Why do we need air cleaner in winter?

The air cleaner is becoming one of the everyday life electronic appliances to operate all around the world.
It is needed in spring when yellow sand and pollen blow in wind,
in winter and at the turning of seasons not to catch a cold, and in summer to remove mold and bacteria.
Moreover, the simple ventilation doesn't remove the bacteria that cause nasitis, atopic dermatitis or bronchitis.
That's why the air purifier is needed in winter.
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http://www.skin-analyzer.com/