By Claire WhitakerIndoor air can contain particles, gases, and volatile organic compounds (VOCs) from cooking, cleaning, building materials, and outdoor pollution. Portable air cleaners and HVAC filters can help reduce these pollutants, but understanding the different filter types is essential to choose the right equipment for your home.
Mechanical filters and HEPA
According to the EPA’s Guide to Air Cleaners in the Home, portable air cleaners often use a high-efficiency particulate air (HEPA) filter to achieve a high clean air delivery rate (CADR). These mechanical filters trap particles such as dust, pollen, mold spores, and fine particulate matter (PM2.5). The EPA notes that CADR is measured at the unit’s highest speed and indicates how quickly it can clean a room of a specific particle size. For small particles of greatest health concern, the EPA recommends choosing a unit with a high CADR for tobacco smoke, which represents the smallest particles. To match a unit to your room, multiply the room’s length by width to get square footage, and compare it to the manufacturer’s recommended room size. Proper placement also matters; see our air purifier placement guide. The EPA provides a sizing chart: for a 100-square-foot room, a minimum CADR of 65 cfm is suggested, scaling up to 390 cfm for 600 square feet, assuming 8-foot ceilings. For a deeper look at sizing, see our guide on air purifier room size. HEPA filters do not remove gases or odors; for that, a separate activated carbon filter is needed.
Activated carbon filters for gases and VOCs
The EPA explains that to filter gases, including volatile organic compounds (VOCs) and odors, a portable air cleaner needs an activated carbon filter or another absorbent filter designed for gases. There is no widely used performance rating for gas removal, unlike CADR for particles. The EPA advises that activated carbon filters can be effective if they contain a large amount of material; the thicker the filter, the better. Some units combine a HEPA filter and a carbon filter to address both particles and gases. However, the EPA cautions that no air cleaner will eliminate all pollutants, and filtration should supplement, not replace, source control and ventilation.
Pre-filters and filter care
The University of Washington’s Environmental Health & Safety (EH&S) guidance notes that many units include pre-filters, which help extend the life of the HEPA filter and should be replaced more often. The EPA stresses that all filters need regular replacement; a dirty or overloaded filter will not work well. Manufacturers typically recommend replacing HVAC filters every 60 to 90 days, but the EPA says to follow the manufacturer’s instructions and inspect filters regularly. If a filter appears heavily soiled when you change it, consider replacing it more frequently. See our air purifier maintenance guide for step-by-step filter care. When changing filters, the UW EH&S suggests placing the old filter directly into a plastic bag and sealing it to avoid releasing captured particles.
Electronic air cleaners, ionizers, and ozone risks
Some portable air cleaners use electrostatic precipitators or ionizers to charge particles and collect them on plates. The EPA warns that these devices, along with UV lights without adequate lamp coatings and plasma air cleaners, may have the potential to emit ozone, a lung irritant. The California Air Resources Board (CARB) strongly advises against using ozone generators at home, noting that ozone can cause respiratory tract irritation, breathing difficulty, and permanent lung damage. CARB enacted a regulation in 2008 that requires all indoor air cleaners sold in California to be certified for low ozone emissions; consumers can check the CARB list of certified devices. The EPA states that no federal agency has approved ozone generators for use in occupied spaces. Both CARB and the Association of Home Appliance Manufacturers maintain lists of air cleaners that have been shown to emit little or no ozone. If you see an air cleaner marketed as an “ionizer” or “electronic air cleaner,” check whether it is certified and avoid any device that intentionally produces ozone.
UV lights and other technologies
The EPA notes that research has not yet shown that technologies such as ultraviolet (UV) light, photocatalytic oxidation, or plasma can remove gases effectively in portable residential air cleaners. The EPA also lists UV lights without adequate lamp coatings among the technologies that may have the potential to emit ozone, so check any UV-equipped unit against the CARB certified list.
HVAC filters and MERV ratings
For whole-home filtration, the EPA recommends upgrading the filter in your furnace or central HVAC system. Furnace and HVAC filters are usually designed to filter particles, not gases. The EPA advises choosing a filter with a Minimum Efficiency Reporting Value (MERV) of at least 13, or as high a rating as your system can accommodate. Filters with a MERV of 13 or higher are required to demonstrate at least 50% removal efficiency for the smallest particles. The EPA cautions that HVAC filters work only when the system is running, which in many homes is less than 25% of the time during heating and cooling seasons. Running the fan continuously to increase filtration may raise electricity costs and affect humidity control. For more on the trade-offs of continuous fan operation, see our guide on air purifier noise and energy use. The EPA says you may need to consult a professional HVAC technician to find the highest efficiency filter your system can use. The filter must fit snugly in the slot; common thicknesses range from 1 to 5 inches.
The table below summarizes the main filter types and their characteristics, based on guidance from the EPA and CARB. Each type has trade-offs, and no single filter can address every indoor air quality concern. Refer to the product packaging and the CARB certified list for specific performance claims.
| Filter Type | What It Removes | Key Cautions |
|---|---|---|
| HEPA mechanical filter | Fine particles (PM2.5, dust, pollen, smoke) | Does not remove gases or odors; requires regular replacement |
| Activated carbon filter | Gases, VOCs, and some odors | Effectiveness depends on amount of material; no standard performance rating; needs replacement |
| Pre-filter | Helps extend the life of the HEPA filter (UW EH&S) | Must be cleaned or replaced more frequently |
| Electronic air cleaner / ionizer | May capture particles via electrostatic charge | May emit ozone; check for CARB certification; avoid if ozone production is a concern |
| Ozone generator | Intentionally produces ozone | Ozone is a lung irritant; not approved for occupied spaces; avoid entirely |
| UV light | Not shown to remove gases effectively in portable residential units (EPA) | May emit ozone if lamp coatings are inadequate; not a substitute for particle filtration |
| HVAC filter (MERV 13+) | Fine particles throughout the home; required to show at least 50% removal efficiency for smallest particles | Only works when system runs; may require HVAC modifications; replace per manufacturer schedule |
Frequently asked questions
What is CADR and how do I use it to size an air purifier?
The EPA explains that CADR, or clean air delivery rate, measures the volume of filtered air an air cleaner delivers per minute, typically for tobacco smoke, dust, and pollen. To choose the right size, calculate your room’s square footage and compare it to the manufacturer’s recommended room size. The EPA’s sizing chart suggests a minimum CADR of 65 cfm for a 100-square-foot room, scaling up to 390 cfm for 600 square feet, assuming 8-foot ceilings. For small particles like PM2.5, look for a high CADR for tobacco smoke.
How often should I replace air purifier filters?
The EPA advises following the manufacturer’s recommendations. For HVAC filters, manufacturers typically suggest replacement every 60 to 90 days, but you should inspect them regularly. If a filter looks heavily soiled at the time of replacement, change it more often. Pre-filters in portable units may need more frequent replacement to protect the HEPA filter. A dirty or overloaded filter will not work effectively, so timely replacement is key.
Are ozone-generating air cleaners safe?
No. The EPA and CARB both warn that ozone is a lung irritant that can cause respiratory problems and permanent lung damage. No federal agency has approved ozone generators for use in occupied spaces. CARB requires all air cleaners sold in California to be certified for low ozone emissions. Consumers should avoid any device that intentionally produces ozone and check the CARB certified list before purchasing.
For more guidance on choosing and using an air purifier, visit our air purifiers category page.
Sources
- EPA: Guide to Air Cleaners in the Home (2nd edition) PDF
- EPA: Guide to Air Cleaners in the Home (web page)
- California Air Resources Board: Air Cleaner Information for Consumers
- University of Washington EH&S: Portable Air Cleaners and Household Fans
About Claire Whitaker
Claire Whitaker is part of our editorial team and reviews and edits our non-vacuum cleaning gear guides, checking product details against manufacturer specifications and owner feedback.
