Human response to climate consists of physiological and behavioral reactions to extreme events, but in many climate/health analyses, the identification of these events is poorly expressed. For example, most studies rely on some combination of temperature and humidity to identify extreme heat events, but there is a much larger dimension to understand how these episodes could lead to human stress. This presentation will review a suggested approach to identify particularly dangerous heat episodes, and it will link these events to negative human health outcomes in urban areas. Additionally, we will present a means to determine whether these events have become more common and/or more extreme during the past 60 years. We react to weather more in a relative, rather than absolute, fashion, which explains why more people die of heat-related causes in Toronto than in Phoenix. Our approach identifies dangerous air masses approaching a city by their origin. Two such air masses are the “moist tropical,” which is hot and humid, and “dry tropical,” which is even hotter and very desiccating to the human body. The identification of these air masses that have historically led to deterioration in human health during very hot weather is the cornerstone of our “heat health watch warning systems,” presently being used by a number of National Weather Service offices as guidance to call excessive heat warnings. Air masses are characterized by much more than just temperature and humidity, and their typical attributes include other factors that are responsible for negative health outcomes, such as solar load, wind speed, and air quality. The delineation of oppressive air masses also underscores the effects of long consecutive-day episodes, which are particularly dangerous to health since most people cannot adjust well to the cumulative impacts of heat. Our interests also align with those of LBNL in regard to uncovering systematic aspects of climate change. We apply an air mass-based methodology to answer the following sorts of questions: Are these oppressive air masses becoming more frequent through time? Are they going through physical changes where there are temporal trends in temperature, dewpoint, or other meteorological factors? Most traditional climate change studies tend to evaluate trends for the entire period of record, but we have successfully segregated changes that occur only in those air masses that are responsible for heightened human mortality and morbidity. These results, completed for five Midwestern cities and published in a recent Union of Concerned Scientists monograph, will be presented, and we propose an expansion of this type of work on a national level with the collaboration of LBNL.