Humidity's Effect On IAQ

Low Humidity (Dry Air)

When the air is too dry, it can pull moisture from your body, which can lead to a number of health issues. Think of it like a dry sponge absorbing water from a damp surface.

• Skin and Respiratory Issues: Low humidity can cause your skin to become dry, itchy, and flaky. Your lips may also become chapped. More importantly, it can dry out the mucous membranes in your nose and throat. These membranes are your body's first line of defense against viruses, bacteria, and allergens. When they are dry, they are less effective at trapping these invaders, making you more susceptible to colds, the flu, and sinus infections.

• Increased Infection Transmission: Medical research has shown a direct link between low humidity and the increased transmission of airborne viruses. When the relative humidity drops below 40%, the tiny liquid droplets we expel when we cough or sneeze can shrink and become lighter. This allows viruses within these droplets to stay suspended in the air for longer periods and travel greater distances, increasing the risk of transmission to others. Additionally, a dry environment weakens the body's immune response in the respiratory system, making it easier for these pathogens to infect a person. 

• Asthma and Allergies: Dry air can also irritate the airways, which can worsen symptoms for people with asthma or allergies. The air itself can trigger a flare-up, and the lack of moisture can make the lungs more sensitive.

High Humidity (Damp Air)

High humidity can be just as problematic as low humidity. When the air is saturated with moisture, it makes it difficult for your body to cool itself through sweating.

• Heat-Related Illnesses: Your body cools down by sweating. The sweat evaporates from your skin, and this process removes heat from your body. In high humidity, the air is already full of water vapor, so sweat can't evaporate as easily. This can lead to overheating, heat exhaustion, or even heatstroke in extreme cases.

• Mold, Mildew, and Dust Mites: High humidity provides a perfect environment for the growth of mold, mildew, and dust mites. These organisms are common allergens. Inhaling mold spores or dust mite waste can trigger allergic reactions and asthma attacks, even in people who have never had these issues before.

• Bacteria and Viruses: Some studies suggest that certain bacteria and viruses thrive in very humid environments, increasing the potential for their growth and spread.

In summary, maintaining a relative humidity between 40% and 60% is a key factor in creating a healthy indoor environment. This helps your body maintain its natural defenses, and it limits the growth of harmful biological organisms.

Physiological Consequences of Low Humidity

The human body relies on moisture for the proper function of several systems. Low humidity extracts moisture from the body, leading to a cascade of negative effects:

1. Respiratory System Issues

The most immediate impact is on the respiratory tract:

• Dry Mucous Membranes: The nasal passages, throat, and lungs are lined with mucous membranes that require moisture to function effectively. Low humidity causes these membranes to dry out and crack.

• Impaired Cilia Function: The cilia are tiny, hair-like structures that line the airways and constantly move mucus to trap and expel inhaled dust, pollutants, and pathogens. When the mucus dries out, the cilia's function (known as the mucociliary clearance mechanism) is impaired, making the occupant more susceptible to respiratory infections, colds, and flu.

• Increased Irritation: Dry air can irritate the throat, leading to a persistent cough, and exacerbate existing conditions like asthma or bronchitis.

2. Eye and Skin Irritation

Low humidity directly affects the moisture content of the body's exterior layers.

• Dry Eyes: Evaporation of the tear film increases rapidly in dry air, leading to dry, itchy, and irritated eyes. This can be particularly problematic for contact lens wearers or individuals with pre-existing dry eye conditions.

• Skin Dryness and Eczema: Dry air pulls moisture from the skin, resulting in chapping, flaking, and general dryness. It can worsen inflammatory skin conditions like eczema (atopic dermatitis) and psoriasis, leading to discomfort and potential secondary infections from scratching.

3. General Discomfort and Dehydration

Low humidity increases the rate of perspiration evaporation, which can lead to a deceptive sensation of being cooler than the actual air temperature, potentially masking mild dehydration.

• Thirst and Dry Mouth: Occupants often experience increased thirst and a dry mouth, requiring increased water intake to compensate for moisture loss.

Impact on Pathogen Survival and Transmission

Low relative humidity has been scientifically linked to increased survival and transmission rates of various airborne viruses, including influenza.

• Prolonged Viability of Viruses: Studies show that when RH levels drop below 40%, the influenza virus and certain other pathogens (e.g., coronaviruses) remain viable and infectious for longer periods in the air. The stability of the virus's outer envelope is optimized at both very low and very high RH, but the mid-range (40-60%) is generally disruptive.

• Aerosol Size Dynamics: Low RH causes respiratory droplets that carry viruses to rapidly evaporate, resulting in smaller, lighter aerosolized particles. These smaller particles can remain suspended in the air for longer periods and penetrate deeper into the lungs, increasing the risk of infection.

In summary, maintaining indoor RH between 40% and 60% is a critical building science strategy for both occupant comfort and health, serving as a buffer against both pathogen proliferation (associated with low RH) and mold growth (associated with high RH).

Effects on Building Materials and Durability

High RH drives moisture into the building envelope via vapor diffusion and can lead to saturation of materials, accelerating degradation.

1. Microbial Growth and Decay

The most significant consequence is the enablement of microorganism growth (mold, mildew, and bacteria):

• Critical Threshold: When RH at a material's surface exceeds 70% to 80% for a sustained period, even if the air RH is lower, mold spores can germinate. Maintaining air RH above 60% increases the likelihood that surfaces, especially those slightly cooler than the ambient air (e.g., in corners or behind furniture), will achieve this critical surface moisture content.

• Wood Decay Fungi (Rot): Sustained moisture levels above the Fiber Saturation Point (FSP), typically a 28% moisture content in wood, enable wood-decaying fungi (rot) to colonize and break down cellulose and lignin, compromising the structural integrity of wood framing, sheathing, and finishes.

• Insulation Degradation: High humidity saturates fibrous insulation (e.g., fiberglass, mineral wool), drastically reducing its effective R-value and thermal performance.

2. Dimensional Instability and Degradation

Hygroscopic materials (materials that readily absorb and release moisture, like wood, drywall, and plaster) swell as they absorb moisture from the air.

• Warping and Cupping: Wood floors, trim, and structural lumber will expand. If this expansion is restricted, it leads to warping, cupping, or buckling.

• Adhesive and Finish Failure: Excess moisture compromises the bond strength of adhesives, leading to peeling wallpaper, cracking or blistering paint, and delamination of engineered wood products (e.g., plywood, MDF).

• Corrosion: High RH, especially when combined with airborne contaminants, significantly accelerates the corrosion and rust of ferrous metals, affecting HVAC components, plumbing connections, and structural fasteners.

Consequences for Occupant Health (IAQ)

From a health perspective, the risks associated with high humidity are directly tied to the biological and chemical changes it induces in the indoor air quality (IAQ).

1. Mold Exposure and Respiratory Illness

The primary health concern is the exposure to mold and its byproducts:

• Allergens and Asthma: Airborne mold spores and fragments are potent allergens. Inhaling them can trigger allergic rhinitis, coughing, sneezing, and severe asthma attacks in sensitized individuals.

• Mycotoxins: Certain molds produce toxic compounds called mycotoxins. Exposure is linked to various non-allergic and non-infectious health issues, though the exact extent of health damage is subject to ongoing research.

• Microbial Volatile Organic Compounds (MVOCs): These are metabolic byproducts of mold and bacteria that off-gas into the air. They are responsible for the characteristic musty odor associated with moisture problems and can be irritating to the eyes, nose, and throat.

2. Dust Mite and Pest Proliferation

Dust mites thrive in high-humidity environments.

• Dust Mites: These microscopic arthropods are significant triggers for asthma and allergies. Their population explodes when the relative humidity is consistently above 70%, as they absorb moisture directly from the air. Controlling RH is the most effective method of dust mite population control.

• Insects: High moisture provides ideal conditions for many common indoor pests, including silverfish, cockroaches, and some termite species.

3. Thermal Discomfort

High RH inhibits the body’s ability to cool itself through the evaporation of sweat.

• Reduced Evaporative Cooling: The moist air is already close to saturation, slowing the rate at which perspiration evaporates from the skin. This makes the occupant feel much hotter and clammy than the dry-bulb temperature indicates, increasing the risk of heat stress and fatigue.

Maintaining indoor RH within the optimal range of 40% to 60% is the single most critical factor in achieving a durable, healthy, and comfortable building environment.