QQs: Why Do We Get Colds When It's Cold?
MA:  At some point during your childhood, you were probably told not to go out in cold weather with wet hair or without bundling up because you'll catch a cold.

But we know the common cold is caused by viruses, not chilly air.  So why does this old wives' tale hang around?

It's probably because colds are more common in colder weather, but as far as scientists can tell that's not because you feel cold.  There are lots of better explanations for why colder weather increases your odds of getting sick.

The connection between temperature and illness isn't simple.  Even though colds and other respiratory illnesses are more common in colder months not all of them spike in the dead of winter when it's coldest.  And the most direct studies we've done haven't found a direct relationship between feeling cold and catching a cold.

For example, in a randomized controlled trial published in 1958, researchers divided nearly 400 people into rooms that were either 27 degrees Celsius, 16 degrees Celsius, or -12 degrees Celsius (80F, 60F or 10F) then they put virus-infected mucus up some of their nostrils.  

But the temperature didn't make a difference:  in every room, just over 1/3 of the volunteers that received the infected mucus got sick.

One study from 2005 did find that:  people who had their feet soaked in freezing cold water reported more cold symptoms in the days afterward compared to a control group.  

But it's hard to tell how much of that was influenced by subjects thinking they'd be more likely to get a cold.  

If feeling cold really does make you more likely to get sick there are a couple of ideas that might explain it.  

One hypothesis argues that even though the cold doesn't weaken your immune system overall, it might lower the defenses in your respiratory system, specifically.

And in a paper published in 2016 in the journal Medical Hypotheses, a microbiologist suggested that viruses lie dormant for extended periods of time in our bodies, then get activated when the temperature drops.

But there are lots of problems with those ideas and the vast majority of research shows that simply being cold doesn't make you more vulnerable to catching colds.  Instead, there are other aspects of cold weather that might increase your chances of getting sick.

Like the fact that the air is super dry.  

Colder weather is associated with lower humidity, because at lower temperatures, the air can't hold as much water.  When humidity is high, the droplets of virus-infected grossness that we breathe out, or sneeze out,  or cough out of our bodies stay large and drop to the floor relatively quickly.  But in dry air, they break up into smaller particles and can float around for hours.

Plus, the lack of moisture can dry out the mucus lining of your nose which might make it easier for viruses to get past that line of defense.

Another potential problem is that some people don't get enough sunlight in the winter making them run low on vitamin D, since vitamin D help power your immune system, lower levels mean lower defenses against the viruses.

And then there's the fact that human behavior changes during colder months.  We are more likely to stay indoors, which means we are more frequently touching stuff infected people touched, and breathing in the remnants of their sneezes.

[Inhales] Mmm!

So researchers are still trying to pin down all the different ways cold weather may or may not affect how likely you are to catch a cold.  

But the best way to avoid catching one isn't necessarily by throwing on another layer or drying out your hair before running out the door.

It's washing your hands with soap, not touching your face with unwashed hands, and staying away from people you know are infected.