Role of Saharan dust in the relationship between particulate matter and short-term daily mortality among the elderly in Madrid (Spain).

Background: Saharan dust outbreaks are a common phenomenon in the Madrid atmosphere. The current Directive 2008/50 CE governing air quality in European cities, draws no distinction between which particulate matter (PM(10), PM(2.5) or PM(10-2.5)) would be the best indicator on days with/without Saharan dust intrusions. This study sought to identify the role played by Saharan dust in the relationship between particulate matter (PM(10), PM(2.5) and PM(10-2.5)) concentrations and daily mortality among the elderly in the city of Madrid.

Methods: We conducted an ecological longitudinal time-series study on daily mortality among the over-75 age group, from 2003 to 2005. Poisson regression models were constructed for days with and without Saharan dust intrusions. The following causes of daily mortality were analysed: total organic causes except accidents (International Classification of Diseases-10th revision (ICD-10): A00-R99); circulatory causes (ICD-10: I00-I99); and respiratory causes (ICD-10: J00-J99). Daily mean PM(10), PM(2.5) and PM(10-2.5) levels were used as independent variables. Control variables were: other ambient pollutants (chemical, biotic and acoustic); trend; seasonalities; influenza epidemics; and autocorrelations between mortality series.

Results: While daily mean PM(2.5) concentrations in Madrid displayed a significant statistical association with daily mortality for all the above causes on days without Saharan dust intrusions, this association was not in evidence for PM(10) or PM(10-2.5) in the multivariate models. The relative risks (RRs) obtained for an increase of 10 μg/m(3) in PM(2.5) concentrations were: 1.023 (1.010-1.036) for total organic causes; 1.033 (1.031-1.035) for circulatory causes; and 1.032 (1.004-1.059) for respiratory causes. On Saharan dust days, a significant statistical association was detected between PM(10) (though not PM(2.5) or PM(10-2.5)) and mortality for all 3 causes analysed, with RRs statistically similar to those reported for PM(2.5).

Conclusions: The best air quality indicators for evaluating the short-term health effects of particulate matter in Madrid are therefore PM(10) concentrations on days with, and PM(2.5) concentrations on days without Saharan dust outbreaks. This fact should be taken into account in a European Directive regulating ambient air quality in almost all countries in the Mediterranean area.