What the numbers on a monitor actually represent
PM2.5 is fine particulate matter: particles of 2.5 micrometres or smaller, roughly 30 times finer than a human hair. The WHO 2021 global air quality guidelines set an annual mean of 5 µg/m³ and a 24-hour mean of 15 µg/m³ for PM2.5. The US EPA strengthened the annual PM2.5 National Ambient Air Quality Standards to 9.0 µg/m³ in 2024 and kept the 24-hour standard at 35 µg/m³. Those are outdoor limits, but they give a yardstick for indoor readings.
CO2 indoors is mostly exhaled breath. Outdoor air holds about 420 ppm of CO2. Many ventilation guidelines treat about 1000 ppm indoors as the upper comfort band. A non-dispersive infrared (NDIR) CO2 sensor reports a real concentration, not an index. That matters because cheap 'air quality' boxes often estimate CO2 from other signals.
Radon is a radioactive gas from soil and rock. The US EPA radon action level is 4 pCi/L, about 148 Bq/m³. The WHO reference level is 100 Bq/m³. Short-term radon tests run 2 to 90 days; long-term tests run more than 90 days. Radon has no smell and no immediate symptoms, so only a test tells you the level.
The EPA Air Quality Index (AQI) runs from 0 to 500. Bands are 0–50 Good, 51–100 Moderate, 101–150 Unhealthy for Sensitive Groups, and 151–200 Unhealthy. The AQI is built for outdoor air, but it is still the fastest way to judge whether to open windows today.
Where indoor pollutants come from
Gas cooking produces nitrogen dioxide (NO2). The WHO 2021 guideline sets NO2 at 10 µg/m³ annual and 25 µg/m³ 24-hour. A running range hood that vents outdoors is the standard fix. Recirculating hoods with carbon filters do far less for NO2.
Candles, incense, and frying add fine PM2.5 indoors. Cleaning products, paints, and adhesives release volatile organic compounds (VOCs). VOCs are a broad family, not one gas, so no single limit covers them all. Formaldehyde is one common VOC with its own WHO indoor guideline.
Radon enters through cracks in basements and slabs. The US EPA action level of 4 pCi/L is the point where mitigation is warranted. The WHO reference level of 100 Bq/m³ is lower, so some homes act sooner. Both numbers come from the same physical measurement: radon concentration in air.
Ozone is a special case. It is a lung irritant outdoors, and some indoor air cleaners generate it on purpose. The California Air Resources Board (CARB) limits ozone emissions from indoor air cleaning devices sold in California to 0.050 ppm. That limit exists because ozone is harmful to breathe.
How to measure without fooling yourself
Consumer laser PM2.5 sensors count particles by light scattering. They report a mass concentration in µg/m³, but they are not reference-grade monitors. The US EPA has tested many low-cost sensors and publishes performance targets. Use them for trends, not for compliance.
NDIR CO2 sensors are the reliable consumer option for CO2. They report ppm directly. The 420 ppm outdoor baseline and the 1000 ppm indoor comfort band are the two numbers to know. A bedroom that climbs past 1000 ppm overnight is under-ventilated.
Radon testing follows a clear protocol. Short-term kits run 2 to 90 days; long-term kits run more than 90 days. The US EPA recommends a short-term test first, then a follow-up if the result is near the action level. Place the kit in the lowest lived-in level, away from drafts and exterior walls.
The EPA AQI is a daily outdoor reading. It runs from 0 to 500, with 0–50 Good and 51–100 Moderate. When the AQI is high, closing windows and running a purifier makes sense. When it is low, ventilation is usually the better move.
What actually cleans indoor air
Source control comes first. A vented range hood cuts NO2 from gas cooking. Sealing a basement slab reduces radon entry. Choosing low-VOC paints and cleaners lowers VOC load. No purifier can outrun a continuous source.
Ventilation is the second lever. Outdoor air holds about 420 ppm of CO2, so bringing it inside dilutes indoor CO2. ASHRAE Standard 62.1 and Standard 62.2 define ventilation rates for acceptable indoor air quality. ASHRAE Standard 241 (2023) adds equivalent clean airflow for infectious aerosols.
Filtration is the third lever. The US Department of Energy HEPA definition requires capturing at least 99.97% of particles of 0.3 micrometres. The EN 1822 H13 class requires 99.95% at the most penetrating particle size. Both are real, testable claims, not marketing words.
Sizing a purifier uses the AHAM two-thirds rule. The smoke CADR should be at least two-thirds of the room's floor area in square feet. A 300 sq ft room needs a smoke CADR of at least 200 CFM. The AHAM CADR is measured in cubic feet per minute (CFM) separately for tobacco smoke, dust, and pollen under ANSI/AHAM AC-1.
Key figures at a glance
| WHO 2021 PM2.5 guideline | 5 µg/m³ annual; 15 µg/m³ 24-hour | Global Air Quality Guidelines, World Health Organization, 2021. |
|---|---|---|
| US EPA PM2.5 NAAQS | 9.0 µg/m³ annual; 35 µg/m³ 24-hour | Strengthened annual standard, 2024; 24-hour standard retained. |
| EPA AQI bands | 0–50 Good; 51–100 Moderate; 101–150 USG; 151–200 Unhealthy | Air Quality Index runs from 0 to 500. |
| Radon action levels | 4 pCi/L (≈148 Bq/m³) EPA; 100 Bq/m³ WHO | US EPA action level; WHO reference level. |
How to size a HEPA purifier for a bedroom using the AHAM two-thirds rule
Measure the room
Measure the bedroom's floor area in square feet. A 300 sq ft room is the worked example here. Write the number down; the AHAM two-thirds rule uses it directly.
Apply the two-thirds rule
Multiply the floor area by two-thirds. For 300 sq ft, that gives 200. The result is the minimum smoke CADR in CFM. The AHAM two-thirds rule is a sizing shortcut, not a guarantee of clean air.
Check the CADR label
Look for a smoke CADR of at least 200 CFM on the ANSI/AHAM AC-1 label. The label also lists dust and pollen CADR separately. Use the smoke number for the two-thirds rule.
Confirm HEPA class
Check that the filter meets the US DOE HEPA definition: at least 99.97% of particles of 0.3 micrometres. An EN 1822 H13 filter meets 99.95% at the most penetrating particle size. Both are acceptable; the DOE number is the common US reference.
Place and run it
Place the purifier so air can circulate freely around it. Keep it away from curtains and walls. Run it on high when cooking or when outdoor AQI is above 100.
Frequently asked questions
Is a PM2.5 reading of 15 µg/m³ indoors safe?
The WHO 2021 24-hour guideline for PM2.5 is 15 µg/m³. The US EPA 24-hour NAAQS is 35 µg/m³. So 15 µg/m³ is at the WHO 24-hour limit, not below it. Long-term exposure matters more than a single reading.
What CO2 level means my room needs more ventilation?
Outdoor air holds about 420 ppm of CO2. Many ventilation guidelines treat about 1000 ppm indoors as the upper comfort band. A bedroom that stays above 1000 ppm overnight is under-ventilated. An NDIR CO2 sensor is the right tool to check this.
Do I need a short-term or long-term radon test?
Short-term radon tests run 2 to 90 days. Long-term tests run more than 90 days. The US EPA recommends a short-term test first. If the result is near the 4 pCi/L action level, follow up with a long-term test.
Does a HEPA filter remove VOCs?
No. The US DOE HEPA definition covers particle capture: at least 99.97% of particles of 0.3 micrometres. VOCs are gases, not particles. Activated carbon is the common media for VOC removal. Source control and ventilation remain the first steps.
Is an ozone-generating air cleaner safe?
The California Air Resources Board (CARB) limits ozone emissions from indoor air cleaning devices sold in California to 0.050 ppm. Ozone is a lung irritant. Devices that intentionally produce ozone are not recommended for occupied spaces. Check for a CARB certification number.