Introduction
As the most significant environmental threat to human health, air pollution exposure may account for as many as 1 in 5 deaths worldwide.1,2 The burden of disease disproportionately affects residents of low- and middle-income countries where air quality continues to worsen in contrast to high-income countries.1 The World Health Organization (WHO) last issued health-based guidelines for major air pollutants in 2006:Air quality guidelines – global update 2005. Particulate matter, ozone, nitrogen dioxide and sulfur dioxide .3 Since then, methods of assessing air pollution exposure have improved.1 Studies on the relationship between air pollution and health have been conducted in nearly all WHO regions, whereas most prior studies were of North American and European origin.1 New evidence has linked air pollution with asthma, diabetes, reproductive outcomes, various neurocognitive endpoints, and other health conditions with which associations were not previously suspected.1,4,5 In light of these developments, the WHO has released a global update to its air quality guidelines.
The purpose of the guideline is to provide key stakeholders and decision-makers with 1) evidence-based air quality guideline (AQG) levels, or the lowest levels of exposure that result in harms to health, for particulate matter 2.5 (PM2.5), particulate matter 10 (PM10), nitrogen dioxide, ozone, sulfur dioxide, and carbon monoxide in both indoor and outdoor environments; 2) interim target levels to guide the reduction of these pollutants in countries that significantly exceed AQG levels; and 3) good practice statements to guide mitigation efforts for specific types of PM that pose a health risk but lack sufficient evidence to derive AQGs.