Indoor Air Quality Standards: EPA/WHO Guidelines Explained

Last updated: March 2026

Health and Safety Notice: This article provides educational information about indoor air quality standards and guidelines. It does not constitute medical or occupational health advice. For building-specific assessments, consult a certified industrial hygienist or indoor air quality professional.

Indoor air can be two to five times more polluted than outdoor air, according to the EPA — yet most people spend roughly 90% of their time indoors. Understanding the standards and guidelines that define “safe” indoor air is essential for homeowners, employers, building managers, and anyone concerned about long-term health. This glossary-style reference explains the major standards from the EPA, WHO, OSHA, and ASHRAE, with the pollutant-by-pollutant limits you need to know.

Who Sets Indoor Air Quality Standards?

No single agency governs all aspects of indoor air quality. Instead, multiple organizations issue guidelines and enforceable standards that overlap:

Organization	Scope	Enforceable?
EPA	National Ambient Air Quality Standards (NAAQS) for outdoor air; indoor air guidelines and recommendations	NAAQS enforceable; indoor guidance voluntary
WHO	Global Air Quality Guidelines for key pollutants	Advisory (no enforcement mechanism)
OSHA	Permissible Exposure Limits (PELs) for workplace air	Enforceable in workplaces
ASHRAE	Ventilation and IAQ standards for buildings (Standard 62.1 and 62.2)	Adopted into building codes by many jurisdictions
CDC/NIOSH	Recommended Exposure Limits (RELs) for workplaces	Advisory (often stricter than OSHA PELs)

For detailed OSHA workplace air requirements, see our OSHA air quality standards guide and the workplace air quality rights guide.

Particulate Matter (PM2.5 and PM10)

Fine particulate matter is the pollutant with the strongest evidence linking air quality to mortality. PM2.5 (particles smaller than 2.5 microns) penetrates deep into the lungs and enters the bloodstream, contributing to cardiovascular disease, stroke, lung cancer, and COPD.

Current Standards and Guidelines

Standard	PM2.5 Annual Mean	PM2.5 24-Hour	PM10 Annual Mean	PM10 24-Hour
EPA NAAQS (2024 revision)	9.0 ug/m3	35 ug/m3	N/A (revoked)	150 ug/m3
WHO Global Guidelines (2021)	5.0 ug/m3	15 ug/m3	15 ug/m3	45 ug/m3
WHO Interim Target 1	35 ug/m3	75 ug/m3	70 ug/m3	150 ug/m3
WHO Interim Target 4	10 ug/m3	25 ug/m3	20 ug/m3	50 ug/m3

Key update (2024): The EPA strengthened the annual PM2.5 standard from 12.0 to 9.0 ug/m3, the most significant tightening in a decade. States have until approximately 2032 to demonstrate attainment. The WHO guideline of 5.0 ug/m3 remains stricter and represents the level at which health effects are minimized based on current evidence.

Indoor implication: While NAAQS technically applies to outdoor ambient air, indoor PM2.5 levels are directly influenced by outdoor infiltration and indoor sources (cooking, candles, smoking, cleaning). Maintaining indoor PM2.5 below 12 ug/m3 (the previous EPA standard) is a reasonable residential target; below 5 ug/m3 aligns with WHO guidance. For more on PM2.5 health effects, see our PM2.5 health effects analysis.

Carbon Dioxide (CO2)

CO2 is not toxic at typical indoor concentrations, but elevated levels indicate inadequate ventilation — which correlates with higher concentrations of other pollutants and reduced cognitive performance.

CO2 Benchmarks

Level (ppm)	Significance
~420	Current outdoor ambient (2026 baseline)
600-800	Well-ventilated indoor space
1,000	ASHRAE 62.1 threshold for office/classroom ventilation adequacy
1,000-1,500	Marginally ventilated; cognitive performance begins to decline
1,500-2,000	Poor ventilation; drowsiness and complaints common
2,000-5,000	Headaches, significant performance impairment
5,000	OSHA PEL (8-hour TWA); NIOSH REL also 5,000 ppm
40,000+	Immediately dangerous to life and health (IDLH)

ASHRAE Standard 62.1 (2025 Edition)

ASHRAE 62.1-2025 specifies minimum ventilation rates and added refined humidity control requirements. The standard does not set a hard CO2 limit but uses differential CO2 concentration (indoor minus outdoor) as an indicator for demand-controlled ventilation (DCV) systems:

• Offices and classrooms: Steady-state CO2 around 1,000 ppm at design occupancy
• Restaurants and lecture halls: 1,500-2,000 ppm
• Conference rooms and auditoriums: Up to 2,500 ppm (due to high per-person ventilation demand relative to floor area)

CO2 sensors must be placed 3-6 feet above the floor, with at least one per ventilation zone and one per 5,000 sq ft of occupiable space.

For real-time CO2 monitoring in office environments, see our CO2 monitoring guide for offices.

Volatile Organic Compounds (VOCs)

VOCs are a broad class of carbon-based chemicals that evaporate at room temperature. Common sources include paint, adhesives, cleaning products, new furniture, and building materials. Some VOCs (benzene, formaldehyde) are known carcinogens.

Key Limits

Compound	WHO Guideline	OSHA PEL (8-hr TWA)	Common Indoor Sources
Formaldehyde	0.1 mg/m3 (30-min)	0.75 ppm	Pressed wood, insulation, adhesives
Benzene	No safe level (carcinogen)	1 ppm	Gasoline fumes, solvents, tobacco smoke
Toluene	0.26 mg/m3 (weekly)	200 ppm	Paints, adhesives, nail polish
Xylene	N/A	100 ppm	Paints, lacquers, pesticides
Total VOCs (TVOC)	No WHO/EPA standard	No OSHA PEL	Composite of all VOC sources

TVOC note: Neither EPA nor WHO publishes an official TVOC guideline. However, many IAQ professionals use the following informal benchmarks:

• Below 300 ug/m3: Good
• 300-500 ug/m3: Acceptable
• 500-1,000 ug/m3: Marginal; investigate sources
• Above 1,000 ug/m3: Unacceptable; remediate immediately

For a comparison of indoor vs. outdoor VOC levels and their health implications, see our VOC indoor-outdoor comparison.

Carbon Monoxide (CO)

CO is an odorless, colorless gas produced by incomplete combustion. It binds to hemoglobin 200 times more effectively than oxygen, making even moderate exposure dangerous.

Standards

Standard	Limit	Averaging Period
WHO	4 mg/m3 (3.5 ppm)	24-hour
WHO	10 mg/m3 (8.7 ppm)	8-hour
WHO	35 mg/m3 (30 ppm)	1-hour
EPA NAAQS	9 ppm	8-hour
EPA NAAQS	35 ppm	1-hour
OSHA PEL	50 ppm	8-hour TWA

Indoor implication: Any sustained CO level above 9 ppm indoors warrants investigation of combustion sources (furnace, water heater, stove, attached garage). CO alarms typically trigger at 70 ppm, well above the chronic health threshold. For advanced detection systems, see our carbon monoxide detection guide.

Ozone (O3)

Ground-level ozone is primarily an outdoor pollutant, but it infiltrates buildings and is also generated by some “ionic” air purifiers and photocopiers.

Standard	Limit	Averaging Period
EPA NAAQS	0.070 ppm	8-hour
WHO	100 ug/m3 (~0.050 ppm)	8-hour (peak season)
OSHA PEL	0.1 ppm	8-hour TWA

Indoor implication: Ozone-generating air purifiers are discouraged by the EPA because ozone irritates the lungs and reacts with indoor surfaces to produce secondary pollutants (formaldehyde, ultrafine particles). Choose purifiers that do not generate ozone. Our air purifier comparison flags ozone-producing units.

Nitrogen Dioxide (NO2)

Gas stoves are the primary indoor source of NO2. The WHO updated its NO2 guideline in 2021, substantially tightening the recommendation.

Standard	Limit	Averaging Period
EPA NAAQS	53 ppb	Annual
EPA NAAQS	100 ppb	1-hour
WHO (2021)	10 ug/m3 (~5.3 ppb)	Annual
WHO (2021)	25 ug/m3 (~13.3 ppb)	24-hour

Indoor implication: Studies have measured peak NO2 levels of 100-400 ppb near gas stoves during cooking, far exceeding both EPA and WHO annual guidelines in short bursts. Adequate kitchen ventilation (range hood vented to the outside) is the primary mitigation. For guidance on kitchen ventilation, see our kitchen ventilation analysis.

Radon

Radon is a radioactive gas that seeps from soil into buildings through foundation cracks. It is the second leading cause of lung cancer in the United States after smoking.

Standard	Action Level
EPA	4.0 pCi/L (148 Bq/m3)
WHO	100 Bq/m3 (~2.7 pCi/L)

The EPA recommends mitigation (typically a sub-slab depressurization system) for any home with radon levels at or above 4.0 pCi/L. WHO sets a lower action level of 100 Bq/m3 (approximately 2.7 pCi/L). For radon detection methods and monitoring tools, see our radon detection and analysis guide.

Putting It All Together: Recommended Indoor Targets

Pollutant	Protective Indoor Target	Source
PM2.5	Below 5 ug/m3	WHO guideline
CO2	Below 1,000 ppm	ASHRAE 62.1
Formaldehyde	Below 0.1 mg/m3	WHO guideline
TVOC	Below 300 ug/m3	Industry consensus
CO	Below 9 ppm	EPA NAAQS
NO2	Below 25 ug/m3	WHO 24-hour guideline
Ozone	Below 0.050 ppm	WHO guideline
Radon	Below 2.7 pCi/L	WHO action level

Achieving these targets simultaneously requires a combination of source control, ventilation, and air filtration. For a holistic strategy, see our complete guide to home air quality.

Frequently Asked Questions

Are indoor air quality standards legally enforceable for homes?

No. EPA NAAQS apply to outdoor ambient air. OSHA PELs apply to workplaces. ASHRAE standards become enforceable only when adopted into local building codes (which varies by jurisdiction). For residential spaces, all indoor air quality targets are voluntary guidelines — but they are based on strong epidemiological evidence and should be treated as health benchmarks.

Why is the WHO PM2.5 guideline so much stricter than the EPA standard?

The WHO guideline (5 ug/m3 annual mean) reflects the most current evidence that health effects occur at very low concentrations. The EPA must balance health protection with economic feasibility and attainability, which is why the NAAQS (9 ug/m3) is higher but still represents a significant tightening from the previous 12 ug/m3 standard.

What is the most important indoor pollutant to monitor?

For most homes, PM2.5 and CO2 together provide the best picture of indoor air quality. PM2.5 captures particle pollution from cooking, cleaning, outdoor infiltration, and combustion. CO2 captures ventilation adequacy. Adding formaldehyde or TVOC monitoring is valuable if you have new furniture, recent renovations, or use many chemical products.

Does ASHRAE 62.1 apply to my home?

ASHRAE 62.1 applies to commercial and institutional buildings. ASHRAE 62.2 is the residential ventilation standard and specifies minimum whole-house ventilation rates based on floor area and number of bedrooms. Check whether your local building code references 62.2.

How do I measure these pollutants at home?

Consumer-grade indoor air quality monitors now measure PM2.5, CO2, TVOC, temperature, and humidity for $100-$300. For radon, long-term test kits ($15-$30) provide the most accurate average reading. For CO, standard CO alarms are essential but only alarm at high levels — a low-level CO monitor provides more granular data. See our smart air monitors guide for product recommendations.

Are there specific standards for schools and daycares?

Yes. The EPA’s Indoor Air Quality Tools for Schools program provides guidelines for K-12 buildings. Many states adopt ASHRAE 62.1 for school ventilation, and the CDC has published COVID-era ventilation recommendations that emphasize 5+ ACH and CO2 monitoring. See our school air quality guide for more information.

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Sources:

1. U.S. EPA, “NAAQS Table.” https://www.epa.gov/criteria-air-pollutants/naaqs-table
2. WHO, “Global Air Quality Guidelines,” 2021. https://www.who.int/publications/i/item/9789240034228
3. EPA, “Final Reconsideration of the NAAQS for Particulate Matter,” 2024. https://www.epa.gov/pm-pollution/final-reconsideration-national-ambient-air-quality-standards-particulate-matter-pm
4. ASHRAE, “Standards 62.1 & 62.2.” https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2
5. OSHA, “Permissible Exposure Limits - Annotated Tables.” https://www.osha.gov/annotated-pels