Not just, but especially due to the Covid Pandemic, many people deal with the indoor climate, but what does that actually mean if you take the phrase about clean air literally !? Fresh air is free of disturbing particles and substances?  The air that we breathe outside is constantly in motion and very quickly mixes, however,  less so in closed rooms where the pollutants and viruses can concentrate and stay more easily. In fresh air, they are quickly distributed into the atmosphere. Also, exhaled Carbon dioxide can be problematic. Up to 1000 parts per million in the room air is okay.  If two people in a small room sleep with tightly closed, sealed windows, it could reach up to 2000 parts per million after one night, consequences the day after are headaches, fatigue, and reduced ability to concentrate.

People often do not even notice or know when to ventilate

The best way is to exchange the air three times throughout the day by ventilating, especially in view of the coronavirus, air filters in the room are not a substitute for airing the rooms. If a healthy indoor climate is an objective, you cannot pay attention only to the concentration of carbon dioxide and vapors, but also to the humidity and temperature. Air that is too dry can dry out the mucous membranes and so the body has a harder time rejecting and remove the dust particles you inhale.

As a result, it is easier for pathogens to penetrate the body and you may catch a cold more quickly. A bowl of water near your heating unit or on top of the radiator, or room plants should help to ensure good air in the room.

If you have problems with air being too dry, you should consider buying a good room air humidifier. The humidity should not be too high. If moist room air condenses on cold walls, mold may easily develop. 

Especially for allergy sufferers, asthmatics, or people with other lung diseases, mold spores in the air can be a real problem

Ulrich Koepf