PM 2·5 : an important cause for chronic obstructive pulmonary disease?
Authors: Chi Pang Wen, Wayne Gao
Journal: The Lancet Planetary Health (2018)

With the advent of popular technology to measure particulate matter with an aerodynamic diameter of 2·5 μm or less (PM2·5) in the past few decades as a measure for air quality, a proliferation of publications linking ambient air pollution to various health outcomes has occurred. To demonstrate the acute effects of air pollution, such as London smog exacerbating existing asthma or chronic obstructive pulmonary disease (COPD), was quite simple, but to show an association with the development of these chronic conditions was more difficult. In the absence of individual breathing data, researchers had to do ecological studies that relied on area spatial sampling. The assumption that all individuals within an area as large as a city or as small as 1 km2 are considered to breathe the same concentration of PM2·5 is difficult to validate. Conclusions from air pollution studies showing significant health risks appear to be intuitively reasonable, but the causal link is not straightforward.1Cox Jr, LAT Do causal concentration-response functions exist? A critical review of associational and causal relations between fine particulate matter and mortality.Crit Rev Toxicol. 2017; 47: 603-631Crossref PubMed Scopus (25) Google Scholar Establishing the link requires many assumptions, overt or hidden, with most of them only known to the researchers. Not many studies have elaborated on the limitations of those assumptions. For example, concentrations of PM2·5 varied surprisingly widely within an individual over time,2Air Quality Monitoring NetworkHistorical air quality data download, air quality monitoring network. Environmental Protection Administration, Taipei2018https://taqm.epa.gov.tw/taqm/tw/YearlyDataDownload.aspxGoogle Scholar more than those from common biological indices, such as blood pressure or serum cholesterol. This large daily or seasonal variation in PM2·5 exposure is further complicated by changing weather, traffic density, or proportion of time indoors, resulting in a variability of as much as 10–20 times,2Air Quality Monitoring NetworkHistorical air quality data download, air quality monitoring network. Environmental Protection Administration, Taipei2018https://taqm.epa.gov.tw/taqm/tw/YearlyDataDownload.aspxGoogle Scholar and making the choice for a single value for an individual in studies of PM2·5 difficult. Another requirement in studying the relationship of PM2·5 exposure with chronic diseases is the need for high exposure (above a certain threshold) sustained for at least 5–10 years. Further, PM2·5 denotes particle size and not toxicity, and risks from the same concentration of different sources might not be the same,3Adams K Greenbaum DS Shaikh R van Erp AM Russell AG Particulate matter components, sources, and health: systematic approaches to testing effects.J Air Waste Manag Assoc. 2015; 65: 544-558Crossref PubMed Scopus (151) Google Scholar such as those arising from a dust storm versus those from diesel exhaust or second-hand smoke. In The Lancet Planetary Health, Cui Guo and colleagues4Guo C Zhang Z Lau AKH et al.Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort study.Lancet Planet Health. 2018; 2: e114-e125Summary Full Text Full Text PDF PubMed Scopus (167) Google Scholar did a long-term follow-up study of a well known large prospective cohort in Taiwan. They showed that increased ambient PM2·5 was associated with a decline in lung function and increased incidence of COPD. In contrast to earlier studies with inconclusive results,5Schikowski T Adam M Marcon A et al.Association of ambient air pollution with the prevalence and incidence of COPD.Eur Respir J. 2014; 44: 614-626Crossref PubMed Scopus (141) Google Scholar, 6Lepeule J Laden F Dockery D Schwartz J Chronic exposure to fine particles and mortality: an extended follow-up of the Harvard Six Cities study from 1974 to 2009.Environ Health Perspect. 2012; 120: 965-970Crossref PubMed Scopus (663) Google Scholar the study by Guo and colleagues4Guo C Zhang Z Lau AKH et al.Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort study.Lancet Planet Health. 2018; 2: e114-e125Summary Full Text Full Text PDF PubMed Scopus (167) Google Scholar established the association by recording incident COPD at a second visit in patients who did not have COPD at the first visit. Allowing a latency period for COPD to develop, averaging up to 5 years between two visits, was a strength of their study. The biological plausibility of the relationship between PM2·5 and lung health was further supported by a finding of decreased lung function from 590 278 observations. Another strength of the study is that each individual had repeated lung function tests, assessed from numerous parameters, showing internal consistency. Nevertheless, the study is an association study, and findings are suggestive but should not be interpreted as causal. Although PM2·5 was found to be significantly associated with developing COPD in the study by Guo and colleagues,4Guo C Zhang Z Lau AKH et al.Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort study.Lancet Planet Health. 2018; 2: e114-e125Summary Full Text Full Text PDF PubMed Scopus (167) Google Scholar clinicians should not overlook the important role of smoking in COPD development. Smoking and COPD are closely linked: 60–80% of people with COPD are smokers, and half of smokers will develop COPD.7Eisner MD Anthonisen N Coultas D et al.An official American Thoracic Society public policy statement: novel risk factors and the global burden of chronic obstructive pulmonary disease.Am J Respir Crit Care Med. 2010; 182: 693-718Crossref PubMed Scopus (672) Google Scholar Application of the numbers from Guo and colleagues' study4Guo C Zhang Z Lau AKH et al.Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort study.Lancet Planet Health. 2018; 2: e114-e125Summary Full Text Full Text PDF PubMed Scopus (167) Google Scholar to the existing literature7Eisner MD Anthonisen N Coultas D et al.An official American Thoracic Society public policy statement: novel risk factors and the global burden of chronic obstructive pulmonary disease.Am J Respir Crit Care Med. 2010; 182: 693-718Crossref PubMed Scopus (672) Google Scholar, 8Afonso AS Verhamme KM Sturkenboom MC Brusselle GG COPD in the general population: prevalence, incidence and survival.Respir Med. 2011; 105: 1872-1884Summary Full Text Full Text PDF PubMed Scopus (115) Google Scholar (figure) shows that the bulk of COPD diagnoses are linked to smoking (70%)—ten times more than those attributed to ambient PM2·5. The hazard ratio was much larger for second-hand smoking than PM2·5 (1·56 [95% CI 1·40–1·74] vs 1·08 [1·04–1·11]) and even greater for first-hand smoking (3·89 [3·52–4·31]). Thus, smokers should not use air pollution as an excuse for not wanting to quit smoking. Another motivation for reducing the numbers of smoker is that they also cause COPD in non-smokers through second-hand smoke.9Eisner MD Secondhand smoke and obstructive lung disease: a causal effect?.Am J Respir Crit Care Med. 2009; 179: 973-974Crossref PubMed Scopus (23) Google Scholar The American Thoracic Society stated that "the only medical therapies that clearly reduce COPD progression and mortality are smoking cessation".7Eisner MD Anthonisen N Coultas D et al.An official American Thoracic Society public policy statement: novel risk factors and the global burden of chronic obstructive pulmonary disease.Am J Respir Crit Care Med. 2010; 182: 693-718Crossref PubMed Scopus (672) Google Scholar COPD is a serious disease, and cleaner air would certainly reduce its incidence, as shown by Guo and colleagues.4Guo C Zhang Z Lau AKH et al.Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort study.Lancet Planet Health. 2018; 2: e114-e125Summary Full Text Full Text PDF PubMed Scopus (167) Google Scholar However, with air pollution rapidly improving in most developed countries, including Taiwan, and with smoking prevalence remaining high, the focus of efforts to reduce COPD should not be distracted from tobacco control. Reducing existing and future numbers of smokers and thereby reducing second-hand smoking9Eisner MD Secondhand smoke and obstructive lung disease: a causal effect?.Am J Respir Crit Care Med. 2009; 179: 973-974Crossref PubMed Scopus (23) Google Scholar remain the best strategy to combat the epidemic of COPD. We declare no competing interests. Effect of long-term exposure to fine particulate matter on lung function decline and risk of chronic obstructive pulmonary disease in Taiwan: a longitudinal, cohort studyLong-term exposure to ambient PM2·5 is associated with reduced, and faster declines in, lung function. Long-term exposure to ambient PM2·5 is also associated with an increased risk of the incidence of COPD. This study reinforces the urgency of global strategies to mitigate air pollution for improvement of pulmonary health and prevention of COPD. Full-Text PDF Open Access