Several studies show that people are more likely to die or have acute heart trouble when the temperature drops suddenly, or when air pollution levels are especially bad.
Stress of some sort is probably causing the effects in both cases, four teams of researchers told a meeting of the American Heart Association in New Orleans.
"The issue is not just the absolute temperature outside - the issue is how well you protect against the temperature," said Dr. Gad Cotter of Duke University in North Carolina.
Cotter's team studied 300 patients last winter in Israel.
When the temperature dropped below about 7 degrees C (45 degrees F) - a cold snap for Israel's climate - between two and three times the number of people were taken to hospital with acute heart failure than on warmer days.
"On these nights, or the morning after, we would get a huge number of patients with acute heart failure," Cotter told a news conference. "During days when there was high pollution we also had more patients with acute heart failure."
Acute heart failure is a worsening of a chronic and often deadly condition in which the heart loses its ability to pump well. In an acute episode patients can feel breathless or faint.
Dr. Toru Suzuki of the University of Tokyo and colleagues looked at patients around with world with a rare but deadly condition called acute aortic dissection.
"There are only a few thousand (cases) a year in the United States," Suzuki told the news conference. But it is deadly.
"It is very catastrophic. It literally causes the dissection of the aorta" - the main artery leading out of the heart.
The researchers looked at the cases of 969 patients with acute aortic dissection in the United States, Canada, Germany, Israel, Spain, Italy, Japan and Norway over 10 years. No matter what the climate, incidence of acute aortic dissection peaked in the winter, they found. "It suggests you have to take care in changing temperatures and seasons even if you are in a warmer climate," Suzuki said.
Dr. Nino Kuenzli and colleagues at the University of Southern California looked at two previous trials of 800 people over the age of 40 and found those who lived in more polluted areas tended to have thicker carotid arteries - a measure of underlying heart disease.
"The thickness of the carotid ... increased 4 percent to 5 percent with each incremental particulate level," Kuenzli said. In other words, the more visible pollution there was in the air, the worse the level of underlying artery disease.
And the effects were worse in certain people, especially women past menopause and women already on cholesterol-lowering drugs.
"This is quite a clear and strong association between air pollution and the level of atherosclerosis," Kuenzli said.
"It is telling us that air pollution is chronically contributing to the process of cardiovascular disease and death."
Dr. Vanina Bongard of Toulouse University in France looked at pollution levels there and the effects on all heart events such as heart attack, heart death and acute chest pain among the 400,000 people living in Toulouse.
They found no link between two common pollutants - sulfur dioxide and nitrogen dioxide - and heart trouble. But ozone levels were strongly associated with cardiac arrest, heart attack and angina.
"There is a significant link between short-term ozone exposure within one to two days and acute events," she said.