PM2.5: An important cause for chronic obstructive pulmonary disease?

[[abstract]]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.1 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,2 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,2 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,3 such as those arising from a dust storm versus those from diesel exhaust or second-hand smoke