Saharan dust and association between particulate matter and case-specific mortality: a case-crossover analysis in Madrid (Spain)

BACKGROUND: Saharan dust intrusions are a common phenomenon in the Madrid atmosphere, leading induce exceedances of the 50 μg/m(3)--EU 24 h standard for PM10. METHODS: We investigated the effects of exposure to PM(10) between January 2003 and December 2005 in Madrid (Spain) on daily case-specific mortality; changes of effects between Saharan and non-Saharan dust days were assessed using a time-stratified case-crossover design. RESULTS: Saharan dust affected 20% of days in the city of Madrid. Mean concentration of PM(10) was higher during dust days (47.7 μg/m(3)) than non-dust days (31.4 μg/m(3)). The rise of mortality per 10 μg/m(3) PM(10) concentration were always largely for Saharan dust-days. When stratifying by season risks of PM(10), at lag 1, during Saharan dust days were stronger for respiratory causes during cold season (IR% = 3.34% (95% CI: 0.36, 6.41) versus 2.87% (95% CI: 1.30, 4.47)) while for circulatory causes effects were stronger during warm season (IR% = 4.19% (95% CI: 1.34, 7.13) versus 2.65% (95% CI: 0.12, 5.23)). No effects were found for cerebrovascular causes. CONCLUSIONS: We found evidence of strongest effects of particulate matter during Saharan dust days, providing a suggestion of effect modification, even though interaction terms were not statistically significant. Further investigation is needed to understand the mechanism by which Saharan dust increases mortality.The study was supported by the Health Research Fund, Subdirectorate-General for Assessment and Development of Research (Project PI080354) and by the Carlos III Institute of Health (Project SEPY1453/07).S