After years of headlines about air pollution, we’ve been misled on a few things about the world’s biggest environmental health problem. For example, we’re told that “PM2.5” – solid pollution particles measuring 2.5 micrometres or less – can pass through our lungs and into our blood stream.

But, in fact, the vast majority of them can’t.

We’ve also been told NOx gases – including nitrogen dioxide – are the biggest threat to health within cities. However, NOx is responsible for just 14% of deaths attributed to air pollution in Europe.

The biggest killer of all never makes the headlines, isn’t regulated, and is barely talked about beyond niche scientific circles (despite their best efforts to change that narrative): it’s nanoparticles.

PM2.5 may be too small to see, being roughly 30 times smaller than the width of a human hair. But it’s a relative heavyweight. PM2.5 stomps in at 2,500 nanometres (nm), while nanoparticles are 100nm or below. PM2.5 and PM10 (10,000nm) are killers in their own right, typically causing lung and respiratory conditions. Yet nanoparticles can reach, and wreak havoc in, any organ in the body. And because government authorities monitor PM2.5 by mass (million of nanoparticles may not even register a measurement by microgram) – their reports underrepresent the true risks.

The science of why we should be concerned about the total number of particles that we breathe in, not just their mass, has been known for some time. In 2003, Surbjit Kaur was a young researcher finishing her Masters at Imperial College London, when her supervisor suggested she join the Dapple experiment (the Dispersion of Air Pollution and its Penetration into the Local Environment). Kaur designed a personal exposure study, with a team of six volunteers “dressed up like Christmas trees” with various different air pollution sensors, and asked them to travel a set route in central London every day for four weeks.

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The volunteers “were a combination of friends and people in the department” says Kaur, who has since left science and now works as a management consultant. “But I couldn’t really ask people to do this if I wasn’t doing it myself.” She joined them out on the roadside, which centred around Marylebone Road, a major seven-lane highway and home to the Madame Tussauds waxwork museum, and its lengthy queues waiting outside. “We went out there knowing that we were going to get ill because of that constant exposure. We began to feel quite grotty after a while.”

The equipment draped over the volunteers and inside backpacks measured the standard air pollutants, PM2.5 and CO (carbon monoxide). But Kaur also included a brand-new piece of kit that had only just come on the market: a ‘P-Trak’ nanoparticle counter. “We needed to get all sorts of approvals to use them [in the field work] because they looked a bit like Geiger-counters and there were concerns that the public might panic,” she laughs. The device could count nanoparticles right down to 2nm (many times smaller than a human blood cell) by sucking in air, spraying alcohol onto the surface of the particles to make them visible and individually counting by laser beam. Influenced by work emerging from University of Rochester, New York, and the National Public Health Institute, Finland, Kaur had a hunch that counting these “ultrafine particles” could add some interesting data. She wasn’t wrong.

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