Headteacher Gwen Lee had not expected the results to be good but had been unprepared for what the air pollution engineer found.

Levels of dangerous particulate pollution exceeded WHO guidelines in every classroom of the school – and two were more than three times over the limit.

“We have long been aware of air pollution as a problem but I was horrified that the children were breathing in air that was this dirty inside the school,” said Lee, headteacher at Christopher Hatton primary school in Holborn, London.

The school nestles between two of London’s busiest roads and was one of 50 schools identified as the most polluted in the city by the mayor of London, Sadiq Khan.

But even though the school had been aware that air pollution in the area was an urgent issue, they were unprepared for what experts found when they came to test particulate levels inside the classrooms in August.

It found that all the classrooms exceeded external WHO guidelines for one of the most dangerous particulate pollutants – PM10. Two of the classrooms recorded levels more than three times the WHO yearly average limit of of 20μg/m3 and one – the nursery – recorded a staggering figure of 546μg/m3, more than 27 times the WHO average annual limit.

“The idea that children are being exposed to this level of air pollution as they are trying to learn – with all the known health implications that go with that – is simply unacceptable,” said Lee.

She said there had been some building work going on in the nursery at the time which could partially explain that figure, but added the fact that every classroom was over the WHO limit – and three by so much – was deeply worrying.

The measurements were taken by Swedish company BlueAir after it had been contacted by one of the parents at the school who used the company’s purifiers at home to help their child who suffers from asthma.

The school had already been trialling one air purifier and parents had been trying – unsuccessfully – to raise enough money to get more. But when the scale of the problem was flagged up to BlueAir the company agreed to donate nine units, sending an engineer over in August to test the particulate levels inside the classroom before they were fitted.

Gwen Lee, headteacher at Christopher Hatton School in Holborn sits next to one of the BlueAir purifiers. Photograph: Jill Mead/The Guardian

Sara Alsen, from the company, said it often donated purifiers to schools and children’s hospitals in air pollution blackspots in India, Korea and China, but that it was the first time the company had given away purifiers in Europe.

“I never thought we would find this level of pollution inside classrooms in London. The findings really were quite shocking – especially when you consider the age of the children and their vulnerability to pollution.”

Within an hour of the air purifiers being installed the particulate pollution – which was higher than that in the surrounding area on the day – dropped by 96% to 2.9μg/m3, well below WHO limits. The school has had independent tests done which came back with similar results.

Experts say exposure to air pollution is especially dangerous for children with both short and long-term consequences.

Last year the Guardian revealed that every person in the capital is breathing air that exceeds global WHO guidelines for particulate pollutions with unknown long term health implications. And on Tuesday researchers from Queen Mary’s University in London released findings showing that children absorb a disproportionate amount of particulate air pollution during the school day.

Lee said that although she is delighted the air inside the school is now much safer, it is not a long term solution – either for her pupils or tens of thousands of other children in the UK.

“Children have a right to breathe clean air and although this is a great short term solution, not every school is going to get air purifiers and people still have to live and work in this area. We have to do more to address the root cause of this problem as a matter of urgency.”