Beyond the Tailpipe

This blog was written by Ethan Petersen, a 2023 Summer Schneider Fellow for Transportation at NRDC.

Imagine, for a moment, you’re standing at the side of a busy highway on a hot day, breathing in the smog of a thousand rumbling cars whizzing by you. What if I told you that the pollution you’re feeling right now wasn’t only coming from the engines of those cars, but also from their tires and brakes, as well as the road surface itself? 

It’s true—by some estimates these non-engine pollutants, called “non-exhaust emissions,” already make up over half of the health-damaging particulate matter which comes from on-road transportation. These emissions are becoming an increasingly significant problem for air quality near roads as vehicles get heavier and more miles are driven everyday. 

Since the passage of the Clean Air Act in 1970, improvements in air quality has been one of America’s great public health success stories. Tailpipe pollution from cars and trucks has fallen dramatically, and is set to drop further with new federal and state clean car and clean truck standards. These rules, combined with a fast-growing market for electric cars, promises continued progress on tailpipe pollution.  

However, the tailpipe is not the only source of pollution from vehicles. Non-exhaust emissions are also unjustly distributed—Black, Latine, and Asian Americans (regardless of income) and Americans who have low incomes are disproportionately exposed to and affected by particulate air pollution, including roadway air pollution. And these emissions also affect wildlife, and the people that depend on them, as evidenced by EPA’s recent agreement to review chemicals used in almost every tire after a petition from West Coast Native American Tribes claimed the chemical kills salmon. In fact, tire pollution in general is getting a more thorough review from regulators. 

The growing field of research on non-exhaust emissions helps us start to understand this often-neglected, largely unregulated, and persistent source of pollution, which threatens to exacerbate environmental injustice if left unaddressed. Providing safer and more convenient transportation choices like walking, cycling, and public transit, as well as reducing vehicle weight and improving brake and tire technologies, can help avoid non-exhaust emissions and move us toward a just, healthy, and sustainable transportation system. 

What are non-exhaust emissions? 

Non-exhaust emissions are microscopic, airborne particulates released by the wear and tear of vehicle tires, brake pads, brake discs, and the road’s surface itself. The prevalence of these particulates in the air surrounding roads is often measured as PM_2.5 or PM₁₀, which are the concentrations of airborne particulate matter (PM) less than 2.5 or 10 micrometers in diameter, respectively. The health-damaging effects of both PM_2.5 and PM₁₀ from all sources are well documented, and many researchers have speculated that non-exhaust PM has similar impacts on human health as exhaust PM.

Non-exhaust particulates come from brakes, tires, and the road surface; each of these sources has distinct impacts on air quality. Resuspension of dust already on the road’s surface is the most significant contributor to non-exhaust PM by far, however these particles are difficult to characterize and manage because they could come from anywhere before landing on the road. Brakes are the next most significant source, and may also be particularly hazardous because of their small size and high metal content. Tires contribute the least, but they release large amounts of particles which act as microplastics in ecosystems. In fact, tire particles are estimated to be the second-largest source of oceanic microplastics globally, and have been shown to have devastating effects on aquatic ecosystems.

Rates of non-exhaust particulate emissions are highly variable depending on a range of factors, such as the type of brakes, tires, and pavement used, as well as the speed and acceleration of driving. The most important factor, however, is vehicle weight—heavier vehicles place more strain on brakes and tires, which leads to more wear and emissions releases. If Americans continue driving bigger and heavier cars for longer distances, non-exhaust emissions are likely to increase. 

While the zero-emissions vehicle transition is critical to achieving our climate goals, these vehicles still release non-exhaust particulate emissions. With improvements in weight efficiency, regenerative braking, and use of smaller cars, however, zero emission vehicles have the potential to reduce non-exhaust emissions by as much as 27 percent.

Impacts of non-exhaust emissions 

Non-exhaust emissions are a persistent and unregulated source of hazardous airborne PM, which threaten to continue damaging human health if left unaddressed. PM exposure has been linked to a wide range of respiratory, cardiovascular, and neurological effects, and is associated with 100,000-200,000 deaths each year in the United States alone. 

PM from automotive emissions tends to be concentrated near where it was released, so people who live and work near highways and other busy roads are exposed to much greater levels of PM than others in their region. Due to well-documented histories of racist highway development and housing segregation, Black, Latine, and Asian communities are typically closer to busy roads and therefore exposed to higher levels of automotive PM. 

Other health indicators related to systemic racism, such as higher rates of chronic stress, less access to healthy food, and less access to healthcare, also means people in Black, Latine, and Asian communities are more susceptible to the health impacts of PM exposure. Even if exhaust emissions are eliminated, airborne PM concentrations will remain at dangerous levels in communities near highways if non-exhaust emissions are left unchecked. 

Reducing non-exhaust emissions

Non-exhaust emissions will perpetuate decades-long environmental and health inequities if left unaddressed, but they receive little attention outside of specialized circles. In order to build a transportation system which is healthy, equitable, and climate-friendly, we must adopt strategies that consider and address non-exhaust emissions among other forms of pollution and environmental harm: 

• Shift to more efficient forms of mobility, such as walking, cycling, and public transportation. Buses and trains also release non-exhaust emissions, but they have a smaller impact on a per-trip basis. 
  • State Departments of Transportation have the ability to shift funding away from carbon-intensive and polluting uses like highway expansion, and toward climate- and health-friendly projects which improve walking, cycling, public transportation, and EV charging infrastructure.  
  • Other new federal opportunities, like Reconnecting Communities and Neighborhoods or Safe Streets For All, provide the resources for communities to build-out people-focused transportation infrastructure.
• Reduce vehicle weight and size. This does more than just reduce emissions—smaller, lighter vehicles are more efficient and have better road safety outcomes, and smaller EVs can have extended ranges, lower costs, and less demand for battery materials.
• Improve tire, brake, and road technologies with non-exhaust emissions in mind.
  • Regenerative braking in electric vehicles can significantly reduce brake usage, and therefore reduce non-exhaust emissions. Automakers should design systems which encourage its use. 
  • Better tire durability could reduce non-exhaust emissions while saving on maintenance costs for consumers. Specific emissions-reducing technologies, such as specialized tire compounds or particle-capture systems should also be explored. 
  • Although porous pavement requires maintenance, there’s limited evidence that it helps reduce road dust resuspension, in addition to its other environmental benefits. 
• Directly regulate non-exhaust emissions. The European Union has proposed to limit brake and tire emissions from new vehicles under Euro7, its next round of automotive regulations. Regulatory structures for automotive air pollution in the United States should also turn their focus toward policies that can reduce non-exhaust emissions. 

It’s possible to reduce non-exhaust emissions, and many of the most effective options also reduce carbon emissions and improve transportation equity—so building a just, healthy, and sustainable transportation future is within our reach.