Indoor Air Quality: Why It Matters and How to Fix It

Last updated April 29, 2026 · 9 min read

The U.S. EPA estimates Americans spend about 90% of their time indoors, where pollutant concentrations can be two to five times higher than outdoor levels. Outdoor AQI, important as it is, only tells half the story. This guide covers what's in indoor air, what actually moves the needle, and how to spot mitigation that doesn't.

The 90% problem

The reason outdoor AQI matters so much is because indoor and outdoor air are coupled — outdoor PM2.5 leaks in through doors, windows, and the building envelope. In a typical sealed home with central HVAC, indoor PM2.5 sits at 30–70% of outdoor PM2.5; in a leaky home with no filtration, it can match or even exceed outdoor levels because indoor sources stack on top. So far, so coupled.

But indoor air also has its own problem set: cooking, candles, fireplaces, gas stoves, off-gassing from new furniture, mold, radon, dust mites, pet dander, and human-emitted CO₂. Some of these are nearly absent from outdoor air; others (PM2.5 from cooking) can briefly dwarf outdoor concentrations. The CDC and EPA both treat indoor air quality as a major and underaddressed public-health issue.

The major indoor pollutants

Cooking PM2.5

Frying, sautéing, and any high-heat cooking releases a surprising amount of fine particulate matter. Indoor PM2.5 measurements during a typical dinner prep often spike to AQI-200-equivalent or higher — for 30 minutes, in a small kitchen with the burner on. Searing meat is the worst offender; baking and steaming are much milder. The fix is mechanical: a kitchen range hood that vents outside (not a recirculating one), used at a high enough setting and turned on before cooking starts.

NO₂ from gas appliances

Gas stoves emit NO₂ and small amounts of CO. Studies measuring kitchen NO₂ during gas-stove use routinely find concentrations exceeding the EPA's 1-hour outdoor standard, particularly in small or poorly ventilated kitchens. The strongest evidence links residential gas-stove use to increased childhood asthma risk; one widely cited 2023 meta-analysis attributed about 13% of U.S. childhood asthma to gas-stove exposure. Mitigations: vent the range hood outside while cooking, run it for 5–10 minutes after cooking, and consider an induction cooktop on your next remodel.

Volatile organic compounds (VOCs)

VOCs are a huge category — paints, cleaners, air fresheners, scented candles, vinyl flooring, new furniture, dry-cleaned clothes, personal-care products, and many building materials. Total VOC concentrations indoors typically run 2–5× outdoor levels, and "new building" or "newly remodeled" environments can run 10× or higher for the first few months. Many VOCs are simply odorous; some — formaldehyde, benzene, trichloroethylene — are carcinogens at chronic exposure levels. Strategies: ventilate when introducing new materials, choose low-VOC paints and adhesives (often labeled), let new furniture off-gas in a garage before bringing it inside.

Radon

Radon is a naturally occurring radioactive gas produced by uranium decay in soil, which seeps into homes through foundations and accumulates in poorly ventilated lower floors. The EPA estimates radon causes about 21,000 lung-cancer deaths in the U.S. per year, second only to smoking. Levels vary enormously by geography: a home in eastern Pennsylvania might run 8 pCi/L; the same model in coastal Florida might run 0.5. Test kits cost $15-30 and take a few days to weeks. If your level is above 4 pCi/L, professional mitigation (active soil depressurization) reduces it dramatically.

Mold and biological contaminants

Mold needs moisture; control moisture and you control mold. Bathroom and basement humidity above 60% is the rough threshold where problems compound. Other biologicals — dust mite allergen, cockroach allergen, pet dander — are major asthma triggers, particularly for children. HEPA filtration, hot-water laundry, and dust-mite-resistant bedding covers all reduce dose.

Carbon monoxide (CO)

CO from properly maintained appliances is rarely a problem; CO from a malfunctioning furnace, a generator run in an attached garage, or a charcoal grill brought indoors can be lethal within hours. Every home should have at least one battery-backed CO alarm, with units near sleeping areas. Replace alarms per the manufacturer's expiration date.

Outdoor air entering your home

On Moderate AQI days outdoors, indoor PM2.5 typically tracks at maybe half of outdoor — annoying but not dramatic. On smoke days, it becomes a major problem: a 200-AQI outdoor reading in an unfiltered home produces an indoor reading of 100+ within hours. The fix is the same machinery that handles wildfire smoke (covered in detail in Wildfire Smoke):

• Close the building envelope. Windows down, exhaust fans off, weatherstripping in good shape.
• HVAC on recirculate, not fresh-air intake.
• Upgrade to MERV-13 furnace filters if your blower can handle the static-pressure increase (most can; check the manual). Change them more often during smoke events.
• HEPA air purifier in the room you spend the most time in. Match CADR to room volume — a smoke CADR of at least two-thirds the room's cubic-foot volume is the rule of thumb.

Ventilation: the underrated lever

Modern energy-efficient homes are tightly sealed — great for heating bills, less great for indoor air. CO₂ is a useful proxy for ventilation: outdoor air sits around 420 ppm, while a sealed bedroom with two sleeping adults can push past 2,500 ppm by morning. Above 1,000 ppm, cognitive performance starts to degrade in controlled studies; above 2,000 ppm, most people report stuffiness and headaches.

If you have a balanced mechanical ventilation system (an HRV or ERV — heat- or energy-recovery ventilator), it solves this automatically by exchanging air with the outside while keeping conditioned air's heat. If you don't, the cheap version is a $25 CO₂ sensor on your bedside table; if numbers spike at night, crack a window. During smoke events, this tradeoff inverts — keep windows shut, accept slightly elevated CO₂.

What you don't need

The indoor-air market is full of products that don't do what they advertise. Skip these:

• Ozone generators marketed as "air purifiers". Ozone is itself a regulated pollutant that the EPA explicitly warns against generating indoors at any concentration. Some sold under "ionizer" or "plasma" branding generate ozone as a side effect; check for California Air Resources Board (CARB) certification before buying any electronic air cleaner.
• "Air-purifying" houseplants. The often-cited 1989 NASA study used sealed chambers; in real homes, you'd need hundreds of plants to match a $100 HEPA purifier. Plants are great; they're not air cleaners.
• Salt lamps. No mechanism, no effect.
• UV-only purifiers without high-efficiency filtration. UV inactivates some microbes but doesn't capture particles. Combined HEPA-plus-UV units are fine; UV alone is not enough.
• "Negative ion" generators. Marketing copy varies; the actual evidence for health benefit is weak, and many ionizers produce ozone as a byproduct.

A practical setup for most households

1. One CO alarm on every floor, near sleeping areas.
2. One radon test, run once. If > 4 pCi/L, get mitigation.
3. MERV-13 HVAC filter, swapped out 3-4× per year (more often during fire season).
4. One HEPA air purifier in the bedroom, sized to the room.
5. A kitchen range hood that vents outside, used during cooking.
6. An optional CO₂ sensor for awareness on bedrooms and offices.

That stack costs less than a thousand dollars all-in, lasts years, and produces measurable, large reductions in indoor pollutant exposure for everyone in the household. It does more for chronic health than just about any consumer-level air-quality intervention.

Primary sources: EPA — Indoor Air Quality · EPA — Radon · CDC — Indoor Air · CARB — Certified Air Cleaning Devices