Researchers also found that for a few conditions, including aneurysm and high blood pressure, higher temperatures were tied to a drop in ER visits.

"What we know about climate change is that heat waves in California and throughout the world are going to become more severe and more intense," said Rupa Basu, the study's lead author and an epidemiologist at the California Office of Environmental Health Hazard Assessment. "With that, we're realizing this might implicate more health effects" from future temperatures.

That heat waves can lead to more deaths is already known, and one recent report predicts150,000 additional heat-related deaths will occur in U.S. cities by 2100 because of climate change (see Reuters story of May 24, 2012).

Basu and her colleagues looked at the relationship between heat and specific health conditions, rather than deaths, during the warm seasons in California from 2005 to 2008.

During this period there were 1.2 million visits to emergency rooms, the researchers report in the journal Epidemiology.

Basu's team divided the state into 16 climate zones and compared emergency room visits each day in a given area to local variations in temperature and humidity outside.

For each 10-degree increase in temperature, they saw increased ER visits for a variety of conditions - from a 1.7 percent rise in ER visits for heart disease to a 4.3 percent rise in diabetes visits to a 12.7 percent increase in visits for low blood pressure.

Conditions diagnosed as heat illness or heat stroke rose nearly four-fold for every 10-degree climb on the thermometer, and dehydration visits increased by 25 percent.

Although the study could not pinpoint why certain health conditions are more likely to send people to the ER on hotter days, Basu said it likely has to do with how our bodies adapt to heat.

"Blood flow goes from vital organs to subcutaneous, right under your skin, so you're taking blood away from vital organs such as your heart. It's what your body does to cool down, but some susceptible subgroups might not be able to take that diversion and that's why you end up going to the ER," she said.

The researchers speculate in their report, for instance, that a strong pattern of temperature-related visits for heart disease and ischemic stroke (the kind caused by a clot) could be a result of "increased blood viscosity… or changes in coagulation."

A 10-degree increase in temperature was also tied, however, to a 10 percent drop in ER visits for high blood pressure, an 8.4 percent drop for hemorrhagic stroke (the kind caused by a blood-vessel leak) and a 13.6 percent drop in aneurysm visits.

These results might also be explained by the body's response to heat.

Blood vessels expand to cool off the body, and this lowers blood pressure, "which could explain the reductions in ER visits owing to hypertension and the hypertension-associated blood vessel rupture outcomes, and the increase in hypotension (low blood pressure) visits," Basu and her colleagues write.

Basu's study found that certain groups of people appeared to be more vulnerable than others to the effects of heat.

For instance, the increased risks of heading to the ER for ischemic stroke, heart disease, kidney failure and intestinal infections were higher among Hispanics than whites.

Basu said it's important to identify who might be the most susceptible to heat waves, and in what ways, so that health officials can prepare for future events.

SOURCE: bit.ly/VAzI57>Epidemiology, online September 21, 2012.

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