Particulate matter (PM10) air pollution is associated with both respiratory and cardiovascular morbidity, and mortality. Dockery and coworkers1 were among the first to demonstrate that it is the smaller component of this pollution (PM2.5) that is more strongly associated with mortality. Focus has consequently been directed toward sources of PM2.5, such as traffic and other combustion pollution, as major contributors to severe cardiovascular and respiratory events.1,2
This focus may be misleading in suggesting that coarser particles (PM2.5–10 μm aerodynamic diameter) have no major toxic effects; recent European Union projects that sampled coarse and fine particles have demonstrated that coarse particles have a toxicologic capacity equivalent to fine PM (PM2.5) on a mass basis.3,4 These findings are in good agreement with studies of persons with asthma who already have inflammation of their airways, among whom symptoms are worsened by both fine and coarse particles.5
Still, with regard to mortality, the role of larger particles has been less clear. A few studies done in arid regions with high levels of coarse PM have suggested short-term effects of coarse particulate matter on mortality.5,6 However, in a recent systematic review that considered studies in which both PM2.5 and PM10 were measured,5 there were no clear associations of the coarse fraction with mortality.
With this background, Perez and colleagues7 present data on this issue demonstrating that coarse particles from Sahara-Sahel desert regions blowing into Spain were associated with a stronger effect per mass concentration and a risk coefficient several times higher than that which is typical for PM2.5. The authors used satellite images and other information to identify days on which air masses from the Sahara-Sahel region moved into the Barcelona region, and on which air monitors in rural areas recorded at least 50% of the PM10 levels seen at city monitors. Interestingly, there was a very modest increase in the average mass concentrations of PM, but clear increases in mortality, suggesting qualitative differences in the PM fraction from Sahara compared with the more common coarse PM in the Barcelona region.
The Perez study is important in that it indicates effects on daily mortality from coarse PM, an exposure previously associated mainly with nonfatal respiratory events. The authors found convincing associations of mortality with coarse Saharan dust (but not with coarse local dust) even though the days with desert dust were rather few and the confidence intervals were wide. Cause-specific mortality could not be addressed in detail, and the possible cardiovascular, or respiratory events leading to mortality could not be determined.
Coarse particles are more likely to be deposited in the bronchial passages and thereby affect respiratory conditions such as asthma, chronic obstructive pulmonary disease, and pneumonia and other airway infections. In contrast, fine particles seem more likely to reach the alveoli and lead to cardiovascular events. With the strong effect reported by Perez and colleagues, it seems likely that cardiovascular deaths would be overrepresented, which suggests that Saharan dust may provide a new modality by which coarse particles affect organ systems.
Special features of this dust might help explain the mechanisms of this apparent effect on mortality. The investigators have pursued this question by attempting to characterize the coarse and fine PM sampled from the Saharan dust days. Chemical analysis indicated metals related to oxidative stress induction (such as iron, copper, and zinc) were relatively similar in Saharan and non-Saharan dusts.
Saharan dust also carries large amounts of biogenic factors. These include microorganisms such as bacteria and fungi, as well as related protein and lipid components.8–11 The short lag between exposure and deaths (1 day) does not suggest that infections are the mechanism.
Particulate matter can also contain endotoxins, which are components of the bacterial wall. Endotoxins can cause respiratory and systemic inflammatory responses and can exacerbate lung disease. A multicenter European study showed the highest inflammatory effects of Barcelona dust were caused by the coarse PM10–2.5 fraction.10 It may, therefore, be that some of the before mentioned factors associated with the coarse particles from Saharan region (or others yet to be identified) are responsible for the increased mortality. Because windblown dust is spread over large areas, an important question is whether similar effects on mortality are found in other regions as well. Respiratory effects of Saharan dust have been reported as far away as the Caribbean island of Trinidad.12
The increase in mortality in Barcelona associated with windblown Saharan desert dust raises concern over possible underestimation of toxicity from coarse particles when they come from desert sources.13 Joint efforts by epidemiologists and toxicologists may unravel why certain desert dust carries unexpected toxic capacity. This question is of concern for large areas of the globe that periodically encounter high levels of windblown desert dust.
ABOUT THE AUTHORS
THOMAS SANDSTRÄM, leads a research group that addresses the effects of air pollution on respiratory and cardiovascular health. He coordinates the multinational European Commission funded HEPMEAP project, and serves as Chairman of the Scientific Committee of the Swedish Society for Respiratory Medicine. BERTIL FORSBERG, is the head of the Department of Public Health and Clinical Medicine, Occupational, and Environmental Medicine, and also leads a research group studying the health effects of air pollution.
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