The effect of temperature on mortality: risk varies by region. (EH Update).

The relationship between extreme temperature and mortality the United States varies by location, according to a study researchers at the Johns Hopkins Bloomberg School of Public Health. Excessive heat is more likely to increase mortality risk the North, while excessive cold is more likely to increase mortality risk in the South. The study appears in the January 1, 2001, issue of the American Journal of Epidemiology.

"Historically, we know that episodes of extremely hot or cold temperatures increase mortality Global warming and other weather phenomena, such as El Nino, have warranted the need for further investigation into the weather-mortality relation," said lead author Frank Curriero, Ph.D., an assistant scientist in the Department of Biostatistics at the Johns Hopkins Bloomberg School of Public Health. "We explored this relation in cities along the eastern United States, to further characterize the effects of temperature as varied by latitudes as well as other factors particular to a specific city or area."

The researchers chose 11 large metropolitan areas in the eastern United States and compared daily weather and mortality data for 1973-1994. The analysis was performed in two stages. First, the relationship between temperature and mortality risk from cardiovascular, respiratory and other diseases was estimated for each city Then the variation in risk across cities, due to difference in latitude and other variables, was examined.

Within the selected cities, mortality risk decreased as temperature increased from the coldest days; however, after a certain critical temperature threshold, referred to as the minimum-mortality temperature, mortality risk increased in most of the cities as temperature increased. The minimum mortality temperature threshold ranged between approximately 65 and 70[degrees]F for cities in the North (Boston, Chicago, New York, Philadelphia, Baltimore, and Washington, D.C.) and approximately 76 and 90[degrees]F for cities in the South (Charlotte, Atlanta, Jacksonville, Tampa, and Miami).

Results indicate that the northern cities were more sensitive to heat, showing a larger increase in mortality risk for temperatures greater than the minimum-mortality temperature threshold. Southern cities were more vulnerable to cold temperatures, showing a larger increase in mortality risk for temperatures less than the minimum mortality temperature threshold.

Colder temperatures produced higher mortality risk in cities with higher proportions of elderly people, but not in cities with a large fraction of heating systems. Hotter temperatures produced higher mortality risk in cities in which there were larger percentages of persons without a high school education or living in poverty two indicators of socioeconomic status.

"Temperature at either extreme can be hazardous," said coauthor Jonathan Patz, M.D., M.P.H., an assistant professor in the Department of Environmental Health Sciences at the Bloomberg School of Public Health. "The results of our study, however, will help guide public-health interventions aimed at preventing deaths due to extreme heat and cold. These findings tell us where and to whom such interventions should be directed."

The models developed in this analysis may also be useful in estimating weather-related mortality as a consequence of future climate-change scenarios. Researchers are now applying these methods to more U.S. cities to improve upon their predictive capability. The study was supported in part by the Climate Policy and Assessment Division of the U.S. Environmental Protection Agency.