Around the world, outdoor air pollution causes 3.3 million premature deaths a year, predominantly in Asia, according to a new study published in Nature this week. 

Outdoor air pollutants – such as tiny particles less than 0.0025 millimeters in diameter called fine particulate matter – have been linked to long-term impacts on health. To estimate the contribution of different sources of outdoor air pollution to premature mortality, Jos Lelieveld of Max Planck Institute for Chemistry and colleagues combined a global atmospheric chemistry model with population data and health statistics. 

Emissions from residential energy use – such burning wood for in-home heating and cooking – have the largest impact on premature mortality worldwide. They may cause about 1 million deaths a year, mostly in China and India.

In most of the U.S. and a few other countries, emissions from traffic and fossil fuel-fired power plants are the biggest sources of outdoor air pollution. Meanwhile, in the eastern U.S., Europe, Russia, Turkey, Korea, and Japan, agricultural emissions are the largest contributors of fine particulate matter. Farms account for more than 600,000 premature deaths. “I was surprised” by that result, Lelieveld tells Science. “What you tend to think is that [air pollution comes from] mostly traffic, and maybe industry.” However, animal husbandry and fertilizer use generate ammonia, which can be converted to fine particles.

According to model projections based on a business-as-usual emission scenario, premature death from outdoor air pollution could double by 2050. That’s 6.6 million deaths a year. 

In a related study published in Nature Geoscience this week, a team led by Dominick Spracklen from the University of Leeds looked at particulate matter concentrations after the substantial reductions in deforestation-related fires in the Brazilian Amazon since 2004. The researchers combined a chemical transport model with satellite and ground-based measurements in southwest Brazil and Bolivia to study the impact of this reduction on air quality and human health during non-drought years between 2001 and 2012.

Concentrations of particulate matter, they found, have declined by 30% during the dry season (August to October) as a result of fire reductions – preventing roughly 400 to 1,700 premature adult deaths a year across South America. 

Image in text: Mortality linked to outdoor air pollution in 2010. Deaths per area of 100 km × 100 km (color coded). J. Lelieveld et al., Nature 2015