A new study looks at air pollution and its effects on premature deaths in the United States. Premature deaths are defined as those that take place before the average age of death. Researchers found that half of all such deaths related to air pollution resulted from pollution that came from another state. The study is the first to estimate how pollution crossing state lines affects early deaths, says Steven Barrett. He is a professor at the Massachusetts Institute of Technology and a co-writer of a report on the study. Worldwide, an estimated 4.2 million premature deaths are linked to outdoor air pollution. Most of those deaths are from heart disease, stroke and lung disease, as well as acute breathing infections in children, reports the World Health Organization. Efforts to deal with outdoor air pollution have largely centered on relationships between local sources of pollution and local air quality. But Barrett and other researchers found that, in the United States, cross-state pollution is a major concern.