Indoor air quality (IAQ) has become a major category of HVAC accessory product, and the marketing around it ranges from genuinely useful to nearly fraudulent. Distinguishing between them requires a working understanding of what indoor air pollutants are, where they come from, and which interventions actually reduce them. The list of things worth doing is shorter than the list of things sold.
The pollutants that matter
Indoor air can contain a number of pollutants, but a few dominate from a health perspective.
Particulate matter — especially PM2.5, fine particles 2.5 microns or smaller — comes from cooking (especially gas stoves), wood-burning, candles, and infiltration of outdoor air on bad-air-quality days. PM2.5 is linked to cardiovascular and respiratory disease.
Carbon monoxide (CO) from combustion appliances, especially gas furnaces with cracked heat exchangers, malfunctioning gas water heaters, attached garages with cars running, and gas ranges. CO is acutely dangerous and the right detector saves lives.
Radon from soil under the foundation, varying dramatically by region. The EPA recommends every home be tested. Mitigation, where required, is straightforward.
Volatile organic compounds (VOCs) from paints, new furniture, cleaning products, and adhesives. Concentrations spike after renovations and decay over weeks to months.
Nitrogen dioxide (NO2) from gas combustion, particularly gas cooking. Recent research has emphasized this more than past decades.
Mold and bioaerosols from chronic moisture issues, water damage, or poorly maintained humidification equipment.
Excess humidity — not a pollutant per se, but a precondition for mold growth and dust mite proliferation.
The interventions worth doing
Five interventions produce most of the health benefit available from indoor air quality work.
1. Use the kitchen range hood, vented outside, every time you cook
This is the highest-leverage IAQ measure in most homes, and it requires no purchase. Gas ranges in particular produce PM2.5, NO2, and CO that accumulate rapidly indoors. An exhaust hood that actually vents to the outdoors (not a recirculating model that just runs air through a charcoal filter) reduces these by an order of magnitude. Use the highest setting that's tolerable for noise.
If your range hood is a recirculating model and you can vent it through an exterior wall, this is one of the better remodeling projects you can do in health terms.
2. Test for radon, and mitigate if elevated
Radon is the second-leading cause of lung cancer in the United States, after smoking, and the only way to know your home's level is to test. A short-term test kit costs about $20. Long-term tests are more accurate. If your level is at or above the EPA action level of 4 picocuries per liter, a radon mitigation system (typically $1,000 to $2,500 installed) reduces it dramatically.
Radon is region-dependent but is found in homes of all construction types in all states. There is no climate, building type, or foundation type that exempts a home from needing testing.
3. Functional CO detectors near every sleeping area
Carbon monoxide detectors are required by code in most jurisdictions but the requirement is often the only reason they exist. Replace any detector older than 10 years — the sensors degrade. Test them at the same time you change clock batteries. A detector that has expired or has a dead battery is no detector.
Have any combustion appliance (gas furnace, gas water heater, gas range, fireplace) inspected annually by a qualified technician. Cracked heat exchangers in older furnaces are a primary source of indoor CO and are not visible without disassembly.
4. Use a quality air filter on the HVAC system
The single-most cost-effective IAQ intervention for particulates is upgrading the filter on the central HVAC system from the standard fiberglass panel (which protects equipment but does almost nothing for air quality) to a pleated filter rated MERV 11 or higher. The cost difference is roughly $15 per filter; the particulate reduction is dramatic.
A few cautions. Very high-MERV filters (MERV 13+) can restrict airflow on systems not designed for them, which can damage the equipment and reduce efficiency. Check with an HVAC technician before installing MERV 13+. The right answer for most systems is MERV 11 or 12, replaced every two to three months.
Standalone HEPA air cleaners can be excellent for individual rooms (especially bedrooms) but are not substitutes for whole-house filtration. They are most useful for people with significant allergies or respiratory conditions, or during local air-quality events like wildfire smoke.
5. Manage humidity, in both directions
Indoor relative humidity below 30 percent makes occupants uncomfortable, dries mucous membranes, and increases vulnerability to respiratory infection. Above 50 to 60 percent it accelerates mold growth and dust mite proliferation. The healthy range is roughly 30 to 50 percent.
In winter in cold climates, indoor humidity falls naturally and a whole-house humidifier (installed at the furnace) can be useful. In summer in humid climates, dehumidification — either through the air conditioner or a standalone unit — is important. In basements, a standalone dehumidifier often runs nearly continuously and is one of the better single investments for IAQ.
The interventions not worth doing
Several IAQ products are widely sold and provide limited or no benefit, sometimes at meaningful cost.
UV light sterilizers installed in ductwork have some effect on biological contaminants but require continuous operation, replacement bulbs every year, and a meaningful electrical load. The benefit is real but small for most homes.
Ionizers and ozone generators are heavily marketed but the EPA has been clear that ozone-generating devices can produce indoor ozone at levels that themselves harm respiratory health. These are not recommended.
Bipolar ionization has been the subject of skeptical analysis from independent indoor air researchers. Manufacturer-sponsored studies have shown benefits; independent peer-reviewed studies have generally not. The intervention is expensive and the evidence base is thin.
Charcoal-filter-only air purifiers (no HEPA) do little for particulates and modest amounts for some odors and VOCs.
The single most effective action for most homes
Use your kitchen range hood vented to the outside, every time you cook, on the highest setting that is acoustically tolerable. The health benefit exceeds anything you can buy as an accessory.
For more on HVAC system maintenance that supports IAQ, see HVAC Maintenance Schedule.
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