In countries like the UK, people can spend up to 90% of their time indoors – exposing them to invisible and often ignored sources of air pollution. 

Workplaces, shops, schools, hospitals are regulated under health and safety laws. But it is a growing concern that people’s homes are not typically monitored or regulated for their air quality, especially as indoor air pollutants are harmful to our health. 
For people who are already at risk of strokes, lung cancers, asthma, and respiratory infections, the types of chemical and small particle pollutants commonly detected inside buildings are an unknown threat. 

The quality of indoor air has been studied far less than outdoor air, and it is a much more complex pollution problem that scientists are working to understand through real-life measurements, laboratory experiments, and computer simulations.

We spoke to researchers at the National Centre for Atmospheric Science about studying the air we breathe indoors, the different types of pollutants and where they come from, the links to outdoor air quality, and the ways people can reduce their exposure to poor indoor air quality.

Pollutant emissions come from almost all aspects of our lives indoors. The main domestic emission sources are smoking and cooking. Shower gels, cleaning products, and decorating paint are other household sources, along with burning candles and wood burning stoves that might leak smoke. Airborne mould spores, created in damp rooms, can also lead to indoor pollution.

Gas cookers emit nitrogen oxides and carbon monoxide. Many building materials and ordinary health and home products contain volatile organic compounds – also known as VOCs. The VOCs that are emitted indoors are now proving to be an increasing source of VOCs detected outdoors in more densely populated towns and cities. 

Every product releases a different set of VOCs into the air, leaving behind a kind of ‘chemical fingerprint’. These chemicals can then interact with one another in the air, sometimes creating new pollutants in a process known as ‘secondary formation. The chemistry that takes place indoors can be different to the outdoors – light dependent reactions might be triggered by lamps and LED panels rather than by sunlight. 

In similar houses, offices, and shops on the same street the concentrations of harmful chemical and small particle pollutants can vary massively due to differences in occupant and user behaviour alone. As the insulation and airtightness of buildings improves, to limit the amount of energy needed for heating or air conditioning, pollutants may become concentrated in poorer ventilated spaces.

The potential for mixing between polluted indoor and outdoor air is determined by factors such as ventilation in buildings, the amount and type of polluting activities nearby, and even weather conditions – if it is a warm day people have their windows open, which can let airborne chemicals and particles in and out.

Chemical and small particle emissions from cleaning and personal care products, smoking, cooking and heating, furnishings, and building materials can accumulate and concentrate before being released outdoors by opening windows and using extractor fans. 

Something important to consider is that there are similar pollutants found outdoors and indoors, such as particulate matter (PM2.5), nitrogen oxides (NOx), and carbon monoxide (CO), but they might have different toxicities and concentrations. And in some cases, indoor air can introduce new pollutants to the outdoors, such as biological aerosols created in damp and mouldy rooms.

For most of us, we cannot change the materials that public buildings have been built and decorated with. We might not be able to control the ventilation in our workplaces, although this became a focus during the COVID-19 pandemic, or directly lower the concentration of outdoor air pollutants that make their way into our homes. 

But many indoor air pollutants are related to things we do and products we use, and so by altering some of our usual activities or preferences we can start to reduce our individual contribution to poor air quality inside buildings. 

Some of the ways that people can reduce emissions and their exposure to indoor air pollution in their homes are by:

At the National Centre for Atmospheric Science, researchers are making measurements to collect evidence on different indoor and outdoor pollutants – with the aim of contributing to national emissions inventories and informing studies about human health impacts.  

Measurements of different types of emissions from cooking, cleaning, burning candles, fragrance use, spraying aerosols, and solid fuel burning are being made in specially-developed laboratory experiments. Scientists have also been observing emissions in a set of ‘real-world’ houses, where air samples were taken along with a record of the activities being done by the occupants.  

Chemistry-based computer models are also being used to build virtual indoor spaces, simulating the sources and losses of air pollutants, and to assess how indoor air pollutants may interact and evolve into secondary pollutants. 

As part of the INGENIOUS project, led by Professor Nic Carslaw at the University of York, researchers from the National Centre for Atmospheric Science have been investigating the indoor air quality in over 300 homes in Bradford. 

They are using sensors to measure airborne chemicals, gases, and small particles in three rooms in each home, and have collected samples in about 150 homes in order to do a detailed analysis of VOCs and organic pollutants in the particle phase. 

To collect the samples, researchers needed to pump air through a filter paper. The usual technique is too loud for indoor use, so researchers developed a small soundproofing unit that allowed them to do extensive sampling in real homes for the first time. 

A new research facility at the University of York, also led by Professor Nic Carslaw and involving colleagues from the National Centre for Atmospheric Science, replicates a semi-detached house. One side of the house has been designed in a modern, energy efficient way, and the other side is much more typical of a UK home built in the 1950s – 1960s. 

Researchers will carry out identical activities on each side of the house, while an integrated laboratory between the two sides will monitor air quality under the different ventilation and build conditions. The new facility will help shape the design and build of future homes, and provide a space for detailed study of indoor air pollution. 

The image is from “Clean Air Starts at Home”, an illustrated animation created by Aardman Animation Studios director Dan Binns with atmospheric chemist Professor Ally Lewis at the National Centre for Atmospheric Science and University of York. The animation explores the impacts of air pollution beyond cars and exhaust pipes, and is voiced by comedian Marcus Brigstocke. It was commissioned by Hay Festival and the Natural Environment Research Council, as part of a 2018 project called Trans.MISSION that paired leading scientists with award-winning artists to communicate cutting edge science.