Definition of Humidity

Humidity indicates how moist the air is. Relative humidity is the term used to define the amount of water vapor in the air relative to what the air can hold at a specific temperature. Humidity is the naturally occurring gas state of water in the air. A high/low relative humidity (%) can cause occupant discomfort.

Think about it this way: if you have a cup that is half-full of water, the cup contains 50% of what it can hold. Air works the same way.

Health effects and guidelines

A review of the health effects of relative humidity in indoor environments suggests that relative humidity can affect the incidence of respiratory infections and allergies.

Experimental studies on airborne-transmitted infectious bacteria and viruses have shown that the survival or infectivity of these organisms is minimized by exposure to relative humidities between 40 and 60%. Nine epidemiological studies examined the relationship between the number of respiratory infections or absenteeism and the relative humidity of the commercial or residential space. The incidence of absenteeism or respiratory infections was found to be lower among people working or living in environments with mid-range versus low or high relative humidities.

The indoor size of allergenic mite and fungal populations is directly dependent upon the relative humidity. Mite populations are minimized when the relative humidity is below 50% and reach a maximum size at 80% relative humidity. Most species of fungi cannot grow unless the relative humidity exceeds 60%. Relative humidity also affects the rate of offgassing of formaldehyde from indoor building materials, the formation rate of acids and salts from sulfur and nitrogen dioxide, and the formation rate of ozone.

The influence of relative humidity on the abundance of allergens, pathogens, and noxious chemicals suggests that indoor relative humidity levels should be considered as a factor of indoor air quality. The majority of adverse health effects caused by relative humidity would be minimized by maintaining indoor levels between 40 and 60% relative humidity. 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.


It is recommended that humidity levels be kept between 30-60% to maintain an environment that is comfortable and also deters the growth of hazardous organisms, fungi and bacteria.

If you have a concern about the humidity in your home Contact Us


What’s in the Air

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