Air Quality Glossary | Terms & Definitions

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ACH (Air Changes per Hour): How many times the complete air volume of a room is replaced in an hour. Higher ACH means faster air cleaning. For good air purification, target 4-6 ACH.
Activated Carbon: Porous carbon material that adsorbs gases and odor compounds. Used in filters to address smells and VOCs. Effectiveness depends on the amount of carbon.
Allergen: Substance that triggers an allergic reaction. Common airborne allergens include pollen, dust mite particles, pet dander, and mold spores.
AQI (Air Quality Index): A standardized scale (0-500) for reporting outdoor air quality. Higher numbers mean worse air. Often used to communicate wildfire smoke and pollution levels.

CADR (Clean Air Delivery Rate): Standardized measure of air purifier performance. Measures cubic feet of clean air delivered per minute. Higher CADR means more effective cleaning for larger spaces.
CO2 (Carbon Dioxide): Gas we exhale. Accumulates in poorly ventilated spaces. Not dangerous at typical indoor levels but indicates ventilation quality. Levels above 1000-1500 ppm suggest poor ventilation.

Dander: Microscopic skin flakes shed by pets (and humans). Pet dander is a common allergen. HEPA filters capture dander effectively.
Dust Mites: Microscopic creatures that live in bedding, furniture, and carpets, feeding on skin cells. Their waste products are a common allergen.

HEPA (High-Efficiency Particulate Air): Filter standard requiring capture of 99.97% of particles 0.3 microns in diameter. The gold standard for particle filtration. "True HEPA" meets this standard; "HEPA-like" doesn't.

Ionizer: Device that charges particles so they stick to surfaces. Can produce ozone as a byproduct. Generally less effective and more concerning than HEPA filtration.

MERV (Minimum Efficiency Reporting Value): Rating system for HVAC filters (1-20). Higher MERV means finer filtration. Home systems typically use MERV 8-13. Higher ratings can restrict airflow.
Micron (μm): Unit of measurement. One micron = one millionth of a meter. Human hair is about 70 microns. PM2.5 particles are 2.5 microns or smaller.

Off-gassing: The release of VOCs from new products over time. New furniture, paint, and building materials off-gas, with emissions highest when new and decreasing over weeks/months.
Ozone: A molecule made of three oxygen atoms. At ground level, ozone is a lung irritant. Some air "purifiers" produce ozone intentionally or as a byproduct. Avoid these.

PM2.5: Particulate matter 2.5 microns or smaller in diameter. These fine particles are a key air quality concern. Can penetrate deep into the respiratory system. HEPA filters capture PM2.5 effectively.
PM10: Particulate matter 10 microns or smaller. Includes larger particles like dust and pollen. Also captured by HEPA filters.
Pre-filter: A coarse filter that captures larger particles before they reach the main HEPA filter. Extends HEPA filter life. Often washable or cheaply replaceable.

Relative Humidity: The amount of water vapor in the air as a percentage of what the air could hold at that temperature. Optimal indoor range is typically 30-50%.

VOCs (Volatile Organic Compounds): Gases emitted from various products and materials. Sources include paints, cleaning products, furniture, and building materials. Activated carbon can adsorb some VOCs.
Ventilation: The exchange of indoor and outdoor air. Can be natural (windows) or mechanical (fans, HVAC). Dilutes indoor pollutants by bringing in fresh air.