To see if there was a link between health and small particulate pollution (PM_2.5; that is, particulate matter smaller than 2.5 micrometers), researchers at Harvard University compared death rates and PM_2.5 levels in six U.S. cities. The data reveal that as particulate pollution increases, so does the mortality (death) rate. PM_2.5 pollution and deaths went down in every city between the two sampling periods evaluated—likely a result of stricter pollution limits in the 1990s.