Effects of Local and Saharan Particles on Cardiovascular Disease Mortality

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 Barcelona^1,2 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.

Table: 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 SO_x or NO_x), 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

[email protected]

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.