Pollution is the largest environmental cause of disease and premature death in the world today. Diseases caused by pollution were responsible for an estimated 9 million premature deaths in 2015—16% of all deaths worldwide— three times more deaths than from AIDS, tuberculosis, and malaria combined and 15 times more than from all wars and other forms of violence. In the most severely affected countries, pollution-related disease is responsible for more than one death in four.

Air pollution exposure is the second most important risk factor for ill health in South Asia, contributing to between 13% and 21.7% of all deaths and approximately 58 million disability adjusted life years (DALYs) through chronic and acute respiratory and cardiovascular illnesses.1 Of the top 30 cities in the world with the poorest air quality in 2016, 17 are in South Asia.2 The impact of air pollution transcends boundaries. The “brown cloud”—caused by pollution from carbon aerosols—is a phenomenon captured in satellite images of atmospheric haze over South Asia, as well as China.

The 2015 smoke haze episode was one of the most severe and prolonged transboundary air pollution events ever seen in Southeast Asia (SEA), affecting the air quality of several countries within the region including Indonesia, Malaysia and Singapore. The 24 h mean outdoor PM2.5 (particulate matter (PM) with aerodynamic diameter ≤ 2.5 μm) concentrations ranged from 72–157 μg m−3 in Singapore during this episode, exceeding the WHO 24 h mean PM2.5 guidelines (25 μg m−3) several times over.

An estimated 4.5 billion people are currently exposed to particulate matter (PM) levels at least twice the concentration that the WHO considers safe. Existing evidence linking health to air pollution is largely based on populations exposed to only modest levels of PM and almost entirely composed of observational studies, which are likely to confound air pollution with other unobserved determinants of health.