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Air humidifiers are broadly sold and marking points to "healthy" relative humidity levels as (accordingly to different sources) is 30-50% and during artificial heating (normally, winter) seasons we need to use humidifiers at home.

But was it ever asserted, that in-house relative humidity level <30% can have any negative health effects?

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I can tell you from personal experience that low humidity is very very uncomfortable. It dries out your skin and nasal passages. –  Sam I Am Jan 8 '13 at 20:05
    
Low humidity levels can cause a lot of static electricity build up that can be very inconvenient. I have dry skin and I get static electric "shocks" from everything between my memory foam bed / comforter to the house keys. –  Nivas Jan 10 '13 at 4:26
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2 Answers

up vote 5 down vote accepted

From: "Indirect health effects of relative humidity in indoor environments." by Arundel et al, 1986:

mid-range humidities (40-60%) are more lethal to airborne nonpathogenic bacteria than low or high humidities
...
Measles, influenza, herpesvirus varicellae, and rubella viruses survive longer during exposure to relative humidities below 50%
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influenza infection rates are highest in environments with relative humidities below 40%
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The majority of adverse health effects caused by relative humidity would be minimized by maintaining indoor levels between 40 and 60%. This would require humidification during winter in areas with cold winter climates. Humidification should preferably use evaporative or steam humidifiers, as cool mist humidifiers can disseminate aerosols contaminated with allergens.

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The quote shows a correlation, but no causation? –  gerrit Jan 8 '13 at 14:53
    
@gerrit more quotes added from cited paper re causality –  EnergyNumbers Jan 8 '13 at 15:26
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It is at least asserted that air humidity affects health relevant environmental factors, but if you are interested in marketing for a humidifier or dehumidifier, you can freely pick arguments both in favour of dry and humid indoor environments:

Recent research shows that the absolute air humidity has a strong impact on the survival rate of air-born flue virus. Cold air (which is dry in absolute terms, although relatively humid) or dry air increase the flue virus' life span. Increasing the air humidity will reduce the risk of a flue infection.

Dry air on the other side, will however prevent other factors with negative health impact. Most fungi spores will only thrive under high humidity, so keeping a dry climate will avoid mold growth.

These are of course only two examples allowing opposite argumentation for a dry or humid climate. There are endless other examples of germs or e.g. pest carrying insects, which presence or amount can be controlled by manipulating air humidity.

Another issue is the vaporization of toxic solvents from building materials, which increases in dry air, but the "real" solution to that problem is probably to avoid such building materials completely instead of minimizing the risk by increasing air humidity.

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