Indoor air quality

Although indoor air quality is linked to outdoor air quality, there are some specific issues to be considered, especially since most people spend much of their time indoors.

Outdoor conditions

Outdoor air quality will inevitably affect indoor air quality. Outdoor air can be brought inside through doors and windows, and air-conditioning and ventilation systems. The level of pollution brought inside depends on how close the building is to roads or industrial processes. For example, Wheeler et al. (2020) assessed samples of dust from roof cavities in the Latrobe Valley in Victoria and found that the chemical components of the dust could be traced to coal burned during the Hazelwood mine fire. The composition indicated the level of exposure during the event.

Controlling the timing of ventilation or other activities can significantly affect indoor air quality. For example, indoor air-conditioning systems need an intake of air from outside. Often, workplaces and schools are located near busy roads, which affect the pollution levels of the air intake. However, restricting the timing of air circulation until after the morning commute reduced the levels of particulate matter and volatile organic compounds by around 40% (MacNeill et al. 2016).

Access to near-real-time outdoor measurements of pollen or smoke levels would enable residents to assess the best time for opening or shutting doors and windows. An online app, ‘AirRater’, was developed by the University of Tasmania to provide this information in some jurisdictions (Johnston et al. 2018). Users can also supply information about their symptoms (e.g. hayfever, asthma), which AirRater uses to identify hotspot areas for poor air quality.

Bushfire smoke

During the bushfire season, when the air quality outdoors can be classed as ‘extremely poor’, recommendations to the public often involve staying indoors with doors and windows closed. Reisen et al. (2019) found that the level of protection offered by following these recommendations varied from just 12% to 76% and depended on the age and quality of the house. ‘Leaky’ houses allowed smoke to penetrate indoors easily and offered little protection. Some houses are so leaky that remaining indoors can present real health risks, such as during the 2019–20 summer bushfire period when the air quality was classed as ‘extremely poor’ for many consecutive days (see Summer 2019–20 bushfires).

When air quality in the home is poor, people are advised to move into a ‘clean air room’. Clean air rooms are buildings that have high-efficiency particulate air (HEPA) filters, such as large public buildings and shopping centres. Measurements inside a Port Macquarie library between August and October 2019 (incorporating some prescribed burns and peat fires) showed how running a HEPA filter made indoor levels of fine particulate matter (PM2.5) 83% lower than outdoor levels (Wheeler et al. 2021). Having access to a clean air room is recommended as part of a smoke health protection strategy (Vardoulakis et al. 2020).

Indoor sources of air pollution

As well as influences from outside, indoor sources of air pollution can also affect air quality. Sources of indoor air pollution include:

  • building materials, carpets and furnishings
  • home renovation activities such as painting
  • perfumed personal care products (e.g. soaps, shampoos), air fresheners and cleaning products
  • home office devices such as printers and photocopiers
  • heating and cooling systems
  • smoking
  • fuel-burning devices for heating or cooking
  • mould.

Recent studies examined exposure to volatile organic compounds (VOCs) inside the home with the use of air fresheners, and fragranced laundry and cleaning products. Residents reported asthma (Steinemann & Goodman 2019) and migraine symptoms (Steinemann & Nematollahi 2020) when exposed to fragranced products. Discontinuing the use of fragranced laundry products reduced VOC concentrations from dryer vents by 99% over 2 weeks (Goodman et al. 2020).

Pollutants occurring indoors are not naturally removed by the processes that occur outdoors, such as wind and rain. Chemical reactions to break down indoor air pollutants do still occur, but at slower rates than outdoors. This is because these reactions rely on the production of oxidants under the influence of light, and there is less direct sunlight within the house (Weschler & Carslaw 2018). The chemical breakdown of some indoor pollutants can create other pollutants, such as VOCs breaking down to produce ozone and fine particles.

Buildings that are more energy-efficient often recirculate air, which limits the amount of fresh air entering the building and can trap pollutants inside for longer. Air exchange rates – that is, the number of times the air in a room is completely replaced per hour – are important.

Airborne pathogens

Australian scientists have been at the forefront of research into how poor ventilation systems have been implicated in spreading coronavirus (Morawska et al. 2021). Incidents of infection within the hotel quarantine system may have been caused by contaminated air being transferred from room to room by opening doors at the same time, or via air-conditioning systems that connect between rooms. Virus-containing aerosol exhaled by an infected person can remain in the air for minutes to hours, depending on the air exchange rate in the building. The air exchange rate can be inferred using cheap carbon dioxide (CO2) sensors; humans breathe out CO2, and levels will quickly rise if many people are together in a room with poor ventilation. Morawska et al. (2021) suggested that better ventilation would deliver clean and disinfected air (potentially via ultraviolet treatment) optimised for the number of room occupants and the types of activities occurring. Indoor air quality standards of the future should also consider airborne pathogen counts. Information about building standards is in the Urban chapter (see the Urban chapter).