Source control comes first
Gas cooking produces nitrogen dioxide (NO2). The WHO 2021 guideline sets NO2 at 10 µg/m³ annual and 25 µg/m³ 24-hour. A vented range hood that exhausts outdoors is the standard control. Recirculating hoods with carbon filters do far less for NO2.
Candles, incense, and frying add fine PM2.5 indoors. The WHO 2021 24-hour guideline for PM2.5 is 15 µg/m³. Running a vented hood during cooking keeps indoor PM2.5 closer to that level. A HEPA purifier helps but does not replace ventilation at the source.
Cleaning products, paints, and adhesives release volatile organic compounds (VOCs). Choosing low-VOC products lowers the load. The US EPA publishes indoor air quality guidance on VOCs. Formaldehyde has its own WHO indoor guideline.
Radon enters through cracks in basements and slabs. The US EPA action level is 4 pCi/L, about 148 Bq/m³. Sealing a basement slab and installing a vent pipe are standard mitigation steps. The WHO reference level is 100 Bq/m³, lower than the US action level.
Ventilation against the 420 ppm outdoor baseline
Outdoor air holds about 420 ppm of CO2. Bringing it inside dilutes indoor 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.
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. These standards are the reference for mechanical ventilation design.
Opening windows is the simplest ventilation. It works best when the EPA AQI is below 50 (Good). When the AQI is above 100, closing windows and running a HEPA purifier is the better move.
A short worked example: a 300 sq ft bedroom with two occupants can exceed 1000 ppm CO2 overnight with the door closed. Cracking a window or running a balanced ventilation system brings CO2 down toward the 420 ppm outdoor baseline. A purifier alone does not remove CO2.
HEPA filtration and sizing with the AHAM two-thirds rule
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.
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.
The two-thirds rule is a sizing shortcut, not a guarantee. Real performance depends on placement, filter condition, and air mixing. Keep the purifier away from curtains and walls so air can circulate.
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.
Humidity, radon mitigation, and when to act
The US EPA recommends keeping indoor relative humidity between 30% and 50%. High humidity supports mold growth and dust mites. Low humidity can irritate airways. A hygrometer is the tool to check this.
Radon mitigation is warranted at or above the US EPA action level of 4 pCi/L. The WHO reference level is 100 Bq/m³. A qualified mitigator can install a vent pipe and fan system. Short-term radon tests run 2 to 90 days; long-term tests run more than 90 days.
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. Use the AQI to decide between ventilation and filtration on a given day.
A combined approach works best. Control sources, ventilate when outdoor air is clean, and filter when it is not. Keep humidity in the 30–50% band. Test radon and act at the action level. No single device does all of this.
Key figures at a glance
| EN 1822 H13 class | 99.95% at the most penetrating particle size | European standard for HEPA and ULPA filters. |
|---|---|---|
| AHAM two-thirds rule | Smoke CADR ≥ ⅔ of floor area in sq ft | Example: 300 sq ft room needs ≥200 CFM smoke CADR. |
| AHAM CADR test | Measured in CFM for tobacco smoke, dust, and pollen | ANSI/AHAM AC-1 test method. |
| EPA humidity recommendation | 30% to 50% relative humidity | US EPA indoor air quality guidance. |
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
Does a HEPA filter remove CO2?
No. The US DOE HEPA definition covers particle capture: at least 99.97% of particles of 0.3 micrometres. CO2 is a gas. Ventilation is the way to lower indoor CO2 toward the 420 ppm outdoor baseline.
How do I know what size purifier to buy?
Use 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 under ANSI/AHAM AC-1.
Are ozone-generating air cleaners 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.
When should I mitigate radon?
The US EPA action level is 4 pCi/L, about 148 Bq/m³. The WHO reference level is 100 Bq/m³. If a long-term test stays at or above the action level, mitigation is warranted. A qualified mitigator can install a vent pipe and fan system.
What humidity should I keep indoors?
The US EPA recommends keeping indoor relative humidity between 30% and 50%. High humidity supports mold growth and dust mites. Low humidity can irritate airways. A hygrometer is the tool to check this.