Air Quality - Indoor Versus Outdoor

Outdoor air quality, in a more technical context often called ambient air quality, refers to the air outside, the open environment. Indoor air quality determines what we are breathing most of the time, given that most people spend > 90% of their time indoors. IAQ_OAQ_prettypic
Indoor air is influenced by outdoor pollution, but also by indoor sources like cooking and cleaning, as well as individual behavior and technical setups, such as ventilation.

The relationship between in- and outdoor air quality can be summarized as:

Indoor air    =    outdoor air    +    indoor sources    –    removal through ventilation and filtration

Outdoor air quality is regulated and monitored under the Environmental Protection Agency’s National Ambient Air Quality Standards (NAAQS).

Regulated key pollutants are:

  • Ground-level ozone (O3)
  • Nitrogen dioxide (NO2)
  • Fine and coarse particulate matter (PM2.5 and PM10)
  • Sulfur dioxide (SO2)
  • Carbon monoxide (CO)

Outdoor air quality drives regulatory compliance and risk on a regional scale.

Indoor air quality on the other hand is not regulated, and very individual; it depends on ambient and the building’s condition, as well as a person’s behavior. Common indoor sources are shown in the exhibit below.

A simplified cutaway diagram of a house illustrates how pollutants enter indoor air from multiple locations. The central area labeled “Inside” lists common indoor pollution sources: cooking, cleaning products, pet dander, and paints and solvents. Four surrounding areas contribute contaminants to the indoor space, shown with red arrows pointing inward: • Attic (top of the house): insulation fibers, dust, and rodent leftovers. A downward arrow shows pollutants entering from above. • Outside (left side): pollen, vehicle exhaust, dust, and smoke. A right-pointing arrow shows pollutants entering from outdoors. • Garage (right side): carbon monoxide, pesticides, gasoline, and fertilizers. A left-pointing arrow indicates pollutants entering from the garage. • Crawl space (beneath the house): mold, dust, lead, radon, moisture, and insecticides. An upward arrow shows pollutants entering from below. The diagram emphasizes that indoor air quality is affected by sources both inside the home and from surrounding areas, with directional arrows indicating airflow or pollutant movement into the living space.

Image source: NCTCOG

Key indoor pollutants and their common sources in indoor spaces are:

Pollutant

Sources

Note!

PM2.5

Cooking, smoking

These overlap with the NAAQS for ambient air quality.

Carbon monoxide (CO)

Incomplete burning of fuels - natural gas, propane, wood, or gasoline (e.g., household appliances, idling vehicles, misuse of outdoor heating and cooking equipment)

Volatile organic compounds (VOCs)

Outgassing furniture, paints, solvents, HVAC systems, cleaning agents, and cooking

Significant source of VOCs, which are ozone precursors, and therefore contribute to ozone pollution, especially in urban areas.

Biologicals

Mold, pollen, viruses from humid places around the house, pets

 

Radon

Soil and bedrock processes in the ground

 

Pollutants present in ambient air

Outdoor air infiltration

Outdoor air can enter a building through gaps and cracks, at windows and other entry points.


Older buildings are more exposed to outdoor heat and pollution, because they often have more cracks and gaps which leads to higher air leakage, while newer ones are built tighter with better insulation and sealing, but they may trap indoor pollutants if not ventilated properly.
Both areas of air quality can affect human health. Whereas outdoor air quality is episodic, and impacts the entire population, indoor air quality affects people continuously and drives actual human exposure. It can be better or worse than outdoors, and the good news is that you have more control over it.

Air Quality and Building Standards

Building standards as well as maintenance play a huge role in indoor air quality. Exhibit XXX shows common air leaks in buildings, on the left for the wintertime, on the right for summer.

air_leaks_house_summer_winter

Image source: XXX

Not all buildings let in outside air the same way. The amount of heat, humidity, and pollution that enters indoors depends on how a building is designed, built, and maintained.

Open the sections of the accordion below for more information about the individual building type.

Single-Family Homes (Detached Houses)
  • Typically have more exposure to the outdoors (all sides + roof)
  • Often leakier, especially if older or not well sealed
  • Common leak points: 
    • Attic openings
    • Windows and doors
    • Ducts in attics or crawl spaces

More outside air—including heat, humidity, and air pollution—can enter the home.

Apartments & Multifamily Buildings

Share walls, floors, and ceilings with other units, therefore usually they have less direct outdoor exposure per unit.

Air can come from: 

  • Hallways 
  • Neighboring units 
  • Building ventilation systems 

Less outdoor air may enter directly, but indoor air quality can be affected by neighbors and shared spaces.

Commercial Buildings - Offices, Stores

Indoor conditions are usually more controlled—but depend heavily on system performance and maintenance.

  • Use mechanical ventilation (HVAC systems) to control airflow
  • Often designed to limit uncontrolled air leaks
  • Frequent door openings can still allow outdoor air in

Overall

Drafty buildings let in more heat, humidity, and pollution. Well-sealed buildings offer better protection—but still need proper ventilation to stay healthy.

In North Texas, high summer temperatures and strong sunlight create ideal conditions for ozone formation—making building protection especially important. Buildings also offer protection from extreme heat, another common challenge in hot Texas summers.

How Does This Now Impact Health?

Closing the loop, indoor air quality affects most humans most, considering that we are spending about 90% of our time indoors. 

health_IAQ_OAQ

Outdoor air quality contributes, and considering that it is affecting everyone, it is in the public interest to continuously improve outdoor conditions.

For more information on indoor air quality and what you can do to improve it, visit the following websites:

Improving Indoor Air Quality | US EPA 

Add links

Sign up for air pollution alerts and learn how you can help improve air quality at www.AirNorthTexas.org.

Air North Texas Logo a blue circle with waves in the middle and Air North Texas text around the bottom linking to the Air North Texas site.

Please contact Daniela Tower with any other questions.