A small number of studies have implicated long-term exposure to PM in reducing survival. The first of such
studies was the Harvard Six Cities study which was published in 1993 (9). This study found that subjects living in
communities with elevated fine particulate matter concentrations had a higher risk of dying over a 14-year follow-up
period than subjects living in cleaner communities. A very large U.S. cohort study subsequently supported these
findings (10) and further follow-up of these two cohorts has lent additional support (11, 12). Using data from these
long-term studies, it has been estimated that effects of ambient PM exposure on life expectancy can be substantial, a
reduction of one year or more (