For decades, the conversation about traffic-related air pollution has centered almost entirely on what comes out of the tailpipe. Exhaust emissions, combustion byproducts, diesel particulates — these are the villains most people picture when they think about cars and air quality. Regulatory frameworks around the world have been built around controlling them, and by most measures those regulations have worked. Exhaust emissions from new vehicles are dramatically cleaner than they were twenty years ago.

What almost nobody is talking about is what comes off the wheels.

Brake dust and traffic-related particulate matter

As exhaust emissions have declined, researchers and regulators have begun turning their attention to a different category of vehicle pollution: the particles shed by brakes, tires, and road surfaces during normal driving. These are called non-exhaust emissions, and the numbers are striking. According to the European Environment Agency, brake and tire wear now accounts for approximately 77 percent of PM10 and 60 percent of PM2.5 from road transport across EU member states as of 2023. In the United Kingdom, the Air Quality Expert Group found that non-exhaust emissions contribute well over half of all particle pollution from road traffic. A 2023 study funded by the California Air Resources Board, placing monitors near two Southern California highways, confirmed that air pollutants from brake and tire wear at those locations exceeded those from engine exhaust.

The European Commission has projected that by 2050, non-exhaust emissions could constitute up to 90 percent of all particles emitted by road transport as cleaner engines become the norm.

This is not a minor footnote to the tailpipe story. It is the story.

What these particles actually are

Brake dust is generated by the friction between a brake pad and the rotor every time a vehicle slows down. The particles released are not inert debris — they are a chemically complex mixture whose exact composition depends on the brake pad formulation. Research has identified copper, iron, manganese, antimony, zinc, and various organic compounds including polycyclic aromatic hydrocarbons as common constituents. Copper is particularly notable. A 2025 study published in a peer-reviewed journal found that brake-wear particles from copper-enriched brake pads induced greater oxidative stress, inflammation, and cellular disruption in human lung tissue than diesel exhaust particles used as a comparison — a finding that challenges the assumption that non-exhaust particles are somehow less concerning than combustion products.

Tire wear particles tell a related but distinct story. Over its lifespan, a tire loses between 10 and 20 percent of its mass to wear, ground into particles that range from visible rubber crumbs down to fine and ultrafine fragments smaller than 0.1 microns. A 2024 study in Environmental Science and Technology found that tire wear chemicals were detectable throughout urban air in the Pearl River Delta region, with the compounds distributed primarily in the gas phase — meaning they are not just particles you might filter, but chemical compounds that behave more like gases once airborne. The proprietary nature of tire formulations complicates research; manufacturers are not required to disclose all ingredients, and some include zinc, lead, and other compounds with known health implications.

Road surface abrasion adds a third stream: particles from the pavement itself, often mixed with tire rubber and brake debris, resuspended into the air by passing vehicles.

The size problem, and why these particles reach deeper than most people realize

Particle size determines how far into the respiratory system a particle travels. Coarse particles, generally 2.5 to 10 microns, can reach the airways but are often captured there. Fine particles below 2.5 microns, the PM2.5 fraction, penetrate into the smaller airways and lung tissue. Ultrafine particles below 0.1 microns can enter the bloodstream directly.

Brake and tire wear generates particles across all of these size ranges. Research on brake emissions shows that braking initially releases larger particles, followed immediately by smaller ones below 1 micron. The fine and ultrafine fractions are particularly concerning because they carry the heavy metal content — the copper and iron — in a form small enough to deposit deep in the lungs, where it can trigger localized oxidative stress and inflammatory responses. One study in the research literature found that brake-wear particles caused a decrease in tight junction proteins in lung epithelial cells and induced measurable production of reactive oxygen species, with the intensity of the response correlated with the metal content of the particles.

The implications for human health remain an area of active and somewhat unsettled research. Laboratory studies have identified mechanisms for cellular damage and inflammation. Epidemiological evidence linking specific non-exhaust particle exposure to health outcomes in the general population is more limited, in part because it is methodologically difficult to isolate these particles from the broader mix of traffic-related pollutants. What can be said with reasonable confidence is that the metals these particles carry — particularly copper and iron — are biologically active in ways that plain carbon soot is not, and that the size fractions generated reach places in the lungs where the body's defenses are less effective.

Electric vehicles do not solve this

This is perhaps the most important piece of context for anyone who assumes that the shift to electric vehicles will eventually make this a historical footnote. Electric vehicles produce no tailpipe emissions. They do not reduce brake and tire wear in any meaningful way, and in some cases may worsen it.

Tire wear is driven primarily by vehicle mass and driving behavior, not by powertrain type. Electric vehicles are generally heavier than their combustion-engine equivalents, largely due to battery weight, which means they exert more force on road surfaces and may wear tires faster. Regenerative braking does reduce how often friction brakes engage, which can decrease brake particle emissions under certain driving conditions, and the Ricardo report published for the UK Department of Transport in late 2025 found measurable differences in brake wear emissions between EV and combustion-engine vehicles. But tire wear, road surface abrasion, and resuspended road dust persist regardless of what powers the vehicle, and those sources collectively represent a substantial fraction of non-exhaust emissions.

Regulators have begun to catch up. The European Union's Euro 7 standards, finalized in 2023, represent the first time limits on brake and tire wear emissions have been introduced in vehicle regulation anywhere in the world. In the United States, no equivalent regulations currently exist, and the EPA's near-road monitoring network focuses primarily on nitrogen dioxide rather than the particulate matter most associated with non-exhaust sources.

What this means if you live near a road

The research is fairly consistent on one point: concentration of brake and tire particles falls sharply with distance from the roadway. Exposure is highest within the first few hundred meters of a heavily traveled road, and decreases meaningfully beyond that. For most particles in the coarser size ranges, the road corridor is where the risk is concentrated.

Ultrafine particles, however, are another matter. Because they are so small, they can infiltrate buildings through ordinary gaps in construction — around windows, through ventilation systems, and via any pathway that connects indoor and outdoor air. Once inside, they can remain suspended for extended periods, drifting through rooms and settling into fabrics and dust. Research cited by IQAir notes that ultrafine particles smaller than 0.1 microns slip through tiny cracks and can stay suspended in indoor air for hours, moving from room to room in a way that coarser particles do not.

For people who live within a few blocks of a busy arterial road, a highway on-ramp, or any corridor with significant stop-and-go traffic — which maximizes brake use — the indoor air quality question is not purely academic. The particles being generated outside are finding their way in.

Where air purification fits in

True HEPA filtration captures particles down to 0.3 microns, which covers the fine particle fraction of brake and tire wear — the PM2.5 range where most of the metal-bearing particles that have raised health concern in laboratory studies are found. Running a HEPA-equipped air purifier continuously in the rooms where you spend the most time creates a consistent cycle of particle removal, reducing the concentration of fine particles that have infiltrated from outdoor sources before they accumulate in the air you are actually breathing.

The iAdaptAir by Air Oasis combines True HEPA filtration with activated carbon for gas-phase compounds, UV-C light, and bipolar ionization in a CARB-certified ozone-free unit designed for continuous operation. For people living near roads with heavy traffic, the value of running it consistently rather than occasionally is meaningful — these are not episodic pollution events but a steady background source that does not stop when traffic slows. The air quality indicator ring on the iAdaptAir gives you real-time feedback on particle concentration in the room, which is genuinely useful when outdoor sources are contributing to indoor air quality in ways that are otherwise invisible.

The traffic pollution conversation has been overdue for an update. Cleaner engines were a genuine achievement. But as long as vehicles have tires touching roads and brake pads meeting rotors, the particles they shed will keep finding their way into the air we breathe — indoors included.

Shop Air Oasis and find the iAdaptAir sized for your space. Breathe Better, Live Better.