PM_0.1 particles from aircraft may increase risk of vascular disease

Hansell and colleagues reported an association between exposure to aircraft noise and increased risks of stroke, coronary heart disease, and cardiovascular disease in areas close to Heathrow airport.1 They considered several confounding variables, including air pollution. However, air pollution was represented by PM₁₀—the mass concentration of suspended particles smaller than 10 μm—which does not reflect air pollution from aircraft.

Aircraft emit PM_0.1 particles, which are so low in mass that variations in PM_0.1 or even PM_2.5 do not correlate well with those of PM₁₀.2 3 Yet PM_0.1 particles can penetrate deeper into the lungs and translocate more easily into the bloodstream than PM₁₀,4 5 so they may have led to the reported increased risks of vascular disease.

Unlike noise, which dissipates immediately, PM_0.1 emissions persist for hours. When emitted at low altitude, PM_0.1 would reach ground level within tens of minutes owing to turbulent mixing. Over the following hours, PM_0.1 would coagulate into PM_2.5 particles and persist in the air for several days.3 These PM_2.5 particles have adverse health effects but are not equivalent to PM_0.14—for example, their larger size means reduced lung penetration. Therefore the PM_0.1 coagulation lifetime of hours is comparable to Hansell and colleagues’ 8-16 hour averaging periods, and the reported association between vascular health risks and aircraft activity may have reflected exposure to aircraft emitted PM_0.1 particles.

The authors probably used PM₁₀ data because data on other pollutants were not available. Even when available, such data may reflect ground traffic rather than aircraft activity. Thus the aircraft-activity data they used to estimate noise exposure might have been the best available proxy for aircraft PM_0.1 pollution. The distinction between noise and PM_0.1 is important for policy makers: noise disperses immediately, but PM_0.1 particles may have diluted effects over a longer period and wider area.