Are air humidifiers necessary, does low humidity have negative heath effects?

Question

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?

Answer 1

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
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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.

Answer 2

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.