The humidity in my apartment sometimes drops below 5%, with lowest recorded to be 3%. I have bought an air humidifier, but even that does not increase the humidity by more than 8%.
What are the effects and health risks of such low humidity levels?
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According to below chart (in which the source from 1986 is also stated) the optimal indoor air humidity for human health is between 40-60%, as close to 50% as possible. The low humidity levels you described (5%) would apparently be optimal for bacteria and viruses and negatively affect allergic rhinitis and asthma. Such low levels of indoor air humidity can also cause respiratory infections and is associated with increased ozone production.
If you have any health or specifically respiratory problems I suggest to visit your doctor and mention your indoor humidity problem.
Be sure that your humidity sensor is properly functioning and placed correctly. I'm not sure in what climate you live but the values mentioned seem extremely low to me for an indoor climate. An air humidifier suitable for your room size might help to increase indoor air humidity. The (small) humidifier you described is probably not strong enough for the size of your room.
Houseplants increase humidity in the air through a process known as transpiration. Plants absorb water through their roots, then circulate the moisture through stems and leaves with a vascular system that's comparable to human veins and capillaries. Water reaches the leaves, evaporates into the air and increases indoor humidity. 1