Weather affects human health through the interaction of multiple meteorological variables and human response. As a result, air mass-based studies that categorize weather conditions using a suite of meteorological variables are ideal for evaluating health-weather relationships. Previous air mass studies have demonstrated that oppressive summertime conditions, characterized generally by high heat index values, can be associated with offensive air masses that consistently increase daily mortality rates in U.S. metropolitan areas.

The current study uses time series mortality to initially address two questions (1) what are the time trends in daily mortality of these offensive air masses around the country and (2) is there evidence that Heat Health Watch Warning Systems (HHWWS) and other adaptive measures have helped reduce the mortality impact of these offensive air masses. The study uses results from analyses of these initial questions to develop estimates of the possible future mortality impacts of offensive air masses under different climate change scenarios while allowing for the continued

implementation and development of HHWWS and other adaptive measures. These results show significant regional variation in impacts but suggest that summertime heat-attributable mortality will increase dramatically with climate change without any further adaptation, and that continued implementation of HHWWS and other adaptive measures could provide a significant mortality reduction.