Half of premature deaths related to air pollution in U.S. states are caused by pollution that originated from another state, according to a study 11 years in the making.

The study, published in Nature last week, is the first to calculate how pollution crossing state lines impacts early deaths in each state, said co-author Steven Barrett, an associate professor of aeronautics and astronautics at the Massachusetts Institute of Technology in Cambridge.

Globally, an estimated 4.2 million premature deaths are linked to outdoor air pollution, mainly from heart disease, stroke, lung disease and acute respiratory infections in children, according to the World Health Organization.

Efforts to address outdoor air pollution have largely focused on relationships between local sources of pollution and local air quality. What Barrett and colleagues found is that cross-state pollution accounts for about half of all premature, pollution-related deaths.

The computer model his team developed for the study took weather patterns and atmospheric chemistry processes into account and tied those to data on human exposures and health risks. They used this to track how each state in the contiguous United States affects pollution and health outcomes in every other state.

The model included data from 2005, 2011 and 2018 on different sources and types of pollutants, such as sulfur dioxide, ozone and fine particulates, from seven emissions sectors, including electric power generation, road transportation, marine, rail, aviation, and commercial and residential sources.

The team found that electric power plants — which emit sulfur dioxide from smokestacks — were the biggest contributor to deaths related to pollution from other states. In 2005, sulfur dioxide from power plants was involved in 75 percent of cases of premature deaths from out-of-state pollutants.

One bright note is that regulatory changes to curb emissions since 2005 have reduced the number of early deaths related to air pollution by 30 percent. The team also found the proportion of premature deaths from out-of-state emissions is dropping — falling from 53 percent in 2005 to 41 percent in 2018.

Not all states contribute to the problem equally, the study showed.

Many states in the northern Midwest, such as Wyoming and North Dakota, are “net exporters” of ­pollution-related health impacts, in part because of their low populations relative to the amount of emissions they generate.

States on the East Coast, where winds sweep emissions eastward, are “net importers” of air pollution. New York is hit especially hard, with 60 percent of early deaths related to air pollution arising from out-of-state emissions.

— Reuters