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

摘要

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: 100-199); 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/m3 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.