Indoor Air Quality Glossary

A to C

ACH (Air Changes per Hour): The number of times the total air volume in a room is replaced in one hour. Most homes have 0.35 to 1.0 ACH naturally. Higher means more fresh air exchange.

Building Envelope: The physical barriers between inside and outside: walls, roof, windows, doors, and foundation. A tight envelope saves energy but can reduce natural ventilation.

CO (Carbon Monoxide): A colorless, odorless gas from incomplete combustion. Sources include gas stoves, furnaces, water heaters, and attached garages. Lethal at high levels; causes headaches, fatigue, and confusion at low levels below alarm thresholds.

Cross-Contamination: In multi-unit buildings, pollutant transfer between units through shared HVAC, wall cavities, plumbing chases, or structural gaps.

D to H

Dust Mites: Microscopic organisms thriving in warm, humid environments. Feed on shed skin cells. Concentrated in bedding, upholstered furniture, and carpet. A major source of indoor allergens.

Formaldehyde: A common indoor VOC from composite wood products (particleboard, MDF), some insulation, and adhesives. Causes eye, nose, and throat irritation. Classified as a known human carcinogen.

HEPA Filter: High Efficiency Particulate Air filter. Captures 99.97% of particles 0.3 micrometers or larger. Used in air purifiers and some vacuum cleaners.

M to O

MERV Rating: Minimum Efficiency Reporting Value. A 1 to 20 scale measuring HVAC filter effectiveness. Most homes benefit from MERV 11 to 13. Higher is not always better if the system cannot handle the airflow restriction.

N to P

NO2 (Nitrogen Dioxide): A combustion gas from gas stoves and furnaces. Causes respiratory irritation and worsens asthma. Indoor levels can exceed outdoor levels in homes with gas cooking.

Off-Gassing: The release of VOCs from materials as they age. New furniture, flooring, paint, and building materials off-gas most intensely in the first weeks to months.

PM2.5: Particulate matter smaller than 2.5 micrometers. Penetrates deep into the lungs. Sources include combustion, cooking, and outdoor pollution. Associated with respiratory and cardiovascular effects.

R to S

Radon: A naturally occurring radioactive gas entering homes through foundation cracks. Second leading cause of lung cancer. Odorless and invisible. Requires specific testing to detect.

Relative Humidity: Moisture in the air as a percentage of what it can hold at that temperature. Ideal indoor range: 30 to 50%. Below 30% causes dryness. Above 50% promotes mold and dust mites.

Stack Effect: Natural upward air movement in buildings caused by temperature differences. Pulls air from lower levels (basements, crawlspaces) up through the home, carrying pollutants from lower floors to upper floors.

V

VOC (Volatile Organic Compound): Chemicals that evaporate at room temperature from paint, furniture, flooring, and cleaning products. Common VOCs include formaldehyde, benzene, and toluene. A broad category of indoor chemical pollutants.