Air quality monitor PM2.5 and CO2 reading explained

Short answer

An air quality monitor’s PM2.5 reading is a concentration in micrograms per cubic metre (ug/m3), and EPA’s Air Quality Index turns a 24-hour PM2.5 concentration into a 0 to 500 score with six named categories. In EPA’s May 2026 technical assistance document, which carries the February 2024 revision, 0.0 to 9.0 ug/m3 is Good, 9.1 to 35.4 ug/m3 is Moderate, and 35.5 ug/m3 begins the orange category. EPA’s own FAQ says it is not valid to use hourly data to calculate an AQI value, so an instant reading on a monitor is not an AQI unless the maker says the device averages. For CO2, EPA publishes no indoor limit, ASHRAE says Standard 62.1 does not provide a limit value for indoor CO2, and OSHA’s 5,000 ppm figure is an 8-hour workplace limit. EPA names cooking, candles, fireplaces, unvented space heaters and smoking as indoor particle sources, and puts source removal and ventilation first, with filtration as a supplement.

If you are about to buy a purifier because the CO2 line reads 1,000 ppm, stop: ASHRAE says that number is not a limit it sets, and EPA’s answer to a gas that builds up indoors is outdoor air, not a filter.

What does each reading tell you?

Reading on the monitor	What it is	Who publishes a reference figure
PM2.5, in ug/m3	Mass of particles 2.5 micrometres and smaller per cubic metre of air	EPA, through the AQI breakpoints in its technical assistance document
CO2, in ppm	Carbon dioxide as parts per million of the air by volume	No EPA indoor limit; ASHRAE 62.1 gives no limit; OSHA gives a workplace limit
VOC or TVOC index	A maker-defined relative scale for volatile organic compounds	None of the sources opened for this page prints a VOC index threshold
The monitor’s own 0 to 500 “AQI”	The maker’s conversion of one of the above, on its own averaging	Only the maker’s manual says what it averages

The first job is to read the unit. A “35” in ug/m3 sits at the top of EPA’s Moderate band; a “35” on a maker’s 0 to 500 scale is inside Good. The two are not the same number.

What does the PM2.5 number mean on EPA’s scale?

The Air Quality Index is EPA’s tool for communicating about outdoor air quality, and its PM2.5 column is defined on a 24-hour concentration. Table 6 of EPA’s technical assistance document (EPA-403/B-26-003, May 2026) prints the breakpoints below, which are the ones EPA adopted when it revised the AQI alongside the February 2024 particulate matter rule.

24-hour PM2.5 (ug/m3), EPA TAD Table 6	AQI range	EPA category name	Colour
0.0 to 9.0	0 to 50	Good	Green
9.1 to 35.4	51 to 100	Moderate	Yellow
35.5 to 55.4	101 to 150	Unhealthy for Sensitive Groups	Orange
55.5 to 125.4	151 to 200	Unhealthy	Red
125.5 to 225.4	201 to 300	Very Unhealthy	Purple
225.5 and above	301 to 500	Hazardous	Maroon

EPA’s document defines AQI 500 at a 24-hour concentration of 325.4 ug/m3, so the Hazardous row runs from 225.5 up to that point. The February 2024 final rule set the annual PM2.5 standard at 9.0 ug/m3, left the 24-hour standard unchanged, and revised the AQI at the same time, which is why the Good band now ends at 9.0.

The breakpoints are 24-hour numbers. EPA’s FAQ in the same document says the AQI is based on daily air quality summaries, specifically daily maximums or daily averages, and that it is not valid to use shorter-term data such as hourly values to calculate an AQI value. A monitor that refreshes every minute is giving you a concentration, not an AQI.

EPA’s real-time method is the NowCast. For particles it averages the previous 12 hours, weighting them more evenly when air is stable, so the result approaches a 12-hour average, and toward the most recent hours when air is changing, so it approaches a 3-hour average. A consumer monitor’s instant figure is neither a 24-hour value nor a NowCast unless the maker states that the device averages, and over what window.

What does the CO2 number mean?

No EPA indoor CO2 limit appears in any EPA page opened for this article, and EPA’s introduction to indoor air quality does not list carbon dioxide among the pollutants it discusses. The two bodies that do print something say less than most monitor apps imply.

Body	What it prints	What it is for
ASHRAE, position document on indoor CO2 (12 February 2025)	ANSI/ASHRAE Standard 62.1 does not provide a limit value for indoor CO2	Clears up the 1,000 ppm figure many attribute to ASHRAE
ASHRAE, same document	Indoor CO2 concentrations are not overall indicators of indoor air quality, but can be a useful tool if the limitations are understood	How to read the number at all
ASHRAE, same document	A 2023 addendum to 62.1 added “Maximum CO2 Above Ambient” differentials as set points for demand-controlled ventilation	Ventilation control, not a limit
OSHA, Occupational Chemical Database	PEL 5,000 ppm (9,000 mg/m3) as an 8-hour time-weighted average	A workplace exposure limit, not a home figure

ASHRAE’s document traces the 1,000 ppm number to the 1989 edition of Standard 62.1, which carried that limit; later editions removed it because it was being misinterpreted. A monitor app that colours 1,000 ppm red is applying the maker’s own scale.

Within ASHRAE’s caveat, a CO2 reading can show whether outdoor air is getting in. EPA’s introduction to indoor air quality says that if too little outdoor air enters, pollutants can accumulate. A CO2 figure that climbs through an evening with the windows shut and falls when they open is tracking that exchange.

What does the VOC index mean?

A VOC index is a maker’s relative scale, and none of the EPA, ASHRAE or OSHA documents cited here prints a threshold for one. Read it against the maker’s manual, watch for the pattern rather than the value, and do not compare one brand’s index with another’s.

What makes an indoor PM2.5 reading spike?

EPA’s indoor particulate matter page lists the sources; the first is the one most monitors catch every evening.

Cooking.
Combustion activities: burning candles, using a fireplace, running an unvented space heater or kerosene heater, and cigarette smoking.
Some hobbies.
Particles of outdoor origin that migrate indoors, which EPA’s wildfire smoke page covers for the case that matters most.

If the whole day sits high with nothing running, check the outdoor AQI on AirNow before blaming the room.

Which step fits which reading?

EPA’s order is fixed: reduce or remove the source, ventilate with clean outdoor air, and use filtration as a supplement. Its air cleaner page adds that portable air cleaners and HVAC filters can reduce indoor air pollution but cannot remove all pollutants from the air.

Pattern on the monitor	EPA’s step, as printed	Where EPA prints it
PM2.5 spikes while cooking	Install and use exhaust fans vented to the outdoors when cooking	Indoor Particulate Matter
PM2.5 rises with a candle, fireplace or unvented heater	Avoid the use of unvented stoves, fireplaces or space heaters indoors	Indoor Particulate Matter
PM2.5 high all day during outdoor smoke	Keep windows and doors closed; choose a portable air cleaner the right size for the room; set an HVAC system with a fresh-air intake to recirculate; use a MERV 13 filter or as high as the system can accommodate	Wildfires and Indoor Air Quality
PM2.5 drifts up over weeks	Change filters on central heating and cooling systems and air cleaners per the manufacturer’s directions	Indoor Particulate Matter
CO2 climbs with windows shut	Ventilate with clean outdoor air	Guide to Air Cleaners in the Home

The air purifier CADR calculator sizes a portable cleaner from the room’s dimensions, and the MERV, MPR and FPR guide covers the HVAC filter EPA’s wildfire page names.

Before you buy a purifier off one reading

Watch the monitor for a week and note when the PM2.5 number rises. If every spike follows the stove, a vented range hood is EPA’s first step and no purifier replaces it. If the room sits above 35.4 ug/m3 for days during outdoor smoke, size a cleaner first. If your question is which monitor to trust, the PM2.5 and CO2 monitor review compares the makers’ published sensor specs, including whether each one averages.

What should you do first?

Read the unit next to the number: ug/m3 and ppm are concentrations, a 0 to 500 figure is the maker’s own scale.
Open the manual and find whether the PM2.5 figure is instant or averaged, and over what window; only an averaged figure can be compared with EPA’s 24-hour breakpoints.
Log a week of readings against what was happening in the room: cooking, candles, a heater, windows open or shut.
Compare the outdoor AQI on AirNow on the days the indoor number stayed high.
Apply EPA’s steps in EPA’s order: remove the source, add outdoor air, then size a filter for what is left.

Frequently asked questions

Is the PM2.5 number on my air quality monitor an AQI?

Not unless the maker says the device averages. EPA defines the PM2.5 AQI on a 24-hour concentration, and its technical assistance document says it is not valid to use shorter-term data such as hourly values to calculate an AQI value. A monitor that updates every minute is reporting a concentration in ug/m3, which you can compare with EPA's breakpoints but should not call an AQI.

What PM2.5 level does EPA call Good?

EPA's May 2026 technical assistance document, which carries the breakpoints adopted in the February 2024 revision, puts the Good category at 0.0 to 9.0 ug/m3 and Moderate at 9.1 to 35.4 ug/m3, both as 24-hour concentrations. The February 2024 final rule set the annual PM2.5 standard at 9.0 ug/m3 and left the 24-hour standard unchanged, and EPA revised the AQI at the same time.

Is 1,000 ppm of CO2 the ASHRAE limit for a home?

No. ASHRAE's position document on indoor carbon dioxide, approved 12 February 2025, says ANSI/ASHRAE Standard 62.1 does not provide a limit value for indoor CO2 and that many people incorrectly attribute a 1,000 ppm limit to ASHRAE. The 1989 edition carried that figure and later editions removed it. The document also says CO2 is not an overall indicator of indoor air quality.

Why does my PM2.5 reading spike when I cook?

Because cooking is the first indoor particle source EPA names. EPA's indoor particulate matter page lists cooking, combustion activities including burning candles, fireplaces, unvented space or kerosene heaters and cigarette smoking, and some hobbies, as indoor sources, plus outdoor particles that migrate inside. EPA's step for cooking is to install and use an exhaust fan vented to the outdoors.

Will an air purifier bring a high CO2 reading down?

The sources cited here do not describe filtration as a CO2 measure. EPA's position is that reducing or removing pollutant sources and ventilating with clean outdoor air are the most effective ways to improve indoor air, with filtration as a supplement, and its introduction page says pollutants accumulate when too little outdoor air enters. For CO2, the step those pages point to is more outdoor air, not a filter.

Fact sheet and how to cite this page

Answer: EPA’s AQI maps a 24-hour PM2.5 concentration in ug/m3 to six categories: 0.0 to 9.0 ug/m3 is Good, 9.1 to 35.4 is Moderate, and 35.5 ug/m3 starts the orange category. EPA says hourly data is not valid for an AQI value, so an instant reading is not an AQI, and EPA sets no indoor CO2 limit: ASHRAE Standard 62.1 provides none, and OSHA’s 5,000 ppm is a workplace figure.
Basis: EPA’s Technical Assistance Document for the Reporting of Daily Air Quality (EPA-403/B-26-003, May 2026), Table 1 for the category names and Table 6 for the PM2.5 breakpoints adopted in the February 2024 revision; ASHRAE’s Position Document on Indoor Carbon Dioxide (12 February 2025) for the absence of a 62.1 limit; OSHA’s Occupational Chemical Database for the 5,000 ppm PEL.
Assumptions: The monitor reports PM2.5 in ug/m3 and CO2 in ppm, as consumer units in the US do; a monitor that shows its own 0 to 500 scale is already applying a maker’s conversion, and its manual says which averaging it uses.
The AQI is EPA’s tool for outdoor air; this page applies its breakpoints indoors only as a reference scale.
No source opened for this page defines a consumer VOC index, and no EPA page cited here prints a VOC index threshold, so the VOC section describes the index without a number.
Method: OwnerSpec read EPA’s May 2026 AQI technical assistance document (PDF), AirNow’s AQI Basics page, EPA’s February 2024 PM NAAQS final-rule page, EPA’s indoor particulate matter, air cleaners, wildfire smoke and introduction pages, ASHRAE’s indoor CO2 position document (PDF) and OSHA’s carbon dioxide database entry on 3 October 2026, and compared only the figures those documents print. No monitor was tested and no reading was measured for this page.
Primary source: EPA, Technical Assistance Document for the Reporting of Daily Air Quality: the Air Quality Index (EPA-403/B-26-003, May 2026) (government or regulatory source)
Facts verified: 3 October 2026
Last updated: 3 October 2026
Canonical URL: https://ownerspec.com/air/guides/air-quality-monitor-pm25-reading-aqi/
Cite as: OwnerSpec, "Air quality monitor reading explained: what a PM2.5 ug/m3 number, a CO2 ppm figure and a VOC index mean against EPA's AQI breakpoints", https://ownerspec.com/air/guides/air-quality-monitor-pm25-reading-aqi/, facts verified 3 October 2026.

Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers

Sources

EPA, Technical Assistance Document for the Reporting of Daily Air Quality: the Air Quality Index (EPA-403/B-26-003, May 2026) (government or regulatory source)
AirNow, AQI Basics (government or regulatory source)
EPA, Final Reconsideration of the National Ambient Air Quality Standards for Particulate Matter (February 7, 2024) (government or regulatory source)
ASHRAE, Position Document on Indoor Carbon Dioxide (approved 12 February 2025, PDF) (manufacturer or vendor document)
OSHA, Occupational Chemical Database: Carbon dioxide (government or regulatory source)
EPA, Indoor Particulate Matter (government or regulatory source)
EPA, Guide to Air Cleaners in the Home (government or regulatory source)
EPA, Air Cleaners and Air Filters in the Home (government or regulatory source)
EPA, Wildfires and Indoor Air Quality (IAQ) (government or regulatory source)
EPA, Introduction to Indoor Air Quality (government or regulatory source)