An article in “Atmospheric Environment”: microbes and allergens that normally float in moist air will settle and stick to the moisture of our skin and sinuses when the air is dry. This, in turn, increases the risk of infection and asthma.
The article goes on to describe several high tech and expensive ionization systems. In the end, the simple solution is to have a humidifier. My rule of thumb is this: If the heat is on, put on the humidifier.
There is quite a range of options here. While I love my fancy ultrasonic machine, with a bit of maintenance, a simple vaporizer does the trick. Recently, I even found an excellent low maintenance, anti-bacterial, ultrasonic humidifier at Walgreen’s Drug Store for under $45.