EPA NAAQS, Carbon Monoxide, PM2.5, Combustion Gases, and Indoor Air Quality
Carbon Monoxide and PM2.5 Indoors: How EPA Outdoor Standards Should Be Used
The EPA National Ambient Air Quality Standards, often called NAAQS, are outdoor air standards for criteria pollutants. They are useful reference points when discussing carbon monoxide, PM2.5, and other pollutants, but they are not the same thing as a complete indoor air quality investigation.
Article Summary
EPA's National Ambient Air Quality Standards are regulatory standards for outdoor ambient air. Primary standards are intended to protect public health, while secondary standards protect public welfare. The current NAAQS include carbon monoxide and particulate matter, but the averaging periods and regulatory forms matter: they are not simply indoor meter cutoffs.
For indoor air quality complaints, NAAQS values may provide useful context, but they should not be treated as a simple indoor pass/fail checklist or copied directly onto short-term indoor spot measurements. Indoor air testing may also involve ventilation, combustion sources, VOCs, formaldehyde, particles, mold, moisture, odors, allergens, or building-specific conditions.
This article explains how carbon monoxide and PM2.5 standards relate to indoor air testing, when testing may be useful, and why results must be interpreted with building conditions. For Angstrom’s main service page, visit indoor air quality testing.
What Are the EPA National Ambient Air Quality Standards?
The National Ambient Air Quality Standards are EPA standards for outdoor ambient air. They are established for criteria air pollutants that can affect public health and the environment.
These standards are important for outdoor air regulation, air monitoring networks, state implementation planning, and public air quality management. They are often discussed when people ask about carbon monoxide, fine particulate matter, outdoor smoke, traffic pollution, industrial emissions, or regional air quality.
For indoor air quality testing, the standards provide context, but the investigation must still address the actual building. Indoor sources, HVAC systems, combustion appliances, ventilation, occupant activity, cleaning products, cooking, smoking, renovation, and outdoor air infiltration can all affect measured indoor conditions.
EPA Carbon Monoxide Air Quality Standards
Carbon monoxide, or CO, is a colorless and odorless gas produced by incomplete combustion. Indoor sources may include malfunctioning furnaces, boilers, water heaters, fireplaces, gas ovens, gas ranges, generators, attached garages, vehicle exhaust, and other fuel-burning equipment.
EPA's current outdoor NAAQS for carbon monoxide are 9 ppm averaged over 8 hours and 35 ppm averaged over 1 hour, with the standards not to be exceeded more than once per year. These are ambient-air regulatory standards, not residential alarm thresholds and not a substitute for evaluating an active combustion source inside a building.
Carbon monoxide concerns should be treated seriously. If a carbon monoxide alarm is sounding, occupants feel ill, or there is concern about an active CO release, leave the area and contact emergency services or the local utility as appropriate. Professional testing is not a substitute for emergency response.
EPA PM2.5 Air Quality Standard
PM2.5 refers to fine inhalable particulate matter with aerodynamic diameters generally 2.5 micrometers and smaller. Fine particles may come from outdoor air pollution, wildfire smoke, combustion sources, cooking, candles, tobacco smoke, soot, construction dust, vehicle exhaust, or indoor activities.
EPA's current primary annual PM2.5 standard is 9.0 µg/m³, expressed as an annual mean averaged over three years. The primary and secondary 24-hour PM2.5 standard remains 35 µg/m³, based on the 98th percentile averaged over three years. Those regulatory forms are a major reason a short indoor measurement should not be labeled simply "passing" or "failing" the NAAQS.
Indoor PM2.5 measurements can vary rapidly. Cooking, incense, candles, fireplaces, smoking, cleaning, construction dust, open windows, outdoor smoke, and HVAC filtration can all affect measured fine particle levels.
Do Not Mix Outdoor, Occupational, and Indoor Benchmarks
EPA NAAQS
NAAQS regulate outdoor ambient air. CO is regulated using 1-hour and 8-hour averaging periods. PM2.5 uses annual and 24-hour standards with multi-year statistical forms.
Occupational CO Limits
OSHA's CO PEL is 50 ppm as an 8-hour TWA. NIOSH recommends 35 ppm as an 8-hour TWA with a 200 ppm ceiling. These worker-exposure limits are not residential indoor-air targets.
Indoor Building Assessment
Indoor interpretation should consider the source, measurement duration, room, occupants, HVAC operation, outdoor conditions, instrument performance, and whether the concern is safety, source identification, comfort, documentation, or another project objective.
Emergency CO Conditions
If a CO alarm sounds or an active release is suspected, emergency safety procedures take priority. Environmental consulting is not a substitute for evacuation, emergency services, utility response, or qualified combustion-appliance evaluation.
How NAAQS Relate to Indoor Air Quality Testing
NAAQS are outdoor ambient air standards. Indoor air testing is different because it evaluates the air people breathe inside a specific home, apartment, office, school, commercial space, or building.
In an indoor assessment, measured carbon monoxide or PM2.5 levels are interpreted with the testing location, time of day, source activity, HVAC operation, outdoor conditions, ventilation, occupancy, and the reason for the complaint.
For example, an indoor CO reading near a boiler room may require a different response than a low background reading in a living room. A PM2.5 spike during cooking may have a different interpretation than elevated particles after wildfire smoke infiltration, renovation dust, or combustion-related soot.
Testing Options for CO, PM2.5, and Indoor Air Quality
The correct testing scope depends on the concern. A combustion complaint, smoke infiltration complaint, wildfire smoke complaint, renovation dust complaint, or general air quality complaint may require different measurements.
Carbon Monoxide Testing
CO measurements can document concentrations at selected locations and times, but source identification also requires review of combustion equipment, pathways, ventilation, and operating conditions.
PM2.5 Testing
PM2.5 instruments can document fine-particle concentration trends, but the reading alone does not identify chemical composition or prove a specific source.
PM10 Testing
PM10 measurements can help evaluate larger inhalable particles, dust disturbance, construction dust, or general particulate conditions.
CO₂ and Ventilation
CO₂ testing can help evaluate ventilation patterns and occupancy effects in offices, schools, homes, and occupied spaces.
VOC and Formaldehyde Testing
VOC and formaldehyde sampling may be useful when chemical odors, new materials, renovations, or irritation complaints are present.
Smoke and Odor Assessment
Smoke, soot, tobacco, cannabis, wildfire smoke, or odor complaints may require targeted sampling and source investigation.
When Indoor Testing May Be Useful
Indoor air testing may be useful when there is a specific complaint, source concern, building condition, or documentation need. The test scope should match the question being asked.
- Carbon monoxide concerns from gas, oil, propane, wood, or vehicle exhaust
- Combustion gas concerns from boilers, furnaces, fireplaces, water heaters, or generators
- PM2.5 concerns from smoke, soot, wildfire smoke, tobacco smoke, or cooking
- Renovation dust, construction dust, demolition dust, or particle complaints
- Office, school, apartment, or workplace indoor air quality complaints
- Ventilation concerns, CO₂ complaints, stuffy rooms, or occupancy-related symptoms
- Chemical odors from new materials, cabinets, furniture, flooring, adhesives, or cleaning products
- HVAC concerns involving filtration, outdoor air, return air, or source distribution
- Tenant, landlord, attorney, insurance, school, or property-management documentation
- Need for independent written indoor environmental testing data
Carbon Monoxide Testing in Homes and Buildings
Carbon monoxide testing may be requested when there is a documented combustion-source concern, a prior alarm event, an appliance or garage pathway question, or a need to characterize conditions after immediate safety issues have been addressed. Symptoms alone are not specific enough to identify a building source.
Testing may focus on living areas, bedrooms, boiler rooms, furnace rooms, garages, mechanical rooms, fireplaces, appliance locations, and areas where exhaust or combustion byproducts may enter occupied space.
CO alarms are important safety devices, but an indoor air quality assessment may be requested when a building owner, tenant, school, attorney, or property manager needs documentation of measured conditions and possible source pathways.
PM2.5 Testing for Smoke, Soot, and Fine Particles
PM2.5 testing may be useful when fine particles are suspected from wildfire smoke, tobacco smoke, cannabis smoke, cooking, candles, soot, fireplaces, renovation dust, construction dust, or outdoor air infiltration.
Fine particles can move through small openings, HVAC systems, door gaps, window leaks, hallways, and shared building pathways. Measurements can help document whether fine particles are elevated at selected locations and whether levels vary by room, time, source activity, or HVAC operation.
Particle readings should be interpreted carefully. PM2.5 instruments document a particle-size mass concentration or an instrument-derived estimate; they do not identify the chemical or biological composition of those particles, and optical instruments can respond differently depending on aerosol type and humidity. Source attribution may require observation, timing, comparison locations, HVAC review, or targeted laboratory analysis.
Why Outdoor Standards Do Not Automatically Answer Indoor Questions
Outdoor standards are designed for ambient air regulation. Indoor environments are enclosed spaces with specific sources, ventilation rates, materials, occupants, HVAC systems, filtration, and activities.
A measured indoor pollutant level may be influenced by outdoor air, but it may also be caused by indoor sources. For example, indoor PM2.5 may increase during cooking even when outdoor air quality is acceptable. Carbon monoxide may be related to a specific appliance or exhaust pathway rather than regional outdoor air.
A useful report explains the test method, locations, conditions, readings, limitations, and practical interpretation instead of treating one number as the entire answer.
NAAQS and Indoor Air Quality FAQs
What does NAAQS mean?
NAAQS stands for National Ambient Air Quality Standards. These are EPA standards for outdoor ambient air pollutants such as carbon monoxide, particulate matter, ozone, nitrogen dioxide, sulfur dioxide, and lead.
What is the EPA carbon monoxide standard?
EPA’s outdoor carbon monoxide NAAQS include 9 ppm averaged over 8 hours and 35 ppm averaged over 1 hour. Indoor CO concerns should still be evaluated based on the building, sources, alarms, symptoms, and safety conditions.
What is the current annual PM2.5 standard?
EPA's current primary annual PM2.5 NAAQS is 9.0 µg/m³ as an annual mean averaged over three years. The 24-hour PM2.5 standard is 35 µg/m³ based on the 98th percentile averaged over three years. These are outdoor regulatory standards, not simple indoor meter pass/fail values.
Are NAAQS indoor air standards?
No. NAAQS are outdoor ambient air standards. They may provide useful context for indoor testing, but indoor air quality assessments must consider building-specific sources and conditions.
Can indoor air quality testing identify carbon monoxide sources?
Testing can document measured CO conditions and may help identify likely source pathways. Active CO hazards require immediate safety response, appliance review, and qualified mechanical or utility evaluation.
Can PM2.5 testing identify the exact source of particles?
PM2.5 testing documents fine particle levels, but source identification may require observation, timing review, room comparisons, HVAC review, smoke or soot assessment, or additional laboratory analysis.
Important Limitations
Indoor air testing is limited to the locations, time periods, instruments, and conditions evaluated. Results can change with source activity, ventilation, HVAC operation, outdoor air, weather, occupancy, cooking, cleaning, combustion appliance operation, and window or door position.
NAAQS values can provide useful context, but they do not replace emergency response, CO alarms, appliance servicing, medical evaluation, or a building-specific source investigation.
Environmental testing can document measured building conditions and support property decisions, but it is not medical advice. Health questions should be discussed with a qualified healthcare professional.
Current EPA and NIOSH Reference Sources
Current federal ambient-air standards and regulatory forms for carbon monoxide, PM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide, and lead.
U.S. EPA National Ambient Air Quality Standards for Particulate MatterEPA's current PM NAAQS page, including the 2024 revision of the primary annual PM2.5 standard to 9.0 µg/m³.
U.S. EPA Carbon Monoxide's Impact on Indoor Air QualityEPA guidance on indoor CO sources, household conditions, measurement, alarms, and combustion-equipment precautions.
CDC / NIOSH NIOSH Pocket Guide — Carbon MonoxideCurrent NIOSH reference showing a 35 ppm REL as an 8-hour TWA, a 200 ppm ceiling, and the OSHA 50 ppm 8-hour TWA PEL.
Need Site-Specific CO, PM2.5, or Indoor Air Quality Testing?
This article explains regulatory standards and measurement context generally. A site-specific assessment should be designed around the actual source question, rooms, combustion equipment, HVAC operation, outdoor conditions, measurement duration, particle sources, and documentation objective.
Angstrom provides independent environmental assessment, testing, and reporting. For broader building concerns, visit Indoor Air Quality Testing.
