Indoor Air Quality: The Complete Guide to Sources, Health Risks & Solutions
You spend 90% of your time indoors. The EPA estimates indoor air is 2–5× more polluted than outside. From gas stoves to radon, here's what's actually in your home's air and what to do about it.
Key Facts
The Main Indoor Pollutants
PM2.5
Gas cooking, candles, incense, smokers, outdoor infiltration
Cardiovascular disease, lung cancer, respiratory inflammation
Yes — affordable PurpleAir, Awair, IQAir AirVisual
HEPA purifier, ventilate cooking, no smoking indoors
NO₂
Gas stoves and ovens are the #1 indoor source
Asthma trigger, reduces lung function in children, respiratory irritant
Specialty sensors (Atmotube, AirThings View Plus)
Electric induction stove, exhaust fan over gas stove, open windows when cooking
VOCs
Paints, adhesives, furniture, cleaning sprays, air fresheners, nail polish
Eye/throat irritation, headaches, some compounds carcinogenic (formaldehyde, benzene)
TVOC sensors (Awair, AirThings) — broad detection
Low-VOC products, ventilate new furniture, avoid spray cleaners
Radon
Natural uranium decay in soil — enters through basement cracks
#2 cause of lung cancer after smoking; 21,000 deaths/year in USA
Long-term radon test kits (~$20); professional testing
Basement sealing, sub-slab depressurization system
CO (Carbon Monoxide)
Gas appliances, fireplaces, attached garages, generators
Headaches, cognitive impairment at low levels; fatal at high levels
Required by law in most US/EU jurisdictions; cheap detectors work
CO detector, annual appliance servicing, never run engines indoors
Mold spores
Water leaks, high humidity (>60%), bathroom/kitchen condensation
Allergies, asthma triggers, lung infection in immunocompromised
Visual inspection; professional mold test kits for confirmation
Fix water leaks, dehumidifier, exhaust fans, anti-mold paint
Formaldehyde
MDF furniture, plywood, flooring, new carpets, cigarette smoke
Nose/throat irritation; classified human carcinogen (IARC Group 1)
HCHO sensors built into some Awair/IQAir units
Choose CARB Phase 2 / TSCA compliant products; ventilate new furniture
Gas Stoves: The Hidden Indoor Air Crisis
A 2022 Stanford study found that gas stoves leak methane continuously — even when off. But the bigger air quality issue is what happens when they're on:
What gas cooking generates
- • NO2: 100–400 μg/m³ in kitchen during cooking
- • PM2.5: 50–200 μg/m³ from browning/frying
- • CO: 5–30 ppm during peak cooking
- • Benzene, formaldehyde from high-heat cooking
Health context
- • Cooking NO2 spike: outdoor EPA standard for 1-hr exposure
- • Study: homes with gas stoves have 50–400% higher NO2
- • Children in gas stove homes: 42% higher asthma risk (meta-analysis)
- • Effect equivalent to having a smoker in the home
Air Purifiers: What Works
Air purifiers are the most evidence-backed indoor air quality intervention. But the market is full of misleading claims. Here's what's real:
Captures 99.97%+ of particles ≥0.3 μm — including PM2.5, PM10, pollen, pet dander, mold spores. This is the gold standard. Look for 'True HEPA' not 'HEPA-type'.
Adsorbs VOCs, odors, formaldehyde, NO2. Essential complement to HEPA for gas/VOC removal. Needs regular replacement (every 3–6 months for heavy use).
Can kill bacteria and viruses if exposure time is sufficient. Most purifiers pass air too quickly for meaningful UV inactivation. Better suited as a supplemental feature than primary technology.
Generate ozone as a byproduct — ozone is an outdoor pollutant that you don't want to concentrate indoors. May cluster particles (makes them fall) but doesn't capture them. Avoid ozone-generating 'air cleaners'.
Some units generate byproducts including formaldehyde and acetaldehyde. Technology is inconsistent. Not recommended unless specifically vetted.
How to size a HEPA purifier correctly
The key metric is CADR (Clean Air Delivery Rate) — cubic feet per minute of clean air. Rule of thumb: CADR should be at least ⅔ of room area in square feet.
| Room size | Minimum CADR | Example models |
|---|---|---|
| 150 sq ft (bedroom) | 100 CFM | Levoit Core 300, Coway Mighty |
| 300 sq ft (living room) | 200 CFM | Levoit Core 400S, Winix 5500-2 |
| 500 sq ft (open plan) | 333 CFM | Coway Airmega 400, Blueair 211+ |
| 1000+ sq ft | 667 CFM | IQAir HealthPro Plus, Rabbit Air A3 |
The Ventilation Paradox
The classic advice is "open your windows for fresh air." But this is only correct when outdoor AQI is lower than indoor levels. In polluted cities, the calculation reverses:
Open windows when:
- • Outdoor AQI < 50 (Good)
- • Cooking — even on poor outdoor days
- • After using spray cleaners or paints
- • Moving in with new furniture (off-gassing)
- • Early morning when outdoor NO2 is lowest
Keep windows closed when:
- • Outdoor AQI > 100
- • Wildfire smoke nearby
- • Saharan dust intrusion event
- • Peak traffic rush hour (8–10am, 5–7pm)
- • Near a high-pollen source during allergy season
Frequently Asked Questions
Do houseplants improve indoor air quality?
Plants have a negligible effect on indoor air quality in real living conditions. NASA's famous 1989 study was done in sealed chambers with very high pollutant concentrations — you'd need 680 plants per 1,000 sq ft to replicate their results. Plants are great for well-being and humidity, but should not substitute for HEPA purifiers, ventilation, or source control.
Is air freshener bad for indoor air?
Yes. Most conventional air fresheners are VOC-release devices — they mask odors with chemical fragrances (limonene, alpha-pinene) that react with indoor ozone to form formaldehyde and secondary organic aerosols. The EPA lists fragrance compounds as significant indoor VOC contributors. Better alternatives: ventilate to remove odor sources, use baking soda for odor absorption, or choose unscented products.
Should I worry about indoor air quality if I live in a clean-air city?
Yes. Even in cities with low outdoor AQI, indoor sources are often the dominant exposure. Gas cooking in a well-sealed modern apartment generates NO2 spikes exceeding outdoor pollution standards. Radon exposure is independent of outdoor air quality. Building materials in new construction off-gas VOCs for 6–24 months regardless of outdoor conditions.
What is the most cost-effective indoor air quality improvement?
For most homes: (1) A HEPA air purifier in the bedroom (~$100–150 for a quality unit) — you sleep 8 hours there, highest ROI; (2) Always using the stove exhaust fan when cooking; (3) A $20 radon test kit if you have a basement or ground floor. These three steps address the dominant particle, combustion, and radioactive risks respectively at minimal cost.