A team led by Daniel Horton, a climate modeller at Stanford University in California, used 15 global climate models to track changes in the number and duration of atmospheric stagnation events, in which stationary air masses develop and allow soot, dust and ozone to build up in the lower atmosphere. “Much of the air-quality community focuses on pollutants,” says Horton. “This study takes a step back and looks at the weather or climate component that can lead to the formation of hazardous air quality.”

How worsening air quality due to stagnation would affect different regions has been poorly studied, and there are few estimates of human impact. The new study shows just how widespread the effects will be, says Jason West, an environmental scientist at the University of North Carolina at Chapel Hill.

Hot and stuffy Air stagnation arises from three meteorological ingredients: light winds, a stable lower atmosphere and a day with little or no precipitation to wash away pollution.

The researchers then factored in current population size to quantify human exposure to daily stagnation events and air pollution. The impacts are especially intense in India, Mexico and the western United States. By far, the largest uptick in overall human exposure will be in India, says Horton, due to the country’s enormous population, along with the increases in atmospheric stagnation.

Outdoor air pollutants are a major contributor to stroke, heart disease, lung cancer and respiratory diseases including asthma. The World Health Organization estimates that outdoor air pollution caused 3.7 million premature deaths globally in 2012. Nations could mitigate the air pollution impact by limiting emissions of greenhouse gases, particulate matter and the precursors to ozone, including nitrogen oxide, nitrogen dioxide and volatile organic compounds, says Horton.

The latest study does not account for changes in population size or distribution, or for changes in the amount of pollution entering the atmosphere.

But it nevertheless portends dire consequences, says Susanne Grossman-Clarke, an urban climatologist at the Potsdam Institute for Climate Impact Research in Germany. ”Combine these stagnant air masses with extreme heat and a great number of people may end up sitting in emergency rooms,” she says.

This article is reproduced with permission and was first published on June 22, 2014.