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The suitable healthy humidity for human body is between 50% and 60%. In autumn and winter, the humidity in Guangdong is less than 30%, and the air humidity in northern heating areas is lower.
Needless to say, the harm of dry air will not only make your skin crack due to water shortage, but also make it easy for bacteria to spread in the air. The prevalence of influenza in winter is also based on this reason.
Someone asked, what's the difference between humidifiers? Which is better?
The ultrasonic humidifier breaks the liquid water into tiny water mist with a high-frequency vibrating plate (with high voltage, which cannot be touched), and then blows it into the air. At first, white water mist can be seen, but because the particles are very small and the contact surface with the air is very large, it evaporates quickly.
The advantage of ultrasound is high humidification efficiency, while the disadvantage is that impurities in water will be blown into the air, such as compounds, bacteria, minerals, etc. in tap water. Moreover, these water mist with impurities will adsorb bacteria in the air and become bacterial carriers. Don't underestimate these invisible impurities, which are equivalent to the powder like scale floating all over the house. Inhalation by people with sensitive respiratory system is likely to cause a series of respiratory diseases, such as infectious pneumonia. This kind of impurity white fog is generally called "white dust" in the medical field.
The evaporative humidifier uses a fan to blow against a water wheel like a wet towel/paper to accelerate the natural evaporation of water. In fact, its principle is similar to that of drying clothes in a room.

Obviously, compared with the ultrasonic humidifier, the efficiency of the evaporation type is relatively low. After all, one is passive evaporation and the other is active diffusion. In order to improve the efficiency, evaporative humidifiers must increase the size and wind speed, but this will increase the noise.
The biggest advantage is safety and power saving. After all, all impurities will adsorb and stay on the humidified paper, and only water will evaporate. However, the evaporative humidifier has a very obvious disadvantage - the humidification paper needs to be replaced regularly, because it is very easy to breed mold and absorb impurities in the air, as shown in the picture below.

Whether ultrasonic or evaporative humidifiers, water tanks and sinks should be cleaned frequently to prevent water deterioration and bacteria breeding. My practice is to take out the humidification filter element every few days, rinse it with tap water, and then dry it in the sun. However, no matter how clean it is, it is necessary to replace it every 6~12 months.
In fact, there is also a whole room humidifier, which is mainly based on the principle of heating+natural evaporation. It directly uses the tap water pipe to supply water, and can adjust the humidity continuously for 7x24 hours. This equipment is generally used in hospitals, laboratories and high-end residential buildings. It is necessary to lay pipes during decoration (which can be shared with the central air conditioner).