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We’re missing the small picture: The threat from ultrafine particles

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Well, they’re smaller than this… Photo: Christopher Campbell

The vast number of particles we breathe in are smaller than the minimum size most equipment can measure. We often hear about PM2.5; although that term describes particles smaller than 2500 nanometers, the equipment measuring PM2.5 usually measures stuff between 1000 and 2500 nanometers. That’s 0.0001-0.00025 centimeter – small indeed. However, researchers want the rest of us to focus more on ultrafine particles (UFPs) – particles smaller than 100 nanometers. The fact that they are unbelievably small unfortunately does not mean they are harmless – rather the opposite, it seems.

The most important sources of outdoor UFPs are diesel exhaust and other particles from traffic (they are typically around 30-50 nanometers in size), aviation and shipping; industry and power production; and residential heating. Paint pigments, tobacco smoke, candles, cooking and heating are the main culprits indoors. New houses have better isolation; this shuts off more outdoor UFPs, but increases exposure to the indoor ones.

They’re tiny, but many, and their surface area is huge

The way we measure small particles today, is by mass, not by number. But, while the mass of ultrafine particles is too small to measure, there are lots of them. That also means their combined surface area – which interacts with the cells in our blood and respiratory system – is extremely large.

Car traffic
Diesel exhaust and other particles from traffic are a major source of UFPs.
Photo by Ignat Kushanrev on Unsplash

For example, a car’s tailpipe may emit 1 million UFPs per cubic centimeter of air. Let’s say those are all 10 nanometers. Then you still need to gather 1000 of those centimeter cubes to get a total ultrafine particle mass of one single PM10 particle. However, the combined surface area of the UFPs in one cubic centimeter is still a thousand times larger than that of the PM10 particle. And that surface area can have toxic chemicals stuck to it.

Earlier this month, a large number of researchers published a joint white paper summarising research on ultrafine particles. Many of its conclusions are, well, far from ultra-fine. They’re rather disturbing, in fact.

We track large particles, but miss the nasty small ones

First, the number and concentration of ultrafine particles (UFPs) a certain place don’t seem to be linked to the amount of larger particles there. Although small and robust measuring equipment for UFPs is being developed, it’s so far expensive and hard to measure UFPs. Therefore, researchers and policymakers have hoped and assumed that by measuring larger particles they would measure smaller ones too. Not so, it seems. The link between amount of UFPs and larger particles is often there for traffic emissions such as carbon monoxide (CO) and nitrogen oxide (NOx). But residential fuel combustion creates much more UFPs than larger particles, and some industries emit much more larger particles than UFPs.

Residential wood burning may be cosy, but leaves UFPs.
Photo by Hayden Scott on Unsplash

The white paper also highlights that the number and concentration of UFPs vary a lot more from location to location than that of larger particles. Ongoing Belgian and French research projects confirm this. The variation in UFPs between different times of the day – and different times of the year – in the same location is also much larger.

Today, we measure larger particles by gathering data from a limited number of site-specific monitoring stations. This may still work out for temporal variations in UFPs since they might “even out” over a longer time period. But existing monitoring stations won’t tackle the spatial variations. We would need lots of measurement devices to measure those.

They enter our blood and organs…

A third problem the white paper focuses on is that UFPs can hurt us in other ways than larger particles. The damage done depends on how large the concentration of particles is, their size, where in the body they end up, how long they stay, and what they contain. Rather little hard evidence exists about the direct effects of UFPs on humans. But exposure correlates with asthma, eczema, cardiovascular diseases, chronic obstructive pulmonary disease (COPD) and ischemic heart disease.

What we do know is that larger particles (ehrm, can a particle actually be called “large”?) such as PM2.5 and PM10 can’t get further than our upper respiratory system, and stay there for a limited time. But UFPs are so small that they are transported deep into our respiratory systems. They can stay in our lungs or in our blood for months, and cause more systemic damage.

…but we don’t regulate them

The white paper also targets a fourth problem: current air pollution regulations don’t reduce UFPs. Regulators don’t understand UFPs and their link (or lack of such) to larger particles, and we don’t measure UFPs. So regulators tend to think that measuring and regulating larger particles solves the problem. But regulations might just push the bulk of particle emissions below the size thresholds of current regulations. Although particles smaller than that may cause as much, or even more, harm.

And, yes, of course: The exposure is not evenly distributed. A large Canadian study recently found that more people living in locations with higher UFP concentrations get brain cancer. One can only assume what the odds would be for a 13-year-old whose family can only afford a poorly isolated house between a coal power plant and a heavily trafficked road in a megacity in Africa or developing Asia, who works at the powder coating facility nearby without proper protective gear, and who can’t afford to go to the doctor when trouble starts…

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