Heat waves, like the one that scorched the country in July, are more deadly for some people than for others.
Poor blacks and diabetics fare the worst. As you might guess, extreme heat is also hard on the elderly. But as you might not guess, extreme cold has a greater impact.
Also, the increase in risk on extremely hot days is smaller for deaths due to heart disease, such as heart attacks, than for other causes. Conversely, the increase in risk of dying from heart disease on extremely cold days is greater. Deaths from cardiac arrest show the largest increase at such times.
These revelations come from a study of almost 8 million deaths in 50 cities from 1989 to 2000, just completed by researchers at the Harvard School of Public Health (HSPH). Cities ranged from Minneapolis to Miami, from Boston to Honolulu.
Researchers counted the coldest and hottest 1 percent of days in each city as extreme temperature days.
“We aimed to identify specific causes of death that show the largest relative increases on extreme temperature days, an issue that has not been addressed by previous studies,” says Mercedes Medina-Ramon, lead author of the study report. “Recognition of factors that convey susceptibility to die from a specific disease on extreme temperature days may provide relevant information to develop public health programs that will better target those most vulnerable.”
Results of the statistical comparison of weather and deaths over 12 years show that blacks and those with a high school education or less are most likely to die on extremely hot days. That, the researchers conclude, could be related to poorer health, limited access to medical care, and lower quality housing.
In other words, heat waves of the kind that are becoming more common with global warming take their biggest toll on the socially disadvantaged.
Blood, sweat, and clots
Diabetics are also particularly vulnerable to heat, which may be the result of a decreased ability to regulate internal body temperature in high heat.
“This could be explained by a different response to extreme heat stress in diabetics,” notes Medina-Ramon, who is a research fellow in environmental health at HSPH. That, in turn, would involve a flaw in their ability to regulate internal body temperature. Proper regulation, for example, involves an increase in sweat in high heat, or decreased blood circulation to the skin in bitter cold.
“Other studies have shown that this ability may also be diminished in the elderly, for instance having lower sweating rates in heat,” Medina-Ramon points out. Apart from these regulatory mechanisms, the body can show other responses to extreme temperatures. These include changes in heart functioning, breathing, or nervous system activity. “It is possible that a difference in these responses between the elderly and the young may play an additional role in the increased susceptibility to die,” she says.
As to evidence that increases in deaths from heart disease were significantly higher in extreme cold spells, several mechanisms may explain it. “Blood pressure is higher during winter,” according to Medina-Ramon. “It has also been reported that exposure to cold temperatures increases levels of cholesterol and clotting factors, which could lead to thrombosis, or blood clots.”
While extreme cold endangers those with heart problems, the researchers also found that people with irregular heat beats (atrial fibrillation) can be particularly vulnerable to extreme heat. “Atrial fibrillation may contribute to death in several ways, including a five-fold increase in risk of stroke,” Medina-Ramon explains. “An increase in blood viscosity (thickness) with high temperatures may facilitate the formation of blood clots that cause stroke. However, it is important to remember that only about 15 to 20 percent of strokes are related to atrial fibrillation.”
“The recognition that some groups of people are particularly vulnerable to temperature extremes has relevance to public health especially if such people, as is the case of diabetics and the elderly in many countries, are growing portions of the population,” the study report points out. This kind of information is valuable for developing programs that could save lives during spells of extreme temperatures that are bound to occur, and perhaps expand and intensify, in the future.
Besides Medina-Ramon, authors of the report include Antonella Zanobetti, David Paul Cavanagh, and Joel Schwartz (senior author). Their report, “Extreme Temperatures and Mortality,” is available online from Environmental Health Perspectives at http://www.ehponline.org/members/2006/9074/9074.pdf.