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Air Quality Glossary

Essential air quality terms explained: PM2.5, PM10, AQI, HEPA, CADR, and more. Your complete reference guide to understanding air pollution terminology.

Understanding air quality starts with understanding the language. This glossary explains the key terms, acronyms, and concepts you'll encounter when researching air pollution and protection strategies.

A

AQI (Air Quality Index)

A standardized scale (0-500) that communicates how polluted the air is and what health effects might be a concern. Higher values indicate greater pollution and health risk. Developed by the EPA to make air quality data accessible to the public.

Categories:

  • 0-50: Good
  • 51-100: Moderate
  • 101-150: Unhealthy for Sensitive Groups
  • 151-200: Unhealthy
  • 201-300: Very Unhealthy
  • 301-500: Hazardous

See also: AQI Levels Explained

ACH (Air Changes per Hour)

The number of times an air purifier cycles all the air in a room through its filter per hour. Higher ACH means cleaner air. Recommended minimum: 4-5 ACH for general use, 5-6 for allergies/asthma.

Example: If a room has 1,000 cubic feet of air and a purifier moves 250 CFM, the ACH is 15.

Activated Carbon

A highly porous form of carbon used in filters to absorb gases, odors, and volatile organic compounds (VOCs). Does not remove particulate matter—needs to be paired with HEPA for complete air purification.

Effectiveness: Depends on amount (measured in pounds) and contact time with air.

Aerosol

Tiny solid particles or liquid droplets suspended in air. Includes smoke, mist, dust, and pollution particles. Can remain airborne for hours to days depending on size.

Asthma

A chronic respiratory condition where airways become inflamed and narrow, making breathing difficult. Air pollution is a major trigger for asthma attacks. PM2.5, ozone, and NO2 particularly problematic.

B

Berkeley Earth

A scientific research organization that established the widely-cited finding that breathing air with PM2.5 at 22 µg/m³ for 24 hours is roughly equivalent to smoking one cigarette. This research forms the basis for cigarette equivalency calculations.

Website: berkeleyearth.org

Bronchitis

Inflammation of the bronchial tubes (airways to lungs). Can be acute (short-term) or chronic (long-term). Air pollution exposure is a risk factor for chronic bronchitis.

C

CADR (Clean Air Delivery Rate)

A measure of an air purifier's effectiveness, indicating the volume of clean air (in cubic feet per minute) the purifier delivers. Higher CADR = better performance.

Three ratings:

  • Smoke CADR: For smallest particles (most relevant for PM2.5)
  • Dust CADR: For medium particles
  • Pollen CADR: For largest particles

How to use: Match CADR to room size. Divide room volume (cubic feet) by 2 to get minimum recommended CADR.

See also: Air Purifier Guide

Carbon Monoxide (CO)

A colorless, odorless toxic gas produced by burning fossil fuels. Sources include vehicles, gas appliances, wood burning. Interferes with blood's ability to carry oxygen. One of the criteria pollutants measured for AQI.

Health effects: Headaches, dizziness, confusion, death at high concentrations.

CFM (Cubic Feet per Minute)

A measure of airflow rate. Used to express air purifier CADR and ventilation capacity.

COPD (Chronic Obstructive Pulmonary Disease)

A group of progressive lung diseases including emphysema and chronic bronchitis. Long-term air pollution exposure is a risk factor. PM2.5 exposure worsens symptoms and accelerates disease progression.

Criteria Pollutants

Six pollutants regulated by EPA under the Clean Air Act:

  1. Particulate Matter (PM2.5 and PM10)
  2. Ground-level Ozone (O₃)
  3. Carbon Monoxide (CO)
  4. Sulfur Dioxide (SO₂)
  5. Nitrogen Dioxide (NO₂)
  6. Lead

These form the basis of AQI calculations.

D

Diesel Particulate Matter

Fine particles emitted from diesel engines. Particularly harmful form of PM2.5 containing hundreds of chemical compounds, many carcinogenic. Major contributor to urban air pollution.

Health concern: Classified as likely human carcinogen; linked to respiratory disease, cardiovascular effects.

E

Electrostatic Precipitator

An air cleaning device that uses electrical charge to attract and trap particles. Less effective than HEPA filters; may produce ozone as byproduct. Not recommended for residential use.

EPA (Environmental Protection Agency)

U.S. federal agency responsible for environmental protection, including air quality regulation, AQI development, and emission standards. Provides AirNow.gov air quality information.

F

FFP2

European standard for filtering facepiece respirators. Filters at least 94% of particles. Roughly equivalent to N95 masks used in U.S. Suitable for air pollution protection.

See also: N95 Masks Guide

Formaldehyde

A volatile organic compound (VOC) emitted from building materials, furniture, and some air purifiers (photocatalytic types). Respiratory irritant and probable carcinogen. Cannot be filtered by standard carbon filters—requires specialized media.

G

Ground-level Ozone (O₃)

Ozone in the lower atmosphere formed when sunlight reacts with pollutants from vehicles and industry. Not the same as stratospheric ozone layer (which protects us). Ground-level ozone is harmful.

When highest: Hot, sunny summer days (midday peak)

Health effects: Respiratory irritation, reduced lung function, triggers asthma attacks.

Note: Ozone is a gas—not removed by HEPA filters or N95 masks.

H

HEPA (High-Efficiency Particulate Air)

A filter standard that removes at least 99.97% of particles 0.3 micrometers in diameter. The gold standard for air purification.

True HEPA: Meets official standard HEPA-type/like: Marketing terms with no standard—avoid these

Effectiveness: Captures PM2.5, PM10, pollen, mold spores, bacteria, viruses.

See also: Air Purifier Guide

I

Inversion (Temperature Inversion)

Atmospheric condition where warm air sits atop cooler air, trapping pollution near ground level. Prevents normal vertical air mixing. Common in winter and valleys. Can cause severe pollution episodes.

Effect: AQI can spike even without increased emissions because pollution can't disperse.

Ionizer / Ionic Air Purifier

Device that electrically charges particles, causing them to stick to surfaces or collection plates. Less effective than HEPA filtration. May produce ozone—avoid unless certified ozone-free.

K

KF94

Korean standard for filtering facepiece respirators. Filters at least 94% of particles. Often fits smaller faces better than N95. High-quality control. Suitable for air pollution protection.

KN95

Chinese standard for filtering facepiece respirators. Similar to N95 (95% filtration) but different testing protocols. Quality varies significantly by manufacturer—verify authenticity.

See also: N95 Masks Guide

M

MERV (Minimum Efficiency Reporting Value)

Rating system for HVAC filters on scale of 1-16. Higher = better filtration.

Residential use:

  • MERV 1-4: Basic (large particles only)
  • MERV 8-10: Standard residential
  • MERV 13-16: High-efficiency (captures PM2.5) ← Recommended

MERV 13+ performs similarly to HEPA for PM2.5 capture.

See also: Air Purifier Guide

Micrometer (µm or micron)

One millionth of a meter. Unit used to measure particle size. Human hair is ~70 micrometers; PM2.5 is 2.5 micrometers.

Scale:

  • PM10: <10 µm
  • PM2.5: <2.5 µm
  • PM0.1: <0.1 µm (ultrafine)

N

N95 Mask

NIOSH-certified respirator that filters at least 95% of airborne particles 0.3 µm or larger. "N" means not oil-resistant (fine for air pollution). Must fit properly to be effective.

Effective against: PM2.5, PM10, wildfire smoke, dust, pollen, viruses

Not effective against: Gases (CO, ozone, VOCs)

See also: Complete N95 Guide

NIOSH (National Institute for Occupational Safety and Health)

Federal agency that tests and certifies respirators including N95 masks. Approval number (TC-84A-XXXX) indicates authentic N95.

Nitrogen Dioxide (NO₂)

A reddish-brown gas produced by burning fuel, especially in vehicles and power plants. Respiratory irritant; contributes to ozone and PM2.5 formation. One of the criteria pollutants.

Health effects: Airway inflammation, reduced lung function, increased infection susceptibility.

O

Off-gassing

Release of volatile organic compounds (VOCs) from products like new furniture, carpets, paint, and building materials. Can significantly degrade indoor air quality. Most intense when product is new; decreases over time.

Mitigation: Ventilate well when bringing new items indoors; choose low-VOC products.

Oxidative Stress

Cellular damage caused by reactive oxygen species (free radicals). Air pollution, particularly PM2.5 and ultrafine particles, induces oxidative stress—a mechanism by which pollution harms health.

Health impacts: Inflammation, accelerated aging, disease progression.

Ozone Generator

Device that intentionally produces ozone for supposed air purification. NEVER use these—ozone is harmful to lungs. EPA and medical authorities warn against residential ozone generators.

P

P95/P100 Respirators

P95: Filters 95% of particles; oil-resistant (unnecessary for air pollution) P100: Filters 99.97% of particles; oil-resistant

More protective than N95 but also more expensive and restrictive breathing. Overkill for air quality protection—N95 sufficient.

Particulate Matter (PM)

Solid or liquid particles suspended in air. Major component of air pollution.

PM10: Particles <10 µm (coarse) - includes dust, pollen PM2.5: Particles <2.5 µm (fine) - from combustion, chemical reactions PM0.1: Particles <0.1 µm (ultrafine) - penetrates deepest into body

PM2.5 is most health-concerning due to ability to penetrate deep into lungs and bloodstream.

Photochemical Smog

Air pollution formed when sunlight reacts with pollutants (especially from vehicles). Characterized by ground-level ozone, hazy brown appearance. Common in sunny, warm cities with heavy traffic.

Example: Los Angeles smog

R

Respirable Particles

Particles small enough to reach deep into lungs when inhaled. Generally refers to PM2.5 and smaller. Most health-concerning fraction of particle pollution.

S

Sensitive Groups

People more vulnerable to air pollution health effects:

  • Children (developing lungs)
  • Elderly (65+)
  • People with asthma or lung disease
  • People with heart disease
  • Pregnant women
  • Outdoor workers (higher exposure)

These groups should take protective action at lower AQI thresholds.

Sulfur Dioxide (SO₂)

A gas produced by burning coal and oil, especially in power plants. Respiratory irritant; contributes to PM2.5 formation. One of the criteria pollutants.

Health effects: Breathing difficulty, especially in people with asthma.

U

Ultrafine Particles (UFP)

Particles smaller than 0.1 micrometers (PM0.1). Most numerous but least massive component of air pollution. Can penetrate deeply into body, cross blood-brain barrier.

Sources: Combustion processes, especially vehicles

Health concern: Emerging research suggests UFPs may be particularly harmful despite small mass.

V

VOCs (Volatile Organic Compounds)

Gases emitted from certain solids or liquids. Include formaldehyde, benzene, toluene, and hundreds of others. Sources: paints, cleaning products, furniture, building materials, fuels.

Health effects: Eye/respiratory irritation, headaches, some are carcinogenic

Filtration: Require activated carbon (HEPA alone doesn't remove gases).

W

WHO (World Health Organization)

International public health agency that sets air quality guidelines stricter than many national standards:

  • PM2.5: 5 µg/m³ annual mean (U.S. standard: 12 µg/m³)
  • PM10: 15 µg/m³ annual mean (U.S. standard: 150 µg/m³)

Finding: 99% of global population breathes air exceeding WHO guidelines.

Wildfire Smoke

Mixture of gases and fine particles from burning vegetation and structures. Contains particularly high concentrations of PM2.5. Can travel hundreds or thousands of miles, affecting air quality far from fires.

Composition: PM2.5, carbon monoxide, VOCs, formaldehyde, polycyclic aromatic hydrocarbons (PAHs)

Health impact: Wildfire PM2.5 may be 2-10x more toxic than urban pollution PM2.5.

See also: Wildfire Smoke Guide

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This glossary covers essential air quality terminology. For deeper scientific detail, consult EPA, WHO, and peer-reviewed research sources.

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