Frying food and burning wood may turn homes into “toxic boxes” with high levels of air pollution trapped inside, campaigners have warned.

Case studies commissioned by the environment charity Global Action Plan (GAP) showed that ultrafine particle pollution was higher inside than outside in all four of the properties monitored. Tiny particles are thought to be especially harmful to health as they can enter the bloodstream and flow around the body into vital organs.

The 24-hour tests took place in houses in London, Pontypridd, Liverpool and Lancaster. In the latter, the average of 40,000 particles per cubic metre inside the home was more than seven times the outdoor average. In London, the peak indoor pollution resulted from frying sausages and steaks.

An opinion poll for GAP also found that 55% of parents said their children spend more time indoors then they did when they were young, potentially exposing them to more air pollution. Only 11% of parents thought their offspring spent more time outside than they did at that age.

Research published in February found that cooking a Sunday roast on a gas hob can drive small particle pollution in homes far above the levels found in the most polluted cities on Earth for an hour or two.

In 2016, a report from the Royal College of Physicians and the Royal College of Paediatrics and Child Health, led by Prof Stephen Holgate, highlighted the impact of indoor pollution, with sources including boilers, heaters and irritant chemicals from cleaning products.

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Holgate is preparing a more in-depth study on the effects of indoor air pollution on children’s health. “These [case studies from GAP] provide early indicators of the scale of the air pollution challenge that we face in the UK,” he said. “With children spending increasing hours indoors, exposing them to ultrafine particles of pollution, urgent action needs to be taken to address the issue of indoor air pollution.”

“The combination of indoor and outdoor air pollution sources is turning our homes into toxic boxes, with pollution trapped inside,” said Chris Large, a senior partner at GAP, which is organising Clean Air Day on 20 June.

“It’s vital that we raise awareness that air pollution is everywhere, but that there are many things we can do, both indoors and outdoors,” he said. “Some key things you can do are driving less frequently and walking or cycling instead, and opening a window when cooking at home.”

Paul Young, whose end-of-terrace Victorian house in Lancaster hosted one of the case studies, saw peaks in pollution when his woodburner was in use. “Like lots of people we like to create a homely atmosphere, and yet I know that I am contributing to pollution in the rest of the city. We could seriously think about using the woodburner less frequently.”

Advice from GAP on woodburners says: “If you are considering buying a woodburner, ask yourself if you really need one.”

Emma Prior in Liverpool also hosted a case study and is a mother to two teenagers with asthma. “I was really surprised to see the peaks inside my house. Now I’m going to look at prioritising ventilation,” she said.