A rise in air pollution could be linked to an increase in heart attacks, say scientists.
Heart disease is usually associated with genetic factors and poor lifestyles.
But a study of 250 cities around the world found an increase in air pollution led to a rise in heart attacks.
And research in Salt Lake City found that when a steel mill shut down there was a six percent drop in death rates. Mortality rose when the mill re-opened.
Dr. Robert Brook of Louisville University said people who live in cities with poor air quality should not exercise outside in commuting hours.
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