Glossary: Indoor Humidity, Mold & Air Quality Terms Explained
Quick, plain-language definitions for the terms you’ll run into most often when researching indoor humidity, mold, and air quality. Each term links to a full guide where one exists.
Humidity & Physics
Relative Humidity
Relative humidity is the amount of moisture in the air expressed as a percentage of the maximum moisture the air could hold at that temperature. Keeping indoor relative humidity between 30% and 50% helps prevent mold growth, reduce dust mites, and maintain comfortable air for breathing.
Absolute Humidity
Absolute humidity is the actual mass of water vapor present in a given volume of air, typically expressed in grams per cubic meter. Unlike relative humidity, it does not change with temperature, making it useful for comparing moisture levels across different environments or tracking how much water air is actually carrying.
Dew Point
Dew point is the temperature at which air becomes fully saturated with moisture and water vapor begins to condense into liquid water. It is a reliable indicator of how much moisture is actually in the air, and when surfaces in your home fall below this temperature, condensation and mold growth become likely.
Psychrometric Chart
A psychrometric chart is a graph that maps the physical properties of air at different combinations of temperature and moisture content, including humidity, dew point, and enthalpy, all on a single diagram. HVAC engineers and building scientists use it to quickly determine how air will behave and what adjustments are needed to reach comfortable or safe indoor conditions.
Mold & Moisture Damage
Mold Spore
A mold spore is a tiny, seed-like particle released by mold that can travel through the air and grow into new mold colonies when it lands on a damp surface. Keeping indoor humidity below 50% is one of the most effective ways to prevent spores already in your home from taking hold.
Mycotoxin
A mycotoxin is a toxic chemical produced naturally by certain molds that can contaminate indoor air, surfaces, and building materials. Inhaling or touching mycotoxins can cause health problems ranging from irritation and allergic reactions to more serious respiratory or neurological effects, making mold control through humidity management a key part of healthy indoor environments.
Mildew
Mildew is a type of fungal growth that appears as a flat, powdery, or downy patch on surfaces, typically white or gray, and thrives in warm, damp conditions. It commonly forms on bathroom walls, window sills, and fabrics when indoor humidity stays above 60%, and can usually be wiped away more easily than mold.
Efflorescence
Efflorescence is the white, powdery or crusty deposit that forms on concrete, brick, or masonry surfaces when water moves through the material and carries dissolved salts to the surface, where they crystallize as the water evaporates. It signals that moisture is actively migrating through a wall or floor, which can indicate a water intrusion problem that may worsen indoor humidity levels over time.
Rising Damp
Rising damp is moisture from the ground that travels upward through walls or floors by capillary action, leaving visible stains, peeling paint, or a white powdery residue called efflorescence. It commonly affects older buildings without a functioning damp-proof course and can quietly damage structures and degrade indoor air quality over time.
Penetrating Damp
Penetrating damp is moisture that enters a building horizontally through walls, roofs, or floors due to structural faults such as cracked masonry, damaged pointing, or failing window seals. Unlike rising damp, it typically appears as spreading wet patches that worsen during or after rainfall and can lead to mold growth and deteriorating indoor air quality if left unaddressed.
Air Quality & Filtration
MERV Rating
MERV Rating, or Minimum Efficiency Reporting Value, is a standardized scale from 1 to 16 that measures how effectively an air filter captures airborne particles like dust, pollen, and mold spores. Higher numbers mean finer filtration, so choosing the right MERV rating helps balance air quality improvement with maintaining proper airflow through your HVAC system.
HEPA Filter
A HEPA filter is a type of air filter that captures at least 99.97% of airborne particles as small as 0.3 microns, including dust, pollen, mold spores, and pet dander. It is commonly used in air purifiers and vacuums to improve indoor air quality, especially for people with allergies or asthma.
VOC (Volatile Organic Compound)
A volatile organic compound (VOC) is a carbon-based chemical that easily evaporates into the air at room temperature, releasing gases from products like paints, cleaning supplies, adhesives, and furniture. These gases can build up indoors and irritate the eyes, nose, and throat, or cause more serious health effects at high concentrations.
PM2.5
PM2.5 refers to airborne particles smaller than 2.5 micrometers in diameter — fine enough to pass through the nose and throat and reach deep into the lungs. Because these particles can carry harmful pollutants and trigger respiratory or cardiovascular issues, monitoring PM2.5 levels indoors helps assess air quality and guides the use of air purifiers.
AQI (Air Quality Index)
The Air Quality Index (AQI) is a standardized numerical scale used to communicate how clean or polluted the outdoor or indoor air is at a given time. It runs from 0 to 500, with higher numbers indicating greater health risk from pollutants like particulate matter, ozone, and carbon monoxide. Checking the AQI helps people decide when to open windows or limit outdoor activity.
Activated Carbon Filter
An activated carbon filter is a type of air filter that uses porous, treated carbon to trap gases, odors, and chemical vapors as air passes through it. It is commonly used in air purifiers and HVAC systems to reduce volatile organic compounds, smoke, and household odors that standard particle filters cannot capture.
Off-Gassing
Off-gassing is the release of chemicals from solid or liquid materials into the surrounding air as they slowly break down or cure. New furniture, flooring, paints, and adhesives are common sources, and the process can quietly raise indoor chemical levels well above what outdoor air typically contains.
Formaldehyde
Formaldehyde is a colorless, strong-smelling gas released by many common household materials, including pressed wood furniture, adhesives, and certain paints. It is one of the most frequently detected indoor air pollutants and can irritate the eyes, nose, and throat even at low concentrations.
ACH (Air Changes per Hour)
ACH, or Air Changes per Hour, measures how many times the total volume of air in a room is completely replaced within one hour. A higher ACH means fresher air and faster removal of pollutants, humidity, and odors, making it a key factor when sizing ventilation systems or air purifiers.
CFM (Cubic Feet per Minute)
CFM, or cubic feet per minute, is a measurement of how much air moves through a space or device in one minute. It is commonly used to rate the airflow capacity of fans, air purifiers, dehumidifiers, and ventilation systems, helping you match equipment to the size of the area you need to treat.
Equipment
Desiccant Dehumidifier
A desiccant dehumidifier is a device that removes moisture from the air by passing it over a water-absorbing material, such as silica gel, rather than cooling the air to condense moisture. This makes it especially effective in cold environments or very low-humidity applications where refrigerant-based dehumidifiers struggle to perform.
Compressor Dehumidifier
A compressor dehumidifier pulls moist air over a cold coil to condense water out of it, collects that water in a tank, and releases drier air back into the room. It works best in warm spaces like basements and living areas, making it the most common type used in homes.
Whole-House Dehumidifier
A whole-house dehumidifier is a unit installed within a home’s HVAC system that removes excess moisture from the air throughout the entire living space, rather than just one room. Keeping indoor humidity between 30–50% helps prevent mold growth, reduces dust mites, and makes the home feel more comfortable year-round.
Bypass Humidifier
A bypass humidifier is a whole-home humidification device that draws warm air from a furnace’s supply duct, passes it through a water-soaked panel, and returns the moisture-laden air to the return duct without using its own blower. Because it relies on the furnace fan, it only adds humidity when the heating system is running.
ERV (Energy Recovery Ventilator)
An Energy Recovery Ventilator (ERV) is a mechanical ventilation device that brings fresh outdoor air into a building while transferring heat and moisture from the outgoing stale air to pre-condition the incoming air. This exchange reduces the energy cost of heating or cooling a home while also helping maintain stable indoor humidity levels year-round.
HRV (Heat Recovery Ventilator)
A heat recovery ventilator (HRV) is a ventilation device that brings fresh outdoor air into a home while transferring heat from the outgoing stale air to warm the incoming air, reducing energy loss. This makes it possible to ventilate a tightly sealed home year-round without significantly raising heating costs.
Hygrometer
A hygrometer is a device that measures the amount of moisture in the air, expressed as relative humidity. Keeping one in your home helps you know when to run a humidifier or dehumidifier to stay within the comfortable and healthy range of 30–50% relative humidity.
Ozone Generator
An ozone generator is a device that intentionally produces ozone gas, often marketed as an air purifier that eliminates odors, mold, and bacteria. However, ozone at concentrations high enough to be effective against pollutants can also irritate the lungs and is generally not recommended for use in occupied spaces.
Building & Construction
Vapor Barrier
A vapor barrier is a material installed in walls, floors, or crawl spaces to block moisture from passing through and reaching areas where it could cause damage or mold growth. Choosing the right barrier depends on local climate, since blocking moisture movement too aggressively in some regions can trap humidity inside the structure instead.
R-Value
R-value is a number that measures how well a material resists the flow of heat — the higher the number, the better the insulation. In homes, higher R-values in walls, attics, and floors help maintain stable indoor temperatures and reduce the moisture-driven condensation that can lead to mold and poor air quality.
Thermal Bridging
Thermal bridging occurs when a highly conductive material creates a path for heat to bypass a building’s insulation, moving more easily between inside and outside. These cold spots on walls, floors, or ceilings can trigger condensation and mold growth, making them a common hidden cause of indoor moisture problems.
Crawl Space Encapsulation
Crawl space encapsulation is the process of sealing a home’s crawl space with a heavy-duty vapor barrier covering the floor, walls, and sometimes ceiling to block moisture from entering the living areas above. Left unsealed, crawl spaces allow ground moisture to rise into the home, raising humidity levels and encouraging mold growth.