Indoor vs. Outdoor Air Quality: What You Need to Know

When air pollution warnings go out, the standard advice is to stay indoors. But is indoor air actually cleaner? The answer might surprise you—and it has important implications for how you protect your health.

The Indoor Air Quality Myth

Many people assume that being inside means breathing clean air. In reality:

• Indoor air can be 2-5 times more polluted than outdoor air
• In some cases, indoor pollution can be 100 times higher than outside
• We spend approximately 90% of our time indoors

This means indoor air quality often has a bigger impact on your health than outdoor pollution.

Sources of Indoor Air Pollution

Combustion Sources

Cooking: Gas stoves can produce significant amounts of:

• Nitrogen dioxide (NO2)
• Carbon monoxide (CO)
• PM2.5 (especially during high-heat cooking)

Studies have found that cooking a meal can temporarily raise PM2.5 levels above what you'd experience during a bad air quality day outside.

Heating:

• Wood-burning fireplaces
• Kerosene heaters
• Gas furnaces (especially poorly maintained)

Candles and Incense:

• Burning candles produces PM2.5 and soot
• Incense can raise PM2.5 to hazardous levels
• Scented products add VOCs to the mix

Building Materials and Furnishings

Off-gassing: New furniture, carpets, and building materials release volatile organic compounds (VOCs):

• Formaldehyde from pressed wood products
• Benzene from plastics and synthetics
• Various chemicals from paints and finishes

This "new car smell" or "new furniture smell" is actually chemical off-gassing.

Older buildings:

• Asbestos in insulation
• Lead paint dust
• Mold in damp areas

Household Products

Cleaning supplies:

• Aerosol sprays
• Bleach and ammonia products
• Air fresheners

Personal care products:

• Hairspray
• Nail polish and remover
• Perfumes and deodorants

Biological Pollutants

• Dust mites
• Pet dander
• Mold spores
• Bacteria and viruses
• Pollen that comes in from outside

Outdoor Pollution That Enters

When outdoor air quality is poor, that pollution infiltrates your home through:

• Open windows and doors
• Gaps and cracks in the building
• HVAC systems bringing in fresh air
• Attached garages

How Indoor and Outdoor Air Interact

The Infiltration Factor

Outdoor pollution doesn't stay outside. The amount that enters depends on:

Building tightness:

• New, sealed buildings: 10-30% infiltration
• Older, drafty buildings: 50-80% infiltration

Ventilation:

• Natural ventilation increases infiltration
• HVAC systems can filter or introduce pollution

Pollution type:

• Gases infiltrate more easily than particles
• Ultrafine particles enter more easily than larger ones

Typical Scenarios

Clean outdoor air, polluted indoor: In this common scenario, outdoor AQI might be good, but:

• Someone is cooking
• Candles are burning
• The vacuum was just used (stirs up dust)
• Fresh furniture is off-gassing

Polluted outdoor air, cleaner indoor: During high outdoor pollution events, you can maintain cleaner indoor air by:

• Keeping windows closed
• Running air purifiers
• Using recirculated HVAC air
• Minimizing indoor sources

Both polluted: The worst scenario combines:

• High outdoor pollution infiltrating
• Active indoor sources
• Poor ventilation
• No air filtration

Measuring Indoor Air Quality

What to Monitor

PM2.5: The most important health metric. Home monitors are increasingly affordable and accurate.

CO2: Indicates ventilation quality. High CO2 means you're re-breathing air and pollutants may be accumulating.

VOCs: Some monitors detect total VOC levels, indicating chemical pollution from products and materials.

Humidity: Both too high (promotes mold) and too low (respiratory irritation) are problems.

Recommended Levels

| Pollutant | Good | Moderate | Poor | |-----------|------|----------|------| | PM2.5 | <12 µg/m³ | 12-35 µg/m³ | >35 µg/m³ | | CO2 | <800 ppm | 800-1500 ppm | >1500 ppm | | VOCs | <0.5 mg/m³ | 0.5-1 mg/m³ | >1 mg/m³ | | Humidity | 30-50% | 50-60% | >60% or <30% |

Room-by-Room Considerations

Kitchen

Highest risk room for temporary pollution spikes.

Challenges:

• Combustion from gas stoves
• High-heat cooking creates particles
• Smoke and steam

Solutions:

• Always use exhaust fan when cooking
• Open windows (when outdoor air is clean)
• Consider induction cooking
• Clean range hood filters regularly

Bedroom

Most important room for air quality—you spend 8 hours here.

Challenges:

• Dust mites in bedding
• Pet dander if pets sleep here
• Little ventilation during sleep

Solutions:

• HEPA air purifier running at night
• Wash bedding weekly in hot water
• Use dust mite covers on mattress
• Consider keeping pets out

Bathroom

Challenges:

• High humidity promotes mold
• Cleaning products used frequently
• Poor ventilation in many bathrooms

Solutions:

• Always run exhaust fan
• Fix any leaks promptly
• Use lower-VOC cleaning products
• Open window when possible

Living Areas

Challenges:

• Candles, fireplaces, space heaters
• Dust accumulation
• Electronics and furniture off-gassing

Solutions:

• Air purifier for the main living space
• Regular dusting with damp cloth
• Limit candle and fireplace use
• Let new furniture off-gas before bringing inside

Home Office

Challenges:

• Printers emit ultrafine particles and ozone
• Electronics generate some pollution
• Poor ventilation in small spaces

Solutions:

• Keep printers in separate rooms if possible
• Small air purifier for the space
• Open windows periodically (when outdoor air is good)

Strategies for Balancing Indoor/Outdoor Air

When to Open Windows

Open windows when:

• Outdoor AQI is lower than indoor PM2.5
• You've been cooking and need to clear smoke
• CO2 levels are high (stuffy feeling)
• After using strong cleaning products

Keep windows closed when:

• Outdoor AQI is elevated
• During wildfires or high pollution events
• Near heavy traffic during rush hour
• During high pollen seasons (if allergic)

Creating Clean Air Zones

During extended poor outdoor air quality:

1. Pick a room (usually bedroom)
2. Seal it as much as practical
3. Run a HEPA air purifier sized for that space
4. Spend time there especially for sleeping

This "clean room" strategy can significantly reduce your overall exposure.

Balancing Ventilation and Filtration

The key is to:

• Ventilate during good outdoor air quality periods
• Filter continuously, especially during poor outdoor conditions
• Minimize indoor sources
• Monitor both indoor and outdoor levels

Seasonal Considerations

Summer

• Higher ozone levels outside
• Air conditioning keeps windows closed
• But AC circulates air—make sure filters are good

Winter

• Less outdoor ozone
• But heating and fireplace use create indoor pollution
• Tighter buildings mean less fresh air
• CO2 can build up

Fire Season

• Keep everything closed
• Maximize indoor filtration
• Create clean room if extended event

Spring/Fall

• Good times for natural ventilation
• Watch for pollen (if allergic)
• Moderate temperatures allow flexibility

The Bottom Line

Indoor air quality matters as much—often more—than outdoor air quality. The good news is that you have much more control over your indoor environment.

Key takeaways:

1. Don't assume indoor air is clean—measure it
2. Control indoor sources of pollution
3. Filter your air with HEPA purifiers
4. Ventilate strategically based on outdoor conditions
5. Pay extra attention to your bedroom

By managing both indoor sources and outdoor infiltration, you can significantly reduce your overall pollution exposure—regardless of what the outdoor AQI says.

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Understanding how outdoor air quality affects you is the first step. Use our AQI to Cigarettes Calculator to visualize what your outdoor exposure really means.