Researchers have found the first direct evidence that tiny particles of air pollution stick to our red blood cells, meaning they can travel freely around the body.
These particles are produced by vehicle exhausts and from brake and tyre wear and can easily enter the lungs. They have recently been found in the brain and heart, where they are linked to increased risk of disease. However, until now, there has been no conclusive evidence of how pollution particles infiltrate these parts of the body.
In the study, which involved Professor Barbara Maher of Lancaster University and was led by researchers at Queen Mary University of London and published in ERJ Open Research, researchers found an increase in pollution particles stuck to the red blood cells of healthy volunteers, after they spent time on a busy London road.
The research involved 12 adult volunteers. Each volunteer spent four hours in an office building. Then they were asked to spend an hour within ten meters of a busy London road. They each carried a small device called an aethalometer, which measured the amount of particle pollution in the air around them. Then they returned to the office building for another hour.
Eight of the volunteers repeated the experiment on a different day, but this time wearing an FFP2 mask.
Researchers took blood samples from the volunteers after four hours in the office, immediately after their hour by the roadside and again after the hour back in the office. They used microscopes to take detailed photos of the blood samples, allowing them to see and quantify the pollution particles stuck to the red blood cells.
Levels of particle air pollution at the roadside were almost five times higher than in the office building, according to the aethalometers.
When researchers examined the blood samples, they found an increase in the amount of pollution particles stuck to volunteers' red blood cells after they spent time by the roadside. On average, researchers found two to three times as much of the particle material stuck to people's red blood cells after an hour by the road.
In some people, the levels decreased after an hour away from the road, while in others the levels remained high, suggesting there could be differences in how people's bodies deal with the pollution they breathe in.
Although only a small proportion of red blood cells were carrying particles after exposure to roadside air pollution (around two or three out of every thousand), the researchers calculated that in the five litres of blood circulating in the human body, around 80 million red blood cells would be assumed to be transporting particles after standing near a main road for an hour.
When volunteers repeated the experiment wearing an FFP2 face mask, although they were exposed to the same levels of pollution by the roadside, the amount stuck on their red blood cells did not increase after standing by the road. Researchers say this is the first study to show that wearing an FFP2 mask reduces the dose of inhaled pollution particles in humans.
To confirm their findings, the researchers exposed human red blood cells and then mice to diesel exhaust particles in the lab. They found the particles stuck easily to both human and mouse red blood cells, and the more particles they added, the more they found stuck to the blood cells.
They also analysed some of the pollution particles they found on the volunteer's blood cells and found that they contained iron, copper, silicon, chromium and zinc, which are known to be produced by vehicles exhaust, as well as silver, copper and molybdenum, which are produced by brake and tyre wear. The particles were 2.5 micrometres of smaller in size, corresponding to the PM2.5 measure used to monitor air pollution.
Professor Maher used electron microscopy to image and analyse several hundreds of the nanoparticles. Most of the particles are around 10 to 15 nanometres in size - about 10,000 times less than the thickness of a human hair, and very small even compared with the red blood cells, which are around 7000 nm wide.
She said: "Exposure to traffic-derived particulate air pollution has been linked statistically with a range of diseases, including blood clots, heart attacks, lung cancer and Alzheimer's disease. It's been assumed, but never shown before, that the smallest air pollution particles can escape from the lungs and into the bloodstream and then get transported all around the body.
"We showed that after just one hour of traffic exposure, in London's Whitechapel Road, millions of distinctive, metal-rich nanoparticles appeared in the bloodstream of the volunteers, sticking around the edges of their red blood cells.
"Some particles cluster together, some appear stuck to the edges of the red blood cells. All contain metallic elements typical of the ultrafine pollution particles released into the air from vehicles.
"Hitch-hiking on the red blood cells, these inflammatory particles can then travel to every major organ in the body, including the heart and the brain. Our previous work has shown the presence of millions of these pollution particles in the human brain, often associated with damage within the brain cells.
"Our work shows how important it is for human exposure to traffic-derived air pollution particles to be reduced, as a matter of urgency, especially for vulnerable people like young children, pregnant women, and the elderly."
Professor Jonathan Grigg of Queen Mary University of London and lead author of the study said: "In our bodies, red blood cells work by collecting oxygen from our lungs and delivering it throughout the body. With this set of experiments, we have shown that tiny air pollution particles are hijacking our red blood cells, meaning they can also travel almost anywhere in the body.
"We're finding more and more evidence that air pollution particles are making their way into many different organs of the body and now we have clear evidence of how that could be happening.
"This technique means we now have a relatively simple way to measure the amount of pollution entering the body, so now we can test out which factors might increase or reduce the problem. We were surprised to find how well an FFP2 face mask prevents these very tiny particles from reaching and attaching to blood cells."
