Why is it that despite flu vaccines and OTC medicines, most people still become sick during “sick seasons”? One reason may be as simple as fresh-air deficiency. See the data for yourself.

Common wisdom has it that staying indoors, where it’s warm and toasty, is easier on your immune system than being outside in the cold. Problem is, being inside puts you in close constant contact with other people — and their germs.

Adding insult to injury, you’re already more vulnerable to illness now: Winter air is dry, whether you’re indoors or out, so it pulls moisture from cough and sneeze droplets. That makes the drops lighter, so they — and the cold and flu viruses inside them — can hang around in the air longer. The arid air also dries out your nasal passages, creating cracks that viruses are all too happy to invade.

Not only does escaping into the fresh air give you a break from all those germs circulating inside, but going for a stroll can actually boost your immunity: A 2010 Appalachian State University study showed that people who walked briskly for 30 to 45 minutes a day five days a week during the winter had fewer illnesses than their sedentary counterparts.

“Exercise leads to an increase in natural killer cells, neutrophils, and monocytes, which ultimately increases immune function,” says Ather Ali, N.D., M.P.H., assistant director of Complementary/Alternative Medicine Research at the Yale-Griffin Prevention Research Center.

Source: “Secrets of women who don’t get sick.” By Jeannette Moninger. http://www.cnn.com