How human behaviour is the key to tackling indoor air pollution

Monitoring of harmful airborne pollutants such as PM_2.5 and NO₂ has always been conducted outdoors at network stations. However, British people spend 80% of their time indoors – so how much do we know about the air quality inside their homes? 

Associate Professor Samuel Stamp, from The Bartlett School of Environment, Energy & Resources, has spent the last five years addressing this gap in our understanding of indoor air pollution.  

Thanks to recent developments in air quality sensing technologies, Sam and his team have carried out a series of London-based studies where air quality could be measured every minute, and cross-referenced with information from sensors attached to windows, ventilation systems and other building elements.  

“If you want to know one thing about a building, you need to know everything,” says Sam. 

“What we found was the way people use their building is completely different, even in relatively similar apartments in a similar location. 

“Across the studies, we saw how the behaviour of the occupants had an impact on their air quality. We found different pollutants, and different strengths of concentrations across apartments that should in theory have been pretty similar.” 

Ventilation as a negative-sum game 

The buildings chosen for the studies were also predominantly located near busy roads and transport networks in London. This was in recognition of the increasing proportion of people living in cities, and their increased exposure to urban sources of outdoor air pollution. 

It was important to include these factors of increased air and noise pollution from outdoors, when examining the complex interactions and trade-offs between outdoor and indoor air pollution. 

“The window is a key point of interaction between the outdoor and indoor environment,” says Sam. “You’re diluting internal pollutants, but you’re letting in external pollutants at the same time.” 

Due to the airtight nature of low-energy buildings, occupants are faced with potentially greater exposure and higher concentrations of indoor pollutants. These can come from a wide range of sources, including cooking, cleaning products, furniture, or the release of gases from materials used in the construction of the building. 

One key finding, presented in the article ‘Seasonal variations and the influence of ventilation rates on IAQ: A case study of five low-energy London apartments’, compared measurements of formaldehyde to previous studies – demonstrating just how important occupant behaviours are. 

Sam explains. “Generally, warmer temperatures increase the rate at which formaldehyde is emitted from materials, and previous studies have often found concentrations to be higher in the summer than winter.  

“However, we found the opposite was true – concentrations in the summer were much lower than the winter.  

“It turned out that occupants were opening their windows to increase ventilation and deal with overheating – which is another significant issue and health risk in modern, low-energy flats.  

“So this meant the additional ventilation, driven by occupant window opening habits, more than offset any additional emissions. 

“However, occupant behaviours are very varied. We monitored hundreds of windows and whilst some residents open windows every day, others hardly ever open them.” 

Looking for a broader spectrum of ‘typical’ behaviour 

So how can we construct low-energy buildings and carry out retrofits that are optimised for all these different types of occupant behaviour? 

Sam’s research into air purifier use has clarified some of the possible avenues available – while demonstrating even further the impact of human behaviour on both indoor air pollution and its potential solutions. 

“Air purifiers are really effective – we found they were reducing PM_2.5 by 45% after 90 minutes, on average.  

“And that was even when people were opening windows. Air purifiers allowed people to manage overheating, reduce indoor pollutants and also some of the outdoor pollutants at the same time.

“The trouble is, people aren’t very good at perceiving small changes in air quality. We can perceive something like a candle burning, but even harmful levels of particulate matter don’t register. 

“So people weren’t able to use the purifier to consistently reduce exposure. They were mainly using it to get the benefit of a very marginal cooling effect, which comes from the fan inside the purifier.” 

To maximise the potential health benefits of air purifiers, Sam feels that automating these devices, and incorporating them into the mechanical ventilation systems often implemented in low-energy buildings, would be an effective response. Sam and his colleagues have seen similar approaches deployed in public buildings and office settings to good effect. 

Sam’s keen to stress that much more work is needed in this area to ensure that British homes can be healthier and safer. 

“We’re good at considering typical situations in the built environment world. Typical occupants, typical setups. 

“Unfortunately, most places and behaviours aren’t typical.  

“We need to be getting more robust evidence before we plough into the future of building and changing lots of homes. It’s difficult enough to do it the first time – you don’t want to have to go back and change it, like we’ve seen with the cladding issue. If we get the evidence, we won’t make those kinds of mistakes.”