The air quality (AQ) within a building is determined by the presence and concentration of contaminants that affect human health and comfort. Humidity, the amount of water vapor suspended in the air, is not an air pollutant itself but acts as a powerful moderator. Humidity levels dictate the viability, behavior, and concentration of nearly every substance found indoors, profoundly influencing the overall quality of the air you breathe.
The Role of Humidity in Biological Contaminants
Excessive moisture creates an environment where biological contaminants, a major source of poor indoor air quality, can thrive. When relative humidity rises above 60%, it provides the necessary moisture for mold and mildew spores to settle and colonize. As these fungi grow, they release additional spores and mycotoxins into the air, triggering allergic reactions and respiratory irritation.
Bacteria and other microorganisms also benefit from high humidity, supporting their proliferation on surfaces and in the air. Water vapor is a fundamental requirement for their lifecycle, allowing them to rapidly multiply in damp areas like bathrooms or basements. Managing this moisture is a primary strategy for controlling the spread of these airborne biological agents.
Moderate humidity levels, between 50% and 60%, are ideal for the survival of allergenic pests like dust mites. These microscopic creatures must absorb moisture from the air to stay alive and reproduce. Since dust mite waste products are potent allergens, maintaining relative humidity below 50% is a key method for minimizing their populations and resulting health risks.
How Humidity Influences Non-Biological Pollutants
Humidity also influences the physical and chemical behavior of non-living air contaminants like particles and gases. When relative humidity drops below 30% to 40%, the air becomes very dry, affecting the suspension of particulate matter (PM). Fine particles, such as PM 2.5, remain airborne longer in dry air, increasing the risk of inhalation exposure.
The lack of moisture also increases static electricity, which can cause particles to cling to surfaces and then become easily re-suspended into the air. Low humidity can also directly irritate the respiratory system, drying out the protective mucous membranes in the nose and throat. This dry tissue becomes less effective at trapping incoming particles and pathogens, making a person more susceptible to inhaled pollutants.
Conversely, high humidity, particularly above 60%, can increase the rate at which materials release chemical pollutants into the air, a process known as off-gassing. This moisture enhances the desorption of volatile organic compounds (VOCs) from products such as furniture, cleaning agents, and building materials. This results in a higher concentration of gaseous pollutants in the indoor air.
High moisture can also help clear the air of some particles. Water vapor causes fine particulate matter to undergo hygroscopic growth, where particles absorb moisture and increase in mass and size. This increase in weight causes the particles to settle out of the air more quickly.
Managing Indoor Humidity for Optimal Air Quality
A narrow target range is considered the “sweet spot” for minimizing air quality problems caused by biological and non-biological contaminants. Maintaining indoor humidity between 40% and 60% is recommended. This range inhibits mold growth while simultaneously preventing the drying effects of low humidity on respiratory passages.
Controlling moisture levels requires monitoring and appropriate mechanical tools. A hygrometer is a simple, inexpensive device that accurately measures relative humidity in different areas of the home. This monitoring provides the necessary data to adjust control measures.
When humidity levels are too high, dehumidifiers pull excess moisture from the air, which is useful in damp basements or during humid seasons. If the air is too dry, a humidifier adds water vapor back into the environment to mitigate low moisture effects. Proper ventilation, especially using exhaust fans in high-moisture areas like kitchens and bathrooms, also removes localized humidity.