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.
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) add link.
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, very individual, and depends on ambient and the building’s condition, and a person’s behavior.
Key pollutants in indoor spaces are:
Table
Indoor air quality drives actual human exposure. It can be better or worse than outdoors, and the good news is that you have more control over it.
Building standards and maintenance play a huge role in indoor air quality, see exhibit XXX for common air leaks in buildings, here shown for the wintertime.
Exhibit
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, but impacts the entire population, indoor air quality affects people continuously.
Indoor AQ - Infiltration variability by building type
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.
If you’re interested in more details on building types, open the sections in the accordion below.
- 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.
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.
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
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.
How does this now impact health?
Outdoor Heart + Ozone
Building Type and Technical Setup (Filters/ purifiers etc.)
Indoor conditions (temperature + pollutants + humidity)
Health impacts
Steps to improve indoor air quality:
- Source Control
- Airtightness
- Filtration
- Ventilation
- Moisture Control
- Pressure Balancing
- IAQ Monitoring
Improving Indoor Air Quality | US EPA More information