Dry air produces more dust by creating static that attracts the manufacturing of dust through dead skin cells that acts as an adhesive to dander, dirt, biological matter, mold spores and various other fine particulate matter. RH levels at 30-50% will drastically reduce or completely eliminate static. Eliminating a significant static presence in your indoor environment will reduce dust from being manufactured sooner, which means dust build up takes longer to occur.
Providing enough humidity in your indoor environment can help to relieve sinus pressure, ask your doctor.
Providing enough humidity in your indoor environment can combat against dry skin and nose bleeds, ask your doctor.
Providing enough humidity in your indoor environment is excellent for hardwood floors and nice wood furnishings.
Dry air requires higher thermostat settings to make you feel comfortable, where as air that has the proper RH levels will allow you to turn your thermostat down a degree or two to allow you to feel the same comfort at a lower temperature, which in turn can allow you to save money on your energy bill.
As an interesting side benefit, some house plants will respond positively to increased humidity levels versus very dry air.
Less expensive and time consuming than operating several room humidifiers (having to constantly fill a room humidifier up with water takes time, where as automatic water feed does not).

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