Why the Myth Endures
The myth endures because it appears to people as if rainy or cold weather contributes to illness. Certainly there is a strong correlation between rainy or cold weather and viruses like cold or influenza. This is because more people are infected with these viruses during cold and/or rainy months. Plus, being outside in cold rain can cause temporary flu-like symptoms, like shivering or a runny nose. But in these cases, correlation does not imply causation. Viruses cause these illnesses, not weather patterns.
Being Indoors with People
Cold and flu viruses tend to spike any time cold or rainy weather keeps people indoors more often than usual. During these times, people tend to be in closer proximity to one another as well. When you inhale the virus particles that a person breathes or coughs into the air, you can be infected. Whether to exercise indoors or outdoors is not the issue.
Body Temperature Drop
When you get wet outdoors, your body temperature drops. This is offset somewhat by exercise, which increases your body temperature. Even so, especially cold rainy conditions may cool the body enough to cause hypothermia, particularly if your clothes are drenched in water. Hypothermia strains the body, including the immune system, and this may heighten your chances of becoming infected with a virus. In such cases, rain may aggravate your immune system, but it is not the direct cause of illness.
Immunity Benefits of Exercise
Cardiovascular exercise helps boost the function of the immune system. This is true regardless of weather conditions. In other words, your immune system function will improve whether you choose to exercise indoors or outdoors during a cold rain. This can make you less likely to contract a cold or flu virus.
Exercise common sense when carrying out your exercise plan in the rain. MayoClinic.com recommends staying indoors if the outside windchill reaches zero degrees F or lower. By the same token, it’s probably unwise to exercise outdoors in freezing rain, in which quick accumulation can make you more prone to falls and injury. If you have asthma or any other chronic illness, consult your doctor before beginning a new exercise program.