors did not have information about causes of emergency hospital visits limiting this study's ability to identify subgroup morbidity outcomes associated with PM1 exposure. Finally, it is difficult to differentiate the effects of PM1 and PM2·5 on emergency hospital visits in view of the high association between the two pollutants. Historically, evidence from epidemiological studies of air pollution has played a pivotal part in regulatory policy making and standards setting. So far, the standards for PM1 have not been proposed by WHO or any other government agencies. Is it the right time to propose the standards for PM1? The answer is probably no, because the knowledge we have for PM1 is still far from enough. Clearly, more studies are needed for PM1 in both developed and developing countries. More studies are warranted to determine the characteristics of exposure to PM1 to investigate the independent health effects of PM1 from other pollutants—eg, PM2·5—as well as to identify the most susceptible time periods and population groups, and pathophysiological link between PM1 and cardiorespiratory diseases. I declare no competing interests. Effects of ambient PM1 air pollution on daily emergency hospital visits in China: an epidemiological studyExposure to both ambient PM1 and PM2·5 were significantly associated with increased emergency hospital visits. The results suggest that most of the health effects of PM2·5 come from PM1. Full-Text PDF Open Access