Being indoors with the windows closed does not completely protect people from smoke caused by wildfires, according to early findings from researchers at the University of Manchester and National Centre for Atmospheric Science.

Air quality scientists analysed real-time indoor air quality data from 23 homes across Greater Manchester between 1 and 26 July 2026, covering two periods of wildfire activity in the nearby Pennine hills. Every home that was monitored was found to exceed the World Health Organization’s 24-hour guideline for fine particle pollution (PM2.5) on 17 July, when outdoor smoke levels peaked.

Dr Congbo Song, a researcher at the University of Manchester and the National Centre for Atmospheric Science, led the analysis of PM2.5, the fine particles produced by smoke that are small enough to travel deep into the lungs and harm health.

The study found that smoke pollution entered every home monitored during the wildfires, including properties up to 60km from the moorland fires outside Manchester. However, indoor pollution levels were significantly higher in homes where residents had opened windows during the hot weather – as temperatures rose above 33°C.

During the wildfires, average overnight PM2.5 levels in bedrooms increased from a normal level of 2.7 micrograms per cubic metre to 7.8 micrograms per cubic metre when windows were closed, and to 11.8 micrograms per cubic metre when windows were open. “On average, the enhancement of daily PM2.5 during the wildfire period is 8.6 micrograms in bedrooms and 9.2 micrograms in kitchens, compared to the pre-wildfire baseline,” explains Dr Congbo Song.

He adds: “In some homes, overnight concentrations rose to more than 100 micrograms per cubic metre, many times higher than typical levels. The analysis suggests that more than 80% of the monitored households ventilated more overnight during the heatwave-wildfire events, leading to more wildfire smoke particles infiltrating people’s homes.”

While pollution levels generally rose less in homes where windows remained closed, this did not prevent smoke from entering indoors. Closing windows reduced exposure, but PM2.5 concentrations still remained two to four times higher than normal.

The researchers say the findings support UK Health Security Agency advice to stay indoors and keep windows closed when wildfire smoke levels are high. However, they also highlight the difficult trade-off many households face during periods of extreme heat.

Public health guidance to close windows can reduce smoke exposure, but may also cause homes to overheat and limit the ventilation of indoor air pollution from cooking or household products. Both poor air quality and high indoor temperatures can worsen breathing problems and other health conditions, particularly among vulnerable people who do not have access to active air purification and mechanical cooling.

The findings suggest there is a need for clearer, evidence-based guidance to help households balance the risks of smoke exposure and overheating during wildfire events, especially as periods of extreme heat become more common.