Fresh Indoors: Navigating the Challenges of Outdoor Air Pollutants 

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Many people want to know, “which is more polluted, indoor or outdoor air?” The answer is, “It depends.”

Outdoor versus Indoor Air Quality

Weather and seasons impact the link between outdoor and indoor air quality. A number of things to consider:

  • Temperature. When it’s hot, ground-level ozone, a major part of smog, develops. In colder months, people tend to keep windows and doors closed. This reduces air flow and leads to the buildup of indoor pollutants.
  • Humidity. Dampness (i.e., high humidity) promotes mold growth and increases indoor air pollutants like dust mites. On the other hand, air that’s too dry can irritate the nose, throat, and lungs and increase the risk of infections.
  • Wind. When wind dilutes air pollutants, it improves air quality. However, strong winds can also stir up dust and allergens. This increases the amount of pollution in the air.
  • Pollen. As a rule, pollen levels increase during spring and early summer. This increases symptoms for sensitive people.
  • Inversions and air stagnation. Temperature inversions and stagnant air can trap pollutants close to the ground. This can impact indoor air quality, particularly if there isn’t much air flow.
  • Seasonal activities. Humans also impact outdoor and indoor air quality, for instance, burning wood in stoves and fireplaces during the winter.

Combining Ventilation and Filtration

Combining ventilation and air filtration systems is key to achieve good indoor air quality. Heating, Ventilating, and Air Conditioning (HVAC) systems in buildings can control the flow of air, remove pollutants, and dilute indoor contaminants. These systems can create more healthy indoor spaces and reduce the impact of outdoor air on indoor air quality.

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Ventilation systems help control the flow of outdoor air into indoor spaces. They also remove stale indoor air. Proper air flow is needed to dilute indoor pollutants, control humidity, and provide fresh air to people in the building. These systems include:

  • Natural ventilation: This relies on the natural flow of air through windows, doors, and other openings. It can be useful in areas with good outdoor air quality but is not so good in areas with high outdoor pollution levels.
  • Mechanical ventilation: These systems use fans and ducts to actively move air in and out of the building. Mechanical ventilation provides more controlled air flow. This helps maintain good indoor air quality even when outdoor air quality is poor.
  • Balanced ventilation: Balanced systems (e.g., energy and heat recovery ventilators), combine parts of supply and exhaust ventilation. These systems bring in fresh outdoor air and remove stale indoor air while cutting down energy loss.

Air filtration

Systems with air filters trap particles and contaminants on media. These systems can be distinctly helpful to reduce the impact of outdoor air pollution on indoor air quality. Some common systems include:

  • HVAC filters: These filters are part of the building’s HVAC systems. They help remove particles from the air circulating through the system.
  • Portable air purifiers: Stand-alone air purifiers with high-efficiency particulate air (HEPA) filters can be effective to remove particles, allergens, and some pollutants that are gases from indoor air. They can be especially helpful in rooms with poor air flow or during times of poor outdoor air quality.
  • Activated carbon filters: These filters can help remove gaseous pollutants, such as volatile organic compounds (VOCs) and odors. The filters attract and hold pollutants onto the filter’s surface.


Outdoor conditions are constantly changing. BuildingLens uses a variety of tools to manage indoor air quality, responding to changing conditions in real-time. This better protects the health of the people inside our buildings than preset systems with no remote building management controls.


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