Outdoor air pollution usually makes major air quality headlines.

A flight is canceled to Delhi because smog is too dense (1). A red alert for outdoor air pollution is declared due to record levels of dangerous outdoor pollutants (2).

And nearly 7 million people die from air pollution each year due to heart disease and respiratory conditions linked to air pollution (3).In 2020 alone, over 160,000 people died from air pollution in just five of the world’s most populous cities.

In comparison, the difference between indoor and outdoor air pollution doesn’t seem as severe. But research suggests that indoor and outdoor air pollution are closely linked.

Understanding the relationship between indoor vs. outdoor air quality is your strongest weapon against the effects of pollutants on your health.

Your behavior and environment both influence the interaction between indoor and outdoor pollutants, so changing both your habits and your home is crucial to minimizing outdoor air pollution’s effect on indoor air.

Is indoor air quality better than outdoors?

Is indoor air more or less polluted than outdoor air? Indoor air quality can be more deadly vs. outdoor air pollution because it affects you in places where you spend 80% or more of your time each day – and outdoor air pollution that gets indoors can build up to extremely high concentrations (4).

A two-year study of nearly 10,000 participants conducted by the Lawrence Berkeley National Laboratory found that people spend about 87 percent of their day in homes or buildings, and another 6 percent in enclosed vehicles (5).

Several reports by the U.S. Environmental Protection Agency (EPA) have demonstrated that indoor air pollution levels in homes, workplaces, and school classrooms are typically 2-5 times higher than outdoor pollution levels and can quickly become 100 times worse than outdoor air pollution (6,7).

But what makes air quality indoors much deadlier vs. outdoor air pollution?

First, know that outdoor air pollution generally comprises the following pollutants:

• PM10: Particulate matter that’s smaller than 10 microns, such as dust, pollen, and mold (for reference, a human hair is 50-70 microns in size). Many outdoor PM10 particles have natural sources, such as construction and agricultural activities.
• PM2.5: Particulate matter that’s smaller than 2.5 microns. Most outdoor PM2.5 is produced by human activity, such as vehicle exhaust, factory emissions, and smoke from burning wood and biomass fuels. PM2.5 is also a major component of wildfire smoke.
• Ultrafine particles: Ultrafine particles (UFPs) are smaller than 0.1 micron in diameter. These tiny particles can penetrate through lung tissue into the bloodstream and nearly every organ in the body.
• Ozone: Often just called smog, tropospheric (ground-level) ozone (O3) results from heat reacting with pollutants low in the atmosphere. Gases from vehicles and industrial processes, such as nitrogen oxide (NOx) and volatile organic compounds (VOCs) are the most common sources of ozone.

Concentrations of these outdoor pollutants rise and fall constantly because of changes in the weather, climate, and human activity.

For example, outdoor pollutants can build up in the lower atmosphere due to temperature inversions. This happens during periods of cold weather when warm air rises into the upper atmosphere and traps cold air beneath it, causing pollutants to build up at low altitudes (8).

Concentrations may also rise quickly in the mornings during rush hour traffic but subside once traffic diminishes and wind and heat clear the air of excess pollutants by dispersing them on wind currents and allowing them to rise higher into the atmosphere (9).

In this sense, the earth has its own natural air-purifying technology that disperses outdoor pollutants far and wide, helping keep air pollution concentrations at relatively safe levels in most of the world.

But indoor air pollutants aren’t always exposed to similar processes to minimize their concentrations. Ventilation can bring in fresh outdoor air to dilute indoor pollutants, but can also introduce even more pollutants into your indoor air from the polluted outdoor air, especially during extreme air pollution events like wildfires.

Outdoor air pollutants infiltrate your indoor air in ways that may not be immediately obvious – most commonly, through open windows and doors as well as cracks in walls, doors, and window sealants.

Open windows and doors

When your home or building feels stuffy, your first instinct may be to open windows and doors to let in “fresh air.”

But when outdoor pollutant levels and the air quality index is high, ventilating the indoors with outdoor air can make your indoor air pollution worse. In those instances, your air quality indoors vs. outdoors air quality may be better with the windows closed.

In general, it’s recommended to regularly let in lots of outdoor air to decrease concentrations of indoor pollutants and toxic gases, such as ultrafine particles and carbon dioxide (CO2).

But your exposure to outdoor PM10 and PM2.5 increases significantly when polluted outdoor air infiltrates your home or office in such large amounts (10).

A 2016 report from the Board on Population Health and Public Health Practice found that anywhere from 10 to 100 percent of indoor air pollution consists of outdoor air pollution that has infiltrated indoor air (11).

Outdoor ground-level ozone and other outdoor gases can also enter a home or building through ventilation processes and even react with chemicals in your building materials to create harmful chemical byproducts (12).

Research shows that indoor ozone from outdoor sources, even at low levels, can trigger asthma symptoms and lead to respiratory problems (13,14).

A 2009 study in the Journal of Asthma found that increasingly high levels of indoor ozone, especially during hot summer months, were closely correlated with airway obstruction, heightened levels of white blood cells associated with inflammation, and a lower reported quality of life in people with both asthma and allergies (15).

A 2019 study in Frontiers in Immunology also found a link between ground-level ozone and long-term health effects like emergency respiratory disease symptoms, impaired lung function, and higher risks of death from heart and lung diseases (16).

If you’re wondering “at what AQI should I close the windows,” according to the World Health Organization (WHO) guideline it’s best to avoid exposure to an hourly average of PM2.5 over 15 µg/m3. That translates to a U.S. AQI of 57. However, it’s equally important to remember that no amount of air pollution exposure is safe.

Cracks in walls and windows

The most sinister points of entry for outdoor air pollutants are tiny, often undetectable cracks, openings, and gaps in your walls and windows.

Older, less energy-efficient homes are especially susceptible to leaks of outdoor pollutants indoors because they’re less airtight than newer, energy-efficient homes specifically designed to limit the rate of exchange between indoor and outdoor air (17).

Older homes also tend to be more damaged by age and weather. This causes seals and weather-stripping around doors, windows, and other openings to break down, doing little to keep polluted outdoor air from seeping in.

A 2015 study in Science and Technology in the Built Environment found that leaky homes resulted in the highest human exposure levels to PM2.5 and UFPs (18). In some cases, even homes that used MERV 5 air filtration could not filter enough indoor air pollution to prevent exposure.

But newer, airtight homes also have their downsides. Because the exchange rate between indoor and outdoor air in these homes is low, pollutants that do get inside build up more quickly (19).

Some of these pollutants, such as ozone and nitrogen dioxide found in vehicle exhaust, react with off-gassing from building materials and furniture as well as with chemicals in cleaning supplies and other household items, creating new compounds like formaldehyde that make indoor air even more toxic (20).

How do I minimize my exposure to pollution?

Whether you’re concerned about air quality inside vs. air quality outside, you’re always at risk for exposure to airborne pollutants from both indoor and outdoor sources. Try some of the following tips to help reduce your exposure to both outdoor and indoor air pollutants:

• Monitor your indoor air quality. Live air quality data can inform you when your air is becoming too polluted and indicate when you need to mitigate the sources and effects of air pollution. Look for an air quality monitor that uses laser and infrared sensors to measure real-time air pollution levels, especially PM2.5 and CO2 that can rise to dangerous levels indoors.
• Control indoor pollution sources. Mold, dust mites, gases from cooking and heating, and tobacco smoke are the most significant sources of indoor air pollution. To limit these sources:
  • Keep your indoor humidity level around 40% to prevent mold growth.
  • Use a range hood over your stove to capture toxic chemicals produced by cooking.
  • Never smoke indoors. Tobacco contains over 4,000 chemicals, including 60 carcinogens, that leave thirdhand smoke residue on your carpet, walls, and even dust.
• Ventilate your home wisely. Open windows and use fans regularly to disperse indoor pollutants. But if outdoor pollution rises to unhealthy levels, consider keeping your doors and windows closed until air quality improves. In the meantime, a high-performance air purifier with a gas and odor filter can filter both particles and gases from your indoor air.
• Use a high-performance air purifier. Indoor air inevitably becomes polluted from outdoor pollution. When full source control is difficult or impossible, use a high-performance air purifier to filter pollutant particles from your indoor air.
• Use a whole-house or commercial air purification solution. A whole-house air purifier filters both outdoor and recirculated indoor air throughout an entire indoor space rather than in specific areas. HVAC air filtration can also help reduce air pollutants in large facilities like office buildings or school classrooms.
• Use a personal or travel air purifier. Access to clean air isn’t always guaranteed, especially at work or while traveling. A personal air purifier can instantly provide clean air directly to your breathing zone. A car air purifier can also help protect you from air pollution in vehicle exhaust.

You spend 80 percent or more of your time indoors during an average day. Making a few small changes to your habits and your indoor environment can help you breathe cleaner indoor air and protect your health.