Using weather data and death rates for 50 US cities between 1989 and 2000, researchers found that, on average, a two-day cold snap increased death rates by 1.6 percent. Heat waves, on the other hand, triggered a 5.7 percent increase.

"We saw that the effects of cold temperatures are not as big as the effects of hot temperatures," said lead study author Dr. Mercedes Medina-Ramon, of the Harvard School of Public Health in Boston.

This means that relatively milder winters attributable to global warming are unlikely to make up for the health effects of summertime extremes, she told Reuters Health.

Medina-Ramon and colleague Joel Schwartz report their findings in the journal Occupational and Environmental Medicine.

Deaths related to extreme cold are often from heart attacks or cardiac arrest. Extreme heat, along with causing deaths from heatstroke, can contribute to deaths from other causes, like heart attacks.

The elderly and people with known heart disease are among those most at risk during temperature extremes.

Cold snaps, though, are not as deadly, and that may be because most Americans have adequate heat in their homes, according to Medina-Ramon. Air conditioning is less ubiquitous.

Indeed, the study found that heat extremes had the greatest impact on death rates in cities that typically had milder summers, and in those with fewer air-conditioned homes.

"If we spread the use of air conditioning, that would actually decrease deaths," Medina-Ramon said.

However, lower-income families are not necessarily able to bear the cost of air conditioning. What's more, Medina-Ramon said, all of that electricity consumption increases carbon dioxide emissions -- the very thing blamed for global warming.

All of this, she said, points up the need for more-efficient air conditioning systems that eat up less energy.

Less "air conditioning abuse" would also be helpful, Medina-Ramon said, referring to climate-controlled buildings that feel closer to "freezing" than comfortable.