Road traffic not only blows exhaust fumes into the area, but also particles from tires and brakes. Politicians have so far ignored this fine dust, although it is also a health hazard.
Particulate matter not only comes from car exhausts, but also from wear and tear on tires and brakes.
Marylebone Road is not only known for the fact that the wax museum Madame Tussauds rises there. The multi-lane road is one of London’s main thoroughfares and is often congested. It regularly ranks at the top of the list of European roads with the highest levels of air pollution. In order to monitor air quality in real time, there is a measuring station operated by Imperial College London opposite Madame Tussauds.
In September 2019, William Hicks will start a project for his doctoral thesis at this station. He wants to measure the amount of vehicle emissions for a year and use them to create particulate matter time series. Hicks is only marginally interested in the exhaust fumes.
Because vehicles also cause non-exhaust emissions such as tire wear, brake dust and worn road surfaces. They are less well known than exhaust gas emissions, although tons of them end up in the atmosphere, water bodies or soil as coarse and fine dust particles. Environmental policy has so far ignored them, although they are increasing rapidly. And they may be more harmful than the exhaust fumes.
Plastic and metal particles
When the first lockdown was imposed in London in March 2020, traffic on Marylebone Road fell by 32 per cent. With the decline, emissions of gases that are harmful to the environment and health, such as nitrogen oxides and hydrocarbons, fell. “The changes were so drastic that our data looked as if they came from a different location,” says Hicks, the a study published about it.
Because there were fewer vehicles, the typical traffic jams did not occur. This resulted in less braking and less brake dust, which accounts for the majority of non-exhaust emissions. “At the same time, however, we observed that the proportion of tire particles, such as the zinc used for vulcanization, remained consistently high,” says Hicks. This was due to the 15 percent higher speed of the vehicles, but also to the wet weather. Tire and road abrasion produced relatively coarse grains that ended up in the environment as microplastics.
Things are a little different when brake discs and pads meet. They consist of up to 200 substances, including iron, copper and barium. Mechanical grinding creates fine metal particles of up to 2.5 micrometers in size (“PM2.5”) and coarser ones with a diameter of up to 10 micrometers (“PM10”).
Thousands of tons of microplastics and particulate matter
Environmental scientists have already quantified how much particulate matter gets into the air in this way. A simulation of Ford-Werke and the Bergische Universität Wuppertal shows that on a typical taxi route through Los Angeles, about five milligrams of fine dust are produced per brake and kilometer. Converted to Switzerland, with 80 billion kilometers driven, a good 1,600 tons of brake particles ended up in the environment in 2020 alone.
Tires emit even greater amounts of emissions, such as an analysis by Empa displays. Using material flow analysis, the researchers calculate that in 2018 each of the then 8.5 million Swiss people produced an average of around 1.3 kilograms of tire abrasion. Of the 11,000 tons of particles produced in this way, 74 percent ended up on the side of the road, 22 percent ended up in surface water and 4 percent in the soil. According to the analysis, since 1988 around 219,000 tons of tire debris have gotten into the environment.
Tire emissions are not only high when measured in absolute numbers; they also show an upward trend: according to the Empa analysis, they have increased by 10 percent since 1990. These particulate matter emissions have increased more in cities than in rural areas.
Environmental chemist Stuart Grange from Empa’s Air Pollution and Environmental Technology department and his team a comparative study employed for this. For a year, the researchers used filters to collect particulate matter samples in Zurich, Bern, Payerne, Basel-Binningen and Magadino-Cadenazzo and compared the increase in emissions between rural and urban locations. They sent the samples to partner laboratories in Barcelona and Grenoble for chemical analysis.
The laboratories identified 60 chemical elements and compounds in the fine dust mixture. Not all of them can be clearly assigned to a source. But antimony, barium and copper are considered clear indicators of brake dust. They were particularly high at the Bern measuring station, which, like Marylebone Road, is located in a street canyon. Half of the measured increase is non-exhaust emissions, says Grange. “Now that their share is increasing, politicians have to deal more with them.”
Non-exhaust emissions increase
Limit values such as those for exhaust gases have not yet been specified. On the other hand, a clear trend has long been emerging: while exhaust gases are falling, emissions from tire wear and brake dust are increasing.
The Federal Office for the Environment (FOEN) wrote 2021 in a status reportthat in Switzerland exhaust gases such as nitrogen oxide and carbon monoxide decreased by 33 and 48 percent between 2005 and 2019. Fine dust emissions also decreased overall. At the same time, PM2.5 and PM10 emissions from tire wear and brake dust increased by more than 8 and 11 percent, respectively. This is mainly due to the increased volume of traffic.
According to one, the increase in road traffic is OECD study also the reason why a further increase in non-exhaust emissions of around 50 percent is to be expected by 2030. By then, 4 percent of the world’s cars could have been replaced by electric cars, according to the report. But even if it were twice as many, it wouldn’t do much to offset the increase in non-exhaust emissions, says the OECD’s Shardul Agrawala, the report’s lead author. Although electrification reduces exhaust emissions, tire abrasion and brake dust are still there.
According to Agrawala, the term zero-emission vehicle may have misled politicians and the public. Because politicians have not kept pace with technical innovations, mitigation steps are now required. This includes creating incentives for the development of lighter batteries and regulating tire composition. In the end, reducing the number of kilometers driven will help the most, says the OECD expert. Because the trend is towards heavier and heavier cars, he calls for taxation based on weight.
In fact, electric cars weigh up to 24 percent more than comparable combustion engines – mainly because of the batteries. An electric Ford F-150 Lightning is 725 kilograms heavier than its petrol-powered counterpart. Scientists still disagree on how much this additional weight increases the stress on the road surface. If you believe an old rule of thumb from the American Department of Transport, road wear does not increase linearly with heavier axles, but exponentially.
For the lungs, fine dust is not the same as fine dust
On the other hand, electric vehicles cause less brake dust thanks to their regenerative braking. This could be good for health. The small metal particles have the ability to destroy antioxidants in the human body and thus cause inflammation. A team of environmental scientists at the Paul Scherrer Institute (PSI) published a study in the journal “Nature”, which points out that future limit values should not only take into account the amount of fine dust, but also this “oxidative potential” of the particles.
Led by Kaspar Dällenbach, the PSI team used mass spectrometry to determine the chemical composition of the particulate matter at several locations. The researchers then observed in the test tube how fine dust extract with high oxidative potential affects antioxidants in simulated lung fluid.
“The faster the antioxidants are broken down, the higher the oxidative potential,” says Dällenbach. The quickest was the decomposition with the addition of man-made fine dust with a high proportion of brake dust and aged smoke from wood furnaces. Such fine dust puts stress on the cells, which increasingly react with inflammation.
However, it is not yet clear how dangerous these substances really are for humans. This can only be done in epidemiological studies. Dällenbach would welcome them. Because even if fine dust from tire abrasion and brake dust – speaking absolutely – occurs in small quantities, their proportion increases noticeably in relation to conventional fine dust.
If the assumption is confirmed that particles from non-exhaust emissions have a particularly harmful effect on the lungs, they would also be relevant to health in small quantities. Particulate matter would then not only have to be evaluated according to its quantity, but also according to its hazard potential. The legal limit values would then also have to be adjusted accordingly.