Relative humidity of indoor environments has both direct and indirect effects on health and comfort. Direct effects refers to effects on physiological processes, and indirect effects in turn refer to effects that result from the impact of humidity on pathogenic organisms or chemicals. This review article (1986) discusses the indirect ones in greater detail.

Epidemiological studies suggest, that relative humidity can indirectly affect the incidence of allergies and infectious respiratory diseases, and humidification equipment has an effect on them. This is related to the survival and probability of effective contact of infectious or allergenic organisms, such as fungi, mites, bacteria, and viruses. Relative humidity also affects the concentration of chemicals in the air, as it affects their offgassing from i.e. building materials. A review of data has shown that relative humidity for minimizing the adverse indirect health effects lies between 40–60 %.

Indoor relative humidities <40 % are common especially during winter. An increase in the relative humidity above that level should reduce the incidence of respiratory infections, the severity of allergic and asthmatic reactions, and indoor ozone levels. Too high relative humidity in turn can increase the abundance of allergenic mites and fungi, and evaporation rates of harmful chemicals such as formaldehyde, sulfur and nitrogen dioxides in the air.

The use of humidifying systems work against low relative humidity and the adverse health effects caused by it. It has even been suggested that a decrease in morbidity and mortality due to influenza could be decreased; but conventional humidification equipment themselves can have microbial contamination.

“The indirect health effects of relative humidity may be growing in importance as a result of the continuing construction of energy efficient sealed buildings with low fresh air ventilation rates. Energy-conserving buildings require the careful maintenance of good indoor air quality through maintaining, among other factors, optimum relative humidity levels in order to minimize potential health problems.”