Keeping the classroom window closed is not the answer to reduce the amount of pollution coming into schools, a recent study has found.
A project called SAMHE (Schools’ Air quality Monitoring for Health and Education) operated air pollution monitors in nearly 500 classrooms for an academic year and found that days with high outdoor pollution led to higher pollution inside the classrooms.
The Imperial College London researchers found that 6% of school days had outdoor particle pollution above the World Health Organization’s daily guidelines and these days contributed 17% of the annual classroom pollution.
SAMHE, led by Imperial College London, operated air pollution monitors in nearly 500 classrooms for an academic year, and found that days with high outdoor pollution led to higher pollution inside the SAMHE classrooms. Six percent of school days had outdoor particle pollution above the World Health Organisation’s daily guidelines, but these days contributed 17 percent of the annual classroom pollution.
Dr Alice Handy of the SAMHE team said: “Reducing ventilation will not stop outdoor PM2.5 entering classrooms. We saw high levels of particle pollution in classrooms during the weekends around fireworks night when schools and classrooms were closed. Ventilation is also really important to lower carbon dioxide in classrooms, which is linked to attention and can reduce the risk of spreading infections.”
The good news is that school classrooms did provide a degree of protection against air pollution with average particle pollution measuring 30% less than outdoor air. Handy added: “Relatively little of the particle pollution in classrooms seems to be generated there, most comes in from outdoor air.”
Over the school year, about 80% of the classroom pollution came from outside. At home, children can breathe in particle pollution from sources that include cooking, personal care products, wood burners and cigarettes. Pollution in schools can also be lower than on the journey to and from school. Research in London has shown that children who walk along busy roads or travel by car to school have the greatest exposure to traffic pollution.
Handy said: “Our results show how important it is to continue with initiatives such as no-idling zones, school streets, and efforts to burn fewer fossil fuels, to clean up the air both inside and outside schools.”
The team tested out air filters and sensors in an additional 300 classrooms and found they reduced particle pollution by 29% compared with nearby schools in the main part of the SAMHE study.
Dr Samuel Wood of the research team said: “We saw significant reductions, but outdoor air quality was still the leading driver of classroom particle pollution, even with filters installed. Air filters may be able to mitigate against some of the effects of poor outdoor air quality, but they are unlikely to remove them altogether.”
• The headline of this article was amended on 25 July 2025 to better reflect the findings of the study.
