Current scientific studies point out that low temperature outdoors is not the main cause of respiratory diseases, whereas the indoor low humidity is.
First of all, it is really important to understand that humidity in the majority of apartments, houses and offices in the countries with low temperature hardly comes up to 20 % in winter, while the optimum humidity level for living spaces is supposed to be within 35 - 50 %. These recommendations are based on long-time medical and microbiological researches.
There are particles and dust, containing viruses, bacteria and allergens which may persist in the air for a long time. In case of sufficient humidity level they just stick together and go down. Moreover, humid air ruins the structure of viruses, so that they lose their ability to impair human cells.
Another fact is that dry air has bad impact on skin, more specifically, it increases water evaporation from mucosa and skin. Dryness of mucosa causes discomfort, ruins protective barriers of our body, consequently increasing our chances to get sick. Dry skin becomes less elastic, gets cracks and flakes.
Few are aware that parquet, doors and wooden furniture dries up under the influence of dry air. Human hair and pets’ fur are being statically electrified.