Ultrafine particles

When we talk about air pollution, most of us probably think about smog or the smoke associated with wildfires.

But scientists are learning more about a category of pollutants that are smaller in stature yet potentially more dangerous.

They're called ultrafine particles.

A thousand times thinner than a strand of hair, ultrafine particles (UFPs) are emitted by vehicles and industrial activity and according to a recent study led by researchers at McGill University in Montreal, contribute to more than 1,000 premature deaths a year in that city and Toronto.

Owing to their size, these nanoparticles can be breathed into the lungs and pass into the bloodstream.

"They've also been found in people's brains, so they certainly have access to our entire system, in terms of where they can reach," said Scott Weichenthal, an associate professor in the department of epidemiology, biostatistics and occupational health at McGill and lead author of the study.

A 2020 study from the University of Illinois found that "compared to fine particles [ultrafine particles] cause more pulmonary inflammation and are retained longer in the lung."

The study says UFPs have been found to cause cardiovascular disease and are linked to cancer and diabetes in adults; exposure can also lead to greater rates of babies with low birth weights.

A study this year by Brussels-based advocacy group Transport and Environment determined thousands of cases of high blood pressure, diabetes and dementia across Europe could be linked to UFPs emitted by airplanes.

According to Weichenthal, the McGill-led study was the first to examine UFPs and mortality in Canada. 

For regulation purposes, particles are categorized by their diameter. When they have a diameter of less than 10 micrometres (PM10 in shorthand), they can get into the lungs and lead to adverse health effects. Fine particulate matter refers to particles that are 2.5 micrometres or less (PM2.5), and includes smoke, dust and pollen.

Ultrafine particles are even smaller: 0.1 micrometres or less (PM0.1).

UFPs are typically generated by combustion-related activities. Road vehicles are the primary source of those emissions in urban areas. While some of that pollution comes from exhaust, other key sources of UFPs are tires, rotors and brake pads what are known as non-tailpipe emissions. 

Greater uptake of electric vehicles should decrease overall carbon emissions, but a 2023 study by the University of Toronto found that EVs owing to the sheer weight of the battery might generate more UFPs from brakes and tires.

"There's this mounting evidence that electric vehicles are heavier, so they're creating more from that friction of tires on the road," said Marianne Hatzopoulou,
department chair of civil and mineral engineering at the University of Toronto and a Canada Research chair in transportation decarbonization and air quality.

She calls this one of the unanticipated tradeoffs of climate action.

"We're rushing to decarbonize but wait a minute, there's something that we're not focusing on."

Hatzapoulou has studied UFPs for years. After appearing in an episode of CBC's The Nature of Things on air pollution in 2019, she was approached by residents of a downtown Toronto neighbourhood called Bathurst Quay to do an air quality study.

The area is bounded to the north by the bustling Gardiner Expressway, but the study determined that nearby Billy Bishop Airport is a larger source of UFPs.

Based on readings at Hanlan's Point on Toronto Island, winds from the south generally carry about 33 per cent more UFPs averaged over a 24-hour period. But the increase at Bathurst Quay can be more than 85 per cent in that same time frame.

The airport, which is just south of the neighbourhood, sees roughly 50 to 70 departures a day. During periods of aircraft operation, the study found momentary UFP spikes exceeding 100,000 particles per cubic centimetre. For comparison, the World Health Organization describes 20,000 particles/cm3 for a one-hour average as "high."