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We know ozone is bad for our lungs – the increased risk of developing diseases such as asthma was one of the reasons people wore face masks during January’s record-breaking smog in China. But exactly what it did to the lungs has never been clear.

To investigate, Katherine Thompson from Birkbeck College in London and colleagues scattered neutrons off an oily surface. It was designed to mimic the fluid films on lung tissues which make the exchange of oxygen and carbon dioxide more efficient. By looking at where the neutrons ended up, the researchers were able to deduce the chemical changes involved. They found that ozone reacted with the oil at concentrations matching those in polluted town centres, changing the structure of hairpin-shaped chemicals called POPCs.

They found that during the reaction, one of the hairpin’s arms broke off, which increased the oil’s surface tension. This prompted it to bead up rather than spread out. In the lungs, this would reduce the volume of the gases that could be exchanged – that’s “bad news both for people with respiratory problems and healthy people”, says Thompson.

“The real question is whether these events take place in us when we’re exposed to elevated ozone concentrations,” says of King’s College London.

Thompson says the finding should back calls for tighter limits on ozone pollution from cars and factories. In the US, it is estimated that .

The team plans to repeat the work using surfaces that mimic the lungs of people with respiratory diseases.

Journal reference: Langmuir,