Saharan dust, particulate matter and cause-specific mortality: a case-crossover study in Barcelona (Spain)

Studies measuring health effects of Saharan dust based on large particulate matter (PM) fraction groups may be masking some effects. Long distant transport reduces the amount of heavier and larger particles in the Saharan air masses increasing the relative contribution of smaller particles that may be more innocuous. This study investigates the association between different PM fractions and daily mortality during Saharan and non-Saharan days in Barcelona, Spain.

We collected daily PM(1), PM(2.5-1) and PM(10-2.5) fractions, and cause-specific mortality (cardiovascular, respiratory and cerebrovascular) between March 2003 and December 2007. Changes of effects between Saharan and non-Saharan dust days were assessed using a time-stratified case-crossover design.

During non-Saharan dust days we found statistically significant (p<0.05) effects of PM(10-2.5) for cardiovascular (odds ratio for increase of an interquartile range, OR=1.033, 95% confidence interval: 1.006-1.060) and respiratory mortality (OR=1.044, 95% CI: 1.001-1.089). During Saharan dust days strongest cardiovascular effects were found for the same fraction (OR=1.085, 95% CI: 1.017-1.158) with an indication of effect modification (p=0.111). Effects of PM(2.5-1) during Saharan dust days were about the double than in non-dust days for cardiovascular and respiratory mortality, but these differences were not statistically significant.

Our results using independent fractions of PMs provide further evidence that the effects of short-term exposure to PM during Saharan dust days are associated with both cardiovascular and respiratory mortality. A better understanding of which of the different PM size fractions brought by Saharan dust is more likely to accelerate adverse effects may help better understand mechanisms of toxicity.