Car brake wear particles are charged and cause inflammation

Much of the discussion on the role that car emissions play in climate change centres on the emissions that come out of the exhaust pipe. But researchers in the United States say we should also be paying closer attention to the particles that our cars emit when we brake. It turns out that these brake wear particles, which can be just as small as some exhaust particles and trigger inflammatory reactions in the body, actually carry an electric charge; this is both a blessing - because it makes them easy to remove, and a curse, because that can keep them airborne for longer. Jim Smith is from the chemistry department at the University of California, Irvine.

Jim - This is a project that actually has roots back in 2016 to the Volkswagen emissions scandal. As a result of that, Volkswagen gave the state of California a sum of money for research on vehicle emissions and so we applied and we were awarded a two year grant, landed into this area of measuring the properties of brake particles: particles that come from not the tailpipe, but from other sources such as brakes and tires, from road surface wear. They've actually caught up to the emissions that are coming from tailpipes. This is an area of growing concern. We really have no regulations on how cars may or may not be able to emit particles from other sources besides the tailpipe.

Chris - And I suppose as we transition more towards vehicles powered electrically, the proportion of pollution that comes from these other sources like tires and brake pads is going to increase.

Jim - Absolutely, yeah. We love electric vehicles for the impact on our greenhouse gases in the environment, but if you think about it, electric vehicles are not truly zero emission vehicles. You'll always have these other sources unless you have ways of controlling the emissions from these sources.

Chris - And how big are the particles, Jim? Are they on par with what we're worried about with very small particles coming from exhaust pipes, or are they in a totally different regime entirely?

Jim - Actually, surprisingly, they can be quite small. This is something that surprised me a lot. Before I started research, I thought, well, most of these must be large bits of brake material that go into the air. But you can actually create very small particles that can actually penetrate deep into the human lung and actually cause a host of different health ailments. We should really think of these non exhaust emissions as being quite comparable to exhaust emissions in the actual size regime of the particles being produced.

Chris - So what did you actually measure, and what did you find when you were looking into this?

Jim - We went into our machinist workshop and we asked the the machine shop if they could let us use one of their large metal working lathes as a way of spinning the disc of a brake, built a chamber around this lathe, and we were able to replicate the action of braking. We brought all of our equipment in and we started making measurements mostly of the physical and chemical properties of brakes. At that point, it didn't really occur to us to think about the charge or whether or not particles themselves might have anything other than just chemical properties and a different size regime.

Chris - Well you've piqued my interest now. What's special about the charge, then?

Jim - Well, actually, the idea came from one of my colleagues who said, when you rub things together, you actually can create charge on surfaces. He said, well, braking is exactly that: it's rubbing two surfaces together. I wonder if particles produced by brakes could have a charge on them. I thought about that and I said, that makes a lot of sense; let's go try that out. We immediately were able to measure charged particles that were produced from this braking process. It's a very similar mechanism to what would happen with a balloon. If you just rub a balloon on a sweater or your hair like I used to do when I was a kid, and then stick it to the wall, I used to marvel at that. That was such an interesting phenomenon. What you're doing is you're transferring electrons or charges from one surface to another, and essentially that's what is happening with these particles. The act of braking generates this mechanism by which charges get transferred to these particles, and you can think of these particles as little balloons that just got rubbed on a sweater and are now floating in the air with all this static electricity on them.

Chris - And why does that matter, the fact that the particles have a charge?

Jim - Well, there's a couple of things that I think are really interesting about this. For one thing, when you take some of these particles and put them in liquid, they'll actually create a kind of oxidant that actually can be harmful to health. And so if they're charged, this provides us an avenue from which we can actually remove them from the source itself.

Chris - A sort of scavenging system? So as the car is making the particles, rather than them being released into the air or the road surface, you try and grab them before they get away?

Jim - Yeah. So the fact that they're charged means that we can apply an electric field that will provide a force on those charged particles and allow us to sweep them away before they even have a chance to exit the braking system. Our research has shown that up to 80% of the particles coming off of brakes could have a charge on them, so you could immediately remove about 80% of the particles coming from this important source of pollution using electric fields.

Chris - We've predictably dwelled quite heavily on the human health implications, but are there any environmental impacts as well of particles? Can you foresee any role for these sorts of charged particles when they go beyond where they might hurt us directly and be doing things environmentally?

Jim - An insight into this actually comes from research that's being done on dust particles in the atmosphere. It turns out there's been a big mystery in the field of atmospheric science which is, how is it that relatively large bits of dust that come, say, from the Sahara Desert can actually stay in the air for weeks at a time? It turns out these particles, in much of the same way that these brake particles are acquiring charge, actually acquire a little bit of charge in the process of being swept off of the desert. A lot of people know that the Earth has a magnetic field. Well, it turns out the Earth actually has a very weak electric field as well. Even though the earth itself is neutral in charge, the surface of the earth has a very slight negative charge. And the base of a cloud, say, for example, in the atmosphere, will have a very slight positive charge. It's a very small effect, but it can actually have a profound effect on allowing charged particles, particles that have this sort of static electricity on them, to actually remain suspended a lot longer in the air than what would normally be the case if they were neutral or particles that didn't have any charge at all.