The Interaction between Outdoor and Indoor Air Quality

Poor air quality is known to significantly affect human health, and comfort. While outdoor air quality often receives greater attention due to public awareness and legislation, the interaction between outdoor and indoor air quality is important and often overlooked. Understanding this relationship is the first step in creating healthier living and working environments, particularly as we spend around 80% of our lives indoors.

How does outdoor air influence indoor air quality?

Opening windows to let ‘fresh’ air into a space is a natural response to feeling that a room is stuffy, especially if there are unpleasant odours indoors and the air outside is perceived to be clean. Whilst, across much of the UK, opening doors or windows to ventilate indoor air is relatively safe, outdoor air could be more contaminated, particularly in busy urban environments. This is particularly likely if windows are opened at ground floor level, close to heavily trafficked or congested roads, or effuse points from industrial processes.

The age-old issue with raising awareness of air pollution is that we can’t see it and often, we also can’t smell it. Therefore, opening windows to ‘purify’ indoor air can simply be an illusion. As such, in highly polluted locations, such as Air Quality Management Areas (AQMAs), windows may be best left permanently shut and mechanical ventilation systems are used to actively remove contaminants.

Nevertheless, outdoor pollutants can still find their way indoors, especially in older, poorly sealed buildings. Fine particulate matter (PM_2.5), nitrogen dioxide (NO₂), and ozone (O₃) to name a few, can pass through gaps in façades and contribute to poor indoor air. These effects may be particularly prevalent in built up urban areas with ‘street canyons’, with busy roads between tall buildings. This creates a barrier that shields pollutants from the wind or leads to their recirculation, thus reducing their dispersion.

How do indoor pollution sources contribute to the problem?

There are countless household products which contribute to the worsening of indoor air quality, however due to a lack of research and public awareness in comparison to outdoor sources, and the usefulness of these chemicals in our everyday life; little action has been taken to educate the public on the known health effects or to restrict their use within the home.

Volatile organic compounds (VOCs) are released from common household sources such as cleaning and personal care products, air fresheners and candles, flooring, paints and upholstery, and can – often unknowingly – disrupt focus and cause discomfort, headaches, and respiratory conditions.

Also significant is particulate matter (PM₁₀, PM_2.5) which is released from cooking and combustion processes. A new study found that pollutants from gas stoves kill around 40,000 Europeans each year, twice as many as killed from car crashes. Interestingly, air frying was found to be the least polluting cooking method (adding to the long list of reasons you should get one!).

How can we tackle it?

Tackling indoor air quality is a complex issue, however a good starting point is to understand the interaction between indoor and outdoor air pollution as much as possible, to isolate any issues. Below are some measures that could be factored into the building design:

• Ventilation Control: Using high-efficiency particulate air (HEPA) filters in heating, ventilation, and air conditioning (HVAC) systems can ventilate rooms without the need for openable windows that may compromise air quality.
• Sealing and Insulation: sealing windows, doors, and other openings properly can limit the unintentional entry of outdoor pollutants, especially in high-traffic or industrial areas. However, awareness of the potential to trap indoor pollutants is vital.
• Accreditation: there are many guidance documents which set standards for indoor air quality, such as the WELL Building Standard and BREEAM. Gaining the Hea 02 Indoor Air Quality BREEAM credits is ensures that indoor air quality has been considered throughout the design, build and fit-out and that future building occupants can enjoy a safe and healthy indoor air environment.

https://www.theguardian.com/environment/2024/oct/28/pollutants-from-gas-stoves-kill-40000-europeans-each-year-report-finds#:~:text=Gas%20stoves%20kill%2040%2C000%20Europeans%20each%20year% 20by%20pumping%20pollutants,public%20awareness%20of%20their%20dangers

https://www.birmingham.ac.uk/news/2024/air-fryers-confirmed-as-least-polluting-cooking-method

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