Healthy Air Technology
Why can indoor air quality be worse than outdoor air quality?

Why can indoor air quality be worse than outdoor air quality?

27 January 2023

When thinking of air quality and pollution many people’s imaginations will likely conjure images of smoke billowing from chimneys, or fumes from cars but we explain why indoor air quality can be worse than outdoor air quality.

The belief that outdoor air quality is worse is unsurprising as outdoor air quality is given considerably more airtime by the government and the media. In the UK there are legal limits to air pollutant concentrations, designed to protect members of the public, in outdoor air, but no such limits for indoor air.

This is a dangerous situation as indoor air quality is often far worse than outdoor air quality, For example, it has been demonstrated that exposure to PM2.5 (a particularly dangerous pollutant) can be greater whilst cooking an omelette than on an average London street[1] and that the cooking of thanksgiving dinner can cause air pollution concentrations higher than on the streets of Delhi, one of the most polluted cities on Earth[2].

Studies into indoor air quality

Several studies by the US Environmental Protection Agency have reported that indoor air pollutant concentrations in homes, workplaces, and school classrooms are regularly two to five times higher than outdoor air[3]. Studies also show that in some situations, indoor air pollutant concentrations can be orders of magnitude worse, with concentrations 100 times3 and up to 560 times higher[4] reported. Studies also regularly determine that indoor pollutant concentrations exceed World Health Organisation (WHO) guidelines, leading to adverse health outcomes.

Indoor air quality is not solely a measure of the chemical composition of the indoor environment. Mould, bacteria, virus and other pathogens all play a role in the health of indoor environments and is an aspect of indoor air quality which has been brought into sharp focus by the Coronavirus pandemic, where indoor environments have acted to boost transmission and facilitate its spread.

As people spend up to 90% of their time inside[5], overall exposure to air pollution is likely far greater inside than outside, and the risks far less well known. There is, however, an increasing appreciation of the importance of a good indoor environment for people’s health, wellbeing, and productivity.

What influences indoor air quality?

To understand why indoor air quality can be worse than outdoor air quality we need to understand what influences it.

Pollutants in indoor air come from exterior, interior and ground-based sources. Examples of exterior pollutants, which infiltrate inside,  include those emitted from cars and industry. Interior sources include cooking, building materials, cleaning products, wood burning stoves, candles, human beings themselves and shockingly, even some technology used to purify air can be a source of dangerous pollutants. Whilst ground-based sources include gases such as Radon, formed by the decomposition of naturally occurring radioactive materials, as well as gases from landfill. A comprehensive analysis of the types of indoor aerosols, and their sources, can be found here.

Ventilation plays a key role in the control of indoor air quality. It can aide IAQ by conveying indoor pollutants outside, or through the dilution of polluted indoor air with cleaner outside air. However, when there is inadequate ventilation, these beneficial processes are reduced, resulting in the build-up of pollution.

Why indoor air quality can be worse than outdoor air quality

Managing indoor air quality can be complex as rooms and spaces often have different uses, occupancy rates and may be influenced by different sources of pollution depending on their location within a building. This can lead to drastically different indoor air quality environments, in what appear to be similar spaces. For example, between two neighbouring homes in the UK[6], a Volatile organic compound (VOC) differed by over 1,000 times. The author determined this discrepancy was due to differences in occupant cleaning activity.

Indoor environments are partially closed off systems, with indoor air quality reflective of the balance between pollutant emissions and the infiltration of exterior sources, and rates of dilution modulated by the ventilation system. In outdoor environments, there is no comparative barrier to pollutant dispersion which means that dilution occurs quickly. In indoor environments there is also a much higher ratio of air to surface upon which chemical reactions can take place, meaning that the air in indoor environments is more reactive, and secondary pollutants (i.e., those formed due to chemical reactions, instead of being directly emitted) form more readily.

As well as being more closed off, indoor spaces have unique uses and a wide variety of sources. VOCs are known to be emitted by a wide array of products numbering in the thousands[7]. Printers and photocopiers are also known to be a localised source of ozone[8], a respiratory irritant.  Humans themselves are a significant source of VOCs, with humans emitting over 2,000 compounds from their skin and breath alone[9].  Furthermore, rooms with high occupancy rates (or areas with insufficient ventilation) can lead, through people respiring, to increased levels of carbon dioxide that impact the ability to concentrate, limiting productivity.

The UK has some of the least airtight buildings in Europe, meaning that polluted air from the outside can infiltrate into properties more easily.  In urban areas where ambient air pollution is high, this often results in inside concentrations elevated above outside concentrations.  There is however a trend which is seeing buildings become more airtight[10]. This is understood to be beneficial for reducing the infiltration of outdoor pollutants such as nitrogen dioxide (NO2)[11], but does present a concern for levels of common indoor air pollutants such as formaldehyde and VOCs, which will not be conveyed out of the property as quickly and peaks in pollution will take longer to disperse. Tighter buildings, without appropriate ventilation systems, will result in increases in humidity and the associated effects of mould and dust mites, which thrive in humid conditions.

Poor design, negligence, or a lack of understanding of the holistic nature of indoor environments can also lead to a deterioration in indoor air quality. Air fresheners, the use of cleaning products and some air purifiers, can all lead to poorer indoor air quality.  Shockingly some indoor air quality devices have also been shown to increase the concentration of highly reactive chemical species by several orders of magnitude[12]. The use of heating, ventilation and air conditioning (HVAC) systems which recirculate air is a known risk factor in the transmission of virus and other pathogens.

There has been a significant increase in the use of woodburning stoves to heat homes since 2010. Reasons for this trend include the effect of social media, the idea of wood as a renewable fuel and the cost of gas and electricity prices. Whilst most emissions from wood-burners are extracted up the flue, studies have shown that operating a stove inside can increase the concentration of dangerous PM2.5 to three times greater than concentrations outside[13].

Other factors which influence IAQ include natural light, temperature and humidity, with increased humidity and temperature linked to increased formaldehyde emissions from everyday furnishings[14], and increased reactivity of indoor spaces. Natural light, and associated UV rays, play a key role in sterilising surfaces as the DNA of pathogens are destroyed under such conditions. As such, a lack of natural light can play a role in the effective transmission of Coronavirus and other respiratory illnesses.


There is no silver bullet when trying to improve indoor air quality and isolated attempts to try and improve IAQ can have unintended consequences. Without doubt, indoor air quality is given insufficient consideration by the UK government, media and members of the public. Indoor air quality is often far worse than outdoor, and as the majority of people spend significantly longer periods of time inside, indoor air quality is likely far more of a determinant of their health and wellbeing.

To conclude, if you can take one thing from this post, I suggest that when you hear ‘air pollution’ or ‘air quality’, you take a moment to consider not only cars, and industry but also the air fresheners, candles, cleaning materials, everyday dust and perhaps even that black mould in your bathroom, and consider how you can improve your health and wellbeing by improving the air quality in your own home.

Author: Harley Parfitt, Greenavon Air Quality Consultants

About the Author

Harley Parfitt runs Greenavon, an air quality consultancy that specialises in providing air quality assessments, including indoor air quality assessments, for planning applications across the UK. For more information, visit















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