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Effects of Local and Saharan Particles on Cardiovascular Disease Mortality

Pérez, Laura; Tobías, Aurelio; Pey, Jorge; Pérez, Noemí; Alastuey, Andrés; Sunyer, Jordi; Querol, Xavier

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doi: 10.1097/EDE.0b013e3182625d0d
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To the Editor:

Recent studies suggest that outbreaks of Saharan dust over southern European populations may be harmful, after comparing exposure to particulate matter (PM) fractions during Saharan and non-Saharan dust days. Studies conducted in Barcelona1,2 and Rome3 found that the effects of short-term exposure to coarse fraction during dust days were associated with increased total mortality and, more specifically, cardiovascular mortality. However, the specific contribution of Saharan dust versus other local dust remains to be clarified. If Saharan dust is indeed a risk agent, there is a need to develop a source-specific risk function to guide European legislation on setting standards and acceptable risks.

In this study, we estimated the association between daily cardiovascular mortality and PM <10 μm in aerodynamic diameter (PM10) differentiating the effects from Saharan dust and other human-made particulates, during Saharan dust days, in the city of Barcelona between 2003 and 2007. Identification of Saharan dust days was done as in our previous studies.1,2 Additionally, daily PM10 Saharan contributions versus local contributions were calculated from data collected at a regional background site, using a monthly moving 40th percentile to the PM10 time-series after omitting all days with Saharan air mass influence.4 These values were subtracted from the measured daily PM10 concentrations at the regional background site to obtain the amount of African dust brought on any given day under Saharan air-mass influence.4 Finally, the calculated daily African dust was used to distinguish between the urban background PM10 data in Barcelona and daily PM10 Saharan and non-Saharan contributions. We explored the association between daily cardiovascular mortality and PM10 using a time-stratified case-crossover design.5 We used Poisson regression adjusting for temperature, humidity, public holidays, and influenza epidemics, as well as for the 3-way interaction between day of the week, month, and year, to control for seasonality and time trends. This was done to replicate the adjustment made by the case-crossover design with the time-stratified approach for the selection of control days.6

Applying this source apportionment method, 50% of PM10 concentrations on Saharan dust days were due to human-made or local sources (Table). During non-Saharan dust days, the percentage increase of risk (%IR) per 10 μg/m3 at lag 1 due to PM10 was 2.8 (95% confidence interval = 1.6 to 4.1). During Saharan dust days, we found larger estimates, for this same lag, from PM10 attributed to the Saharan contributions (4.0 [−0.4 to 8.7]) and to the local contributions (9.7 [4.3% to 15.3%]) than on non-Saharan dust days.

Levels of PM10 and Percentage Increase in Risk of Cardiovascular Mortality 10 μg/m3 During Non-Saharan Dust Days (Contributing Total PM10 Levels) and Saharan Dust Days (Contributing Local and Saharan Contributions to PM10 Levels)

Using a novel method to differentiate the Saharan dust contribution from our daily total PM10, our results confirm the role of Saharan dust in triggering negative short-term cardiovascular effects. Intriguingly, however, our results also suggest that human-made particulates on those days have stronger effects than on other days, and that the effects observed in past studies may be driven mostly by the increase in effects of these specific contributions. We have no way of knowing the distribution of particle size of the local distribution, and therefore, we cannot determine whether this increase in effects is driven by the very fine traffic-related particles. However, it may be possible that the local particles become more toxic on Saharan dust days. Saharan dust contains Ca-Mg carbonates and other components that may react with pollutant gases (such as SOx or NOx), thus forming new particles.7 Similarly, there could be chemical reactions with other gases or additional condensation of organic compounds on the particles, enhancing toxicity. While the toxicity of particles remains to be understood, these results reinforce the need to re-evaluate the current exemption during Saharan dust days that the European Union has legislated, because this allows for tolerating an average higher exposure to PMs in population, on the specific days when there are increased effects of both local and exogenous particles.

Laura Pérez

Swiss Tropical and Public Health Institute

University of Basel

Basel, Switzerland

Aurelio Tobías

Jorge Pey

Noemí Pérez

Andrés Alastuey

Institute of Environmental Assessment and Water Research (IDAEA)

Spanish Council for Scientific Research (CSIC)

Barcelona, Spain

Jordi Sunyer

Centre for Research in Environmental Epidemiology (CREAL)

Barcelona, Spain

Xavier Querol

Institute of Environmental Assessment and Water Research (IDAEA)

Spanish Council for Scientific Research (CSIC)

Barcelona, Spain.


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2. Pérez L, Tobías A, Querol X, et al.. Saharan dust, particulate matter and case-speciffic mortality: a case-crossover study in Barcelona (Spain). Environ Int. In press.
3. Mallone S, Stafoggia M, Faustini A, Gobbi GP, Marconi A, Forastiere F. Saharan Dust and Associations between Particulate Matter and Daily Mortality in Rome, Italy. Environ Health Perspect. 2011;119:1409–1414.
4. Escudero M, Querol X, Pey J, et al.. A methodology for the quantification of the net African dust load in air quality monitoring networks. Atmos Environ. 2007;41:5516–5524.
5. Janes H, Sheppard L, Lumley T. Case-crossover analyses of air pollution exposure data: referent selection strategies and their implications for bias. Epidemiology. 2005;16:717–726.
6. Lu Y, Symons JM, Geyh AS, Zeger SL. An approach to checking case-crossover analyses based on equivalence with time-series methods. Epidemiology. 2008;19:169–175.
7. Alastuey A, Querol X, Castillo S, et al.. Characterization of TSP and PM2.5 at Izaña and Sta. Cruz de Tenerife (Canary Islands, Spain) during a Saharan dust episode. Atmos Environ. 2005;39:4715–4728.
© 2012 Lippincott Williams & Wilkins, Inc.