Humidity Implicated in Heart Attack Risk for Older Patients

ATHENS, July 13 -- Rising humidity may rival temperature extremes in boosting the risk of fatal heart attacks in patients 70 or older, according to a study here.

During the most humid months here, heart attack deaths were nearly a third higher compared with the driest months, regardless of temperature, said Polychronis E. Dilaveris, M.D., and colleagues at the University of Athens.

While temperature has been linked with heart attack risk in previous research, this study appears to be the first to identify a relationship between relative humidity and acute myocardial infarction deaths," Dr. Dilaveris and colleagues said online today in the journal Heart.

In contrast to most U.S. cities, winter is the most humid season in Athens and summer is the least humid. For example, in 2001 the mean relative humidity was highest in December (73%) and lowest in August (44%), the study reported.

The investigators obtained data on all deaths from acute myocardial infarction in Athens during 2001 (more than 3,000). They also obtained daily climate readings during 2001 from the National Meteorological Society, including temperature, relative humidity, and air pressure measurements.

During the heightened humidity of December, there were 331 deaths from acute MI in the city. During the dry month of August, only 244 died of acute MI. In fact, the average monthly humidity was an independent predictor of heart attack risk (P=.0004), the investigators said.

The relationship between temperature and heart attack risk was more complicated, although temperature was also an independent risk factor, the investigators said. Risk with temperature was U-shaped. It was highest-exceeding 10 deaths per day on average-when the temperature dipped below 40 degrees Fahrenheit or climbed into the 90s (P<.001). Risk was lowest, about seven deaths per day, when the temperature was in the 70s, the study found.

The average of the mean daily temperatures of the preceding seven days was the best predictor of daily mortality. The authors suggested that this relationship indicates that temperature has a cumulative physiological effect, such that consecutive days of low or high temperatures have more prominent an impact on acute myocardial infarct mortality than the mean temperature of a single day.

The links between temperature and humidity and fatal heart attacks were strongest for patients 70 or older (P<.001). The link was only marginally significant for those in their 50s and 60s (P=.043) and for those younger than 50 (P=.049), the study found.

No MI risk was associated with air pressure, the investigators reported.

Cold weather might cause peripheral vasoconstriction, thereby raising blood pressure and heart attack risk, the authors suggested. Cold weather might also favor thrombotic events by altering levels of plasma fibrinogen and other thrombotic factors, they speculated. The authors did not discuss the mechanisms underlying the increased risk of MI in hotter weather.

They also had little to say about the "unclear" biological mechanism linking humidity and heart attack risk. "In temperate climates with less pronounced winter-summer differences, relative humidity possibly has a more influential effect on cardiovascular mortality than previously recognized," they said, adding that further studies would be needed to explore the mechanism.

The results "do not imply that these atmospheric variables are solely responsible for the observed seasonal distribution of acute myocardial infarction deaths," the authors said. Other potentially relevant factors not controlled for in this study include seasonal variations in exercise, eating habits, and atmospheric pollution, they said.

However, the results suggest that public health officials should consider humidity as well as temperature in their efforts to protect older individuals from climate-associated heart attack risk, they concluded.