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Background

Air pollution is a major public health issue, leading to millions of premature deaths worldwide. Cardiovascular diseases account for 60—۸۰% of air pollution-related deaths [1]. In line with the campaign launched in 2015 by the European Society of Cardiology, which aims to ‘‘raise awareness of the detrimental effects that the environment can have on the heart’’, we performed a review of publications reporting on the cardiovascular effects of air pollution. Although both indoor and outdoor air pollution exposure have a strong impact on cardiovascular diseases, we have focused our review on outdoor air pollution. We first introduce some basics on air pollution definition and sources, which are needed to present evidence arising from longand short-term epidemiological studies. We then present some pathophysiological evidence, and propose a schematic overview of mechanistic pathways linking air pollution exposure to clinical events. We also present recent data on the enhancing effects of mitigation measures, and some future directions to overcome the limitations in environmental research.

Air pollution basics

General definition

Air pollution is composed of particulate matter (PM) and gaseous components. PM is classified as coarse particles (diameter < 10m, ≥ ۲٫۵ m), fine particles (diameter < 2.5m, ≥ ۰٫۱ m) and ultrafine particles (nanoparticles, diameter < 0.1m). PM has a different composition depending on its source. Carbonaceous particles are derived from combustion sources, such as traffic emission or residential heating, while inorganic particles are represented by, for example, desert dust and mineral dust from agriculture. Carbonaceous particles are carbon based, but carry on their surface an amount of organic chemicals, such as polycyclic aromatic hydrocarbons and reactive metals [2]. Gaseous pollutants are nitrogen oxides including nitrogen dioxide (NO2) and nitric oxide (NO), ozone, sulphur dioxide (SO2), volatile organic compounds and carbon monoxide (CO). Besides their own toxicity, SO2 and nitrogen oxides also contribute to particle formation through complex atmospheric photochemical reactions involving ammonia from agriculture. As they result from gaseous transformation, these particles are called secondary particles, and are essentially composed of inorganic compounds, such as ammonia, sulphates and nitrates. Ozone is a secondary gaseous pollutant, formed through a photochemical reaction involving sunlight and gaseous precursors such as nitrogen oxides or volatile organic compounds.

Outdoor air pollution sources

In Europe, agriculture is a major source of PM2.5. Nevertheless, particles from agriculture are mainly inorganic particles, which are usually considered to be less toxic than carbonaceous particles from combustion sources, such as road traffic. Assuming this difference in toxicity, road traffic and residential heating have the largest impact on outdoor air pollution-related mortality in Europe [1]. In North America, industry and power generation using fossil fuels are also important sources of PM. In Africa, natural sources, such as desert dust and biomass burning (natural or man-made fires), contribute largely to ambient air pollution concentrations. In Asia, residential heating and cooking are the main sources of particles in both outdoor and indoor emissions [1]. Besides the differences between countries and continents, strong differences exist between the main sources of pollutants within the same country, depending on local sources. In large cities, road traffic is a major contributor to global pollutant emissions, and is also the main source of NO2, arising mainly from diesel vehicles. In Paris, for example, road traffic accounts for 30% of PM emissions, as much as the residential sector, and nearly 60% of emissions of nitrogen oxides [3]. SO2 mainly arises from industrial emissions and maritime transport; however, its contribution to air pollution is decreasing over the years.

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