You turn on the heat for the first time during fall and it feels like suddenly everyone gets sick. It's not your imagination — and it’s not just that the weather is changing.

It’s what heating does to indoor humidity.

When the cold air from outside gets heated indoors, the relative humidity can drop to 20%, according to researchers who say that desert-like air is priming you for illness.

(MORE: If you’re in a mental slump, it may be your allergies)

What happens to you

Dry indoor air thins and dries out the critical mucus lining your nose and throat; it’s your first defense against incoming viruses. That lining and microscopic structures called cilia usually trap and flush out viral particles.

A Yale study even found that mice breathing in dry air were more susceptible to the flu than mice breathing in humid air.

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What happens to a virus

In dry air, virus-carrying droplets get smaller, stay airborne for longer and travel farther, giving them a higher chance of infecting you.

A study using SARS-CoV-2 as the test case found that dry indoor conditions typical of winter kill off viruses slower than more humid summer conditions, increasing the concentration of viral particles in poorly ventilated spaces.

(MORE: Flu season follows the humidity, not the calendar)

Then there’s the cold air

Cold air itself also blunts your nose’s defenses, making a second hit on that crucial lining in your nose and throat. Research published in the Journal of Allergy and Clinical Immunology found that cold air exposure directly impairs the mucous membranes involved in fighting off viruses.

Then there’s the fact that once the heat goes on, many of us will be spending most of our time indoors, with the windows closed, in close proximity with others, increasing our exposure to potential illnesses.

(MORE: Could you be allergic to the cold?)

So what’s the fix?

Improving those indoor conditions is pretty simple and cost effective. Scientists believe keeping the relative humidity in your home between 40 to 60% is the sweet spot for keeping you healthier.

Most SMART thermostats include a hygrometer that can tell you where you stand, or you can buy a digital one for about $20.

If the humidity in your home is below 40%, a humidifier can bump the moisture. Just make sure to clean it properly and regularly to avoid putting more germs in the air!

One last thing: while burping or airing out your home is something we encourage to get fresh air into your home during the colder months, when you crack open a window, you might want to run the humidifier so you’re not dropping that indoor humidity.