Brake dust may cause more problems than blackened wheel covers

March 3, 2017
Rush hour traffic on the downtown connector in Atlanta. Credit: Josh Brown

Though tailpipe emissions could fall in the years ahead as more zero-emission vehicles hit the streets, one major source of highway air pollution shows no signs of abating: brake and tire dust.

Metals from brakes and other automotive systems are emitted into the air as fine particles, lingering over busy roadways. Now, researchers at Georgia Institute of Technology have shown how that cloud of tiny particles could wreak havoc on respiratory health.

In a study published January 31 in the journal Environmental Science & Technology, the researchers described how vehicle-emitted metals such as copper, iron and manganese interact with acidic sulfate-rich particles already in the air to produce a toxic aerosol.

"There's a chain reaction happening in the air above busy highways," said Rodney Weber, a professor in Georgia Tech's School of Earth & Atmospheric Sciences. "Acidic sulfate in the atmosphere comes into contact with those metals emitted from traffic and changes their solubility, making them more likely to cause oxidative stress when inhaled."

The study, which was sponsored by the National Science Foundation and the U.S. Environmental Protection Agency, showed how the metals are emitted mainly in an insoluble form but slowly become soluble after mixing with sulfate.

"Sulfate has long been associated with adverse health impacts," said Athanasios Nenes, a professor and Georgia Power Scholar in the School of Earth & Atmospheric Sciences and the School of Chemical & Biomolecular Engineering. "The old hypothesis was that the acidic sulfate burns your lung lining, which in turn leads the bad health effects. But there is not enough acid in the air alone to really have that impact."

But sulfate plays a key role in making metals soluble before they are inhaled, which could explain the association of sulfate with adverse health impacts, the researchers said.

The researchers collected samples of ambient particulate matter in two locations in Atlanta - one near a major interstate highway and another urban site 420 meters away from the roadway. They analyzed the chemical content, size distribution and acidity of the samples.

Athanasios Nenes, a professor and Georgia Power Scholar in the School of Earth & Atmospheric Sciences and the School of Chemical & Biomolecular Engineering Credit: Georgia Tech

A significant amount of the ambient sulfate found was similar in size to the metal particles, suggesting that the ambient sulfate and metals were mixed within individual particles, which over hours or days would allow the acidic sulfate to convert the metal into a soluble form.

To quantify just how dangerous the aerosol could be, the researchers developed a high throughput analytical system for a chemical assay - called oxidative potential - that simulates the toxic response that such a mix would have on cellular organisms. This instrument was used to generate large data sets on ambient aerosol oxidative potential, which when utilized in an earlier epidemiological study, researchers at Georgia Tech and Emory University found that the chemical assay was statistically associated with hospital admissions in Atlanta for asthma and wheezing.

In the new study, the researchers observed that the peak toxicity indicated by the assay was closely correlated to those particles that contained the largest amount of soluble metals, which occurred only when mixed with highly acidic sulfate.

"That's the smoking gun," Nenes said. "The sulfate essentially dissolves those metals; when you breathe in those particles, the metals could be absorbed directly into the blood stream and cause problems throughout the body. For the first time, a mechanism emerges to explain why small amounts of acidic sulfate can adversely affect health."

While the sample taken from the testing site located farther away from the highway had less particulate metal, there was still enough to cause an increase in the oxidative potential, showing that roadway pollution could travel through the air and potentially cause problems in surrounding areas as well.

Dust from brakes and tires isn't the only source of metals in the air. Incinerators and other forms of combustion also produce mineral dust and metallic particles, which could mix with sulfate to trigger a similar reaction.

The researchers noted that while the amount of particulate sulfate in the southeastern United States has decreased during the past 15 years as sulfur dioxide emissions from power plants have fallen, there's still enough acidic sulfate in the air to keep the pH of particles very low, in the range of 0 to 2, transforming insoluble ambient metals to a soluble form.

"Vehicle tailpipe emissions are going down, but these kinds of emissions from braking will remain to some extent, even if you drive an electric car," Weber said. "Therefore, this kind of process will continue to play out in the future and will be an important consideration when we look at the health effects of particulate matter."

This material is based upon work supported by the National Science Foundation under Grant No. 1360730 and the U.S. Environmental Protection Agency under Grant No. RD834799. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation or the U.S. Environmental Protection Agency.

Explore further: Atmospheric sulfate particles reduced, but as acidic as ever

More information: Ting Fang et al, Highly Acidic Ambient Particles, Soluble Metals, and Oxidative Potential: A Link between Sulfate and Aerosol Toxicity, Environmental Science & Technology (2017). DOI: 10.1021/acs.est.6b06151

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21 comments

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gkam
3 / 5 (10) Mar 03, 2017
Once again, electric transportation is the key here. And it is not just because there are no metal or any other kind of emissions, it is because the use of regeneration takes most of the need for braking away.
KelDude
4.9 / 5 (11) Mar 03, 2017
Electric vehicles can use "regenerative" braking which is magnetism used to stop the vehicle. No brake pads, no pollution. So it's time for most to drive this type of car and abandon the 20th century relic we still depend on. Combustion and braking pollution both removed in one fell swoop.
gkam
2.9 / 5 (10) Mar 03, 2017
Regeneration means it actually uses the momentum of the car to charge the battery, putting that physical load on the motor/generator, slowing down the car. You can have one-pedal driving if you want.
BrettC
2 / 5 (1) Mar 03, 2017
Although, there is still some work to do with the particulates created by electric motors, and emergency breaking still uses break pads.
BrettC
3 / 5 (1) Mar 03, 2017
By the way, what is Tire Dust? I would think the debris left behind by tires would be slight larger than dust, and less prone to traveling very far. Maybe I'm wrong about that, but the article should clarify the less obvious pollution from tires if it's going to include it.
gkam
1.9 / 5 (7) Mar 03, 2017
"Although, there is still some work to do with the particulates created by electric motors"
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WHAT?
Eikka
4 / 5 (6) Mar 03, 2017
Electric vehicles can use "regenerative" braking which is magnetism used to stop the vehicle. No brake pads, no pollution. So it's time for most to drive this type of car and abandon the 20th century relic we still depend on. Combustion and braking pollution both removed in one fell swoop.


Electric cars still need regular brakes, because the regen brakes consume energy rather than return it below a certain speed where their operation becomes inefficient. The regenerative brakes are basically running the motor in reverse, so your braking force is limited by your motor torque. That won't do if you need to brake hard - in a car with only regen brakes at highway speeds, your stopping distance would be half a mile.

Since electric cars do have to have regular disc brakes, they have to be used or else they'll just seize up and stop working when you need them. That means you can't actually build an electric car that woudn't make brake dust.
david_king
5 / 5 (3) Mar 03, 2017
Old-school "Universal" DC electric motors with solid carbon brushes rubbing against copper commutators generate a very fine dust over time. Most modern high performance motors are brushless DC or vectorless 3 phase.
Eikka
3.8 / 5 (5) Mar 03, 2017
Technically speaking, a purely magnetic brake will never stop you because the braking force is proportional to speed. Zero speed, zero braking force. Hence why at some point in the regenerative braking process, the electric car has to start spending energy to apply reverse torque on the motor.

david_king
5 / 5 (3) Mar 03, 2017
The best solution is going to be driverless cars that maintain speed and "think" way ahead. If the cars a mile or two ahead suddenly start applying the brakes, that signal is relayed back to all the other cars so that they can take more modest action in plenty of time.
humy
5 / 5 (4) Mar 04, 2017

Since electric cars do have to have regular disc brakes, they have to be used or else they'll just seize up and stop working when you need them. That means you can't actually build an electric car that woudn't make brake dust.

Yes, but you can make an electric car that uses the regular disc brakes less often (used only for hard braking) and thus makes less brake dust on average than a typical none electric car.
Eikka
3.5 / 5 (2) Mar 04, 2017
Yes, but you can make an electric car that uses the regular disc brakes less often (used only for hard braking) and thus makes less brake dust on average than a typical none electric car.


That is true to some extent.

The final braking to stop should be done with the regular brakes, first to save energy because the regen turns to negative at surprisingly high speeds, and secondly to ensure that the brakes are used regularily so the discs remain clean and the pistons and pads aren't corroded or caked over and jammed.

In the city centers the situation doesn't change very much because most traffic is low speed stop & go type where the regen barely works, and the regular brakes are needed almost as much as before. Of course you can do one-foot driving if you want to replace your brakes earlier, and you don't mind holding up the traffic by being a slow dick.

The biggest effect would be around off-ramps and sub-urban connecting roads.
gkam
1 / 5 (4) Mar 04, 2017
Eikka is guessing.

We do not use the one-pedal driving because of habit, and we want to have secure control. When we apply the brakes, it also activates the regen harder.
Eikka
5 / 5 (1) Mar 04, 2017
The issue with the regenerative braking at low speeds is the fact that an electric motor isn't actually a very good generator because it cannot be optimized for both simultaneously.

First, the efficiency isn't very high. Secondly, the output voltage is a function of the rotational speed and the amount of magnetic excitation applied on the motor. With an induction or BLDC motor, you basically have to put current in so the motor would generate a voltage high enough to charge the battery. When the motor is turning slowly the amount of power needed for excitation is more than the power returned, so you lose energy and there's no point in applying the regenerative brakes. It's just overheating the motor.

That's why in hybrids and electric cars, in the real world the regen brakes hardly matter. In stop & go traffic where it would come most useful, the system doesn't actually work to any great extent. You're looking at only 5...10% returns of energy in mixed city/hwy driving.
Eikka
5 / 5 (1) Mar 04, 2017
Eikka is guessing.


Guessing what?

We do not use the one-pedal driving because of habit, and we want to have secure control. When we apply the brakes, it also activates the regen harder.


That's completely besides the point.

But you uncovered a thid issue: even using the regular brakes, if you're too soft on the pedal they'll jam sooner than later because of rust and caking road dust. You should make a few brisk stops every now and then or the brake pads may seize up and start chafing and squealing on the discs.

It's a typical "old driver" problem. Drive slow, easy on the accelerator, easy on the brakes, and some sunny day the brakes just won't release properly anymore. Then the half-blind and deaf old bat behind the wheel keeps on driving with the discs glowing red.
gkam
1 / 5 (4) Mar 04, 2017
Oh, nonsense. Go get one and find out for yourself.

My gosh, if you lived in Silicon Valley, we would still using vacuum tube TVs.
qquax
5 / 5 (2) Mar 04, 2017
This is the kind of research we won't get any more now that the EPA budget is decimated.
PPihkala
5 / 5 (1) Mar 05, 2017
There is already a solution to the brake dust: Liquid immersed brakes. They will last longer, they are impervious to moisture and dirt and they release no dust. Of course they cost more, but they need less maintenance. So if there is mandate to reduce brake dust, this can be the way to go.
Nattydread
5 / 5 (2) Mar 06, 2017
Electric cars are definitely the solution. I hardly use the disk brakes on my Nissan LEAF by maximizing the regenerative braking.
antialias_physorg
5 / 5 (2) Mar 06, 2017
Electric vehicles can use "regenerative" braking which is magnetism used to stop the vehicle. No brake pads, no pollution.

Also a viable strategy for non-electric vehicles. Just shift down and let the motor do the work for you. I do this all the time. To the point where my garage tells me I should use my brakes more often or they might rust shut. Saves quite some money in skipped brake replacement intervals.

and emergency breaking still uses break pads.

To be fair: How often have you had to use brakes as true emergency brakes in the last decade? If you're a driver who's aware of his surroundings then probably less than a dozen times.

By the way, what is Tire Dust?

Particulates created by tire and road interaction during slippage which causes friction (slippage happens during all phases of movement on the road:acceleration, driving at speed and deceleration)
gkam
1 / 5 (4) Mar 06, 2017
"Just shift down and let the motor do the work for you. I do this all the time. To the point where my garage tells me I should use my brakes more often or they might rust shut. Saves quite some money in skipped brake replacement intervals."
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My dad used to do that, too, but the brakes are made to wear out and save the wear on the engine. They are cheaper to replace or fix.

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