A. Sometimes and sometimes not, weather experts say. And cold air, even if clean, can carry its own health risks.

“Temperature plays some role in the cleanliness of the air,” said Todd Miner, a meteorologist at Pennsylvania State University. One temperature-related case involves the chemical reactions that produce ozone. Higher temperatures combined with sunlight provide the conditions that promote ozone, which has well-known irritating effects on the respiratory tract.

In general, winter weather systems promote the mixing of the layers of the atmosphere, diluting pollutants, and sometimes those systems import cleaner air from Canada, Mr. Miner said. But mixing can be inhibited in temperature inversions, which are more common in winter in most places. In an inversion, the temperature increases with altitude, rather than the more typical decrease. The inversion can cause pollutants to build up near the ground.