Characterizing mortality effects of particulate matter size fractions in the two capital cities of the Canary Islands

Abstract

Most of the studies differentiating the effect of size-classified particulate matter (PM) exposure have been carried out in cities where the average levels of fine particles (PM_2.5) were higher than those of coarse particles (PM_10–2.5). These studies have suggested that PM_2.5 is associated with daily mortality, but there is only limited evidence that PM_10–2.5 is independently associated with mortality. The citizens of the Canary Islands are exposed to PM which is highly influenced by mineral dust because of the islands’ proximity to the Western Coast of Morocco. This offers an excellent opportunity to analyze in detail the short-term association between PM size fractions and total, respiratory and heart disease mortality. A time-series study from 2001 to 2004 was carried out. For each PM size fraction and mortality outcome, Generalized Additive Poisson Model was performed controlling for potential confounding. Different lag structures, unconstrained distributed lag models and two-pollutant models were examined. After assessing the linearity in the relationship, a piecewise linear analysis for exploring the existence of different slopes for different ranges of PM was carried out. The 10 μg/m³ increase in PM_2.5 and PM_10–2.5 levels was associated with a 7.5% (95% confidence interval=0.4–15.0) and a 7.4 (95% CI=1.5–13.7) increase in heart and respiratory disease mortality, respectively. Spline curves were quite linear over the PM concentrations seen on most days (dominated by combustion sources) in these cities, meanwhile on days with higher particulate levels (natural sources) a risk increase above certain PM levels was found, suggesting a curvilinear association and that, at least in some locations, PM_10–2.5 can play an important role in PM-related toxicity. The overall findings suggest that the establishment of new air quality standards for the short-term effect of PM_2.5 and PM_10–2.5 and further limiting levels of PM₁₀ in European Union is advisable.

Introduction

Many studies based on time-series analysis of daily mortality have shown a strong and consistent association between particulate matter (PM) and total, cardiovascular and respiratory mortality (Bell et al., 2004, Brunekreef and Holgate, 2002). Most of them have evaluated this association using ambient fine particles (PM_2.5) or larger particles (PM₁₀), which have an aerodynamic diameter less than 2.5 or 10 μm, respectively. Nevertheless, most of these studies have been carried out in densely populated or industrial cities, with PM levels dominated by combustion sources (Anderson et al., 2004). Fewer studies have evaluated the association between coarse particles (particles with an aerodynamic diameter between 2.5 and 10 μm; PM_10–2.5) and mortality (Castillejos et al., 2000, Brunekreef and Forsberg, 2005, Sandtröm et al., 2005, Sandstrom and Forsberg, 2008).

The objective of the CAS Project (in Spanish, Canarias, Atmósfera y Salud or “the Canary Islands, Atmosphere, and Health”), was to analyze the relation between air pollutants and their short-term health effects in the two Canary capitals. As a part of the project, in the previous paper, some significant association between PM and mortality was observed (López-Villarrubia et al., 2010). Such findings have led to study more comprehensively the size-specific impacts of PM on health. Therefore the results presented in this paper are part of CAS project.

The island factor of the cities of Las Palmas de Gran Canaria and Santa Cruz de Tenerife (approximately 200 km. off the northwest African coast), in addition to the climatic characteristics of the Canary Islands, creates a particular setting that influences the ambient air quality of both cities. The predominance of the trade winds, which blow almost constantly from May to October, facilitates the dispersion of primary pollutants over the ocean from these urban environments, whereas the proximity to the Western Coast of Morocco and the Sahara Desert promotes the seasonality arrival of natural particulate matter on the islands (Bergametti et al., 1989, Gobierno de Canarias, 2008), leading to high concentrations not only of PM₁₀, but also of PM_2.5 (Viana et al., 2002, Querol et al., 2004, Claiborn et al., 2000). Mineral dust input contributes around 10 μg/m³ of PM to the annual PM₁₀ average background in these cities and 3.7 μg/m³ to PM_2.5 (López-Villarrubia et al., 2008). These special features offer an ideal opportunity to analyze the effect on daily mortality of each of the PM size fractions in these two Canary Island cities.

Therefore, in the population and among health professionals there is a perception of the negative effects of particulate matter on health, but until now this issue has not been addressed from the epidemiological point of view.

The main objective of the present study was to assess the acute effects of PM₁₀, PM_2.5 and PM_10–2.5 on the increased risk of daily mortality due to (1) all causes, (excluding accidents and injuries), (2) respiratory and (3) heart disease mortality in residents in Las Palmas de Gran Canaria (L/P de Gran Canaria) and Santa Cruz de Tenerife (S/C de Tenerife) during the period 2001 to 2004. In order to examine this association, various modelling issues which are still of great interest in epidemiological studies, have been addressed: (1) the question of PM size-specific impact assessment, (2) the potential confounding of PM effects by co-pollutants, (3) the temporal relationship between exposure and effect (lagged and cumulative effect), (4) the shape of the concentration-response curve, and (5) the possibility of a non-linear association.