Walking along a busy road could expose you to massive amounts of invisible pollutants, called tire wear particles, or TWPs.
Each year, an estimated 6.1 million tons of these micro- and nano-plastic particles shed from tires as vehicles travel. They’re a significant but often overlooked source of global plastic and chemical pollution.
What’s in TWPs
There are hundreds of different tire formulas, and manufacturers are not obligated to disclose all of their ingredients. In general, TWPs contain:
- Natural and synthetic rubber
- Carbon black for strength and pigment
- Sulfur to harden the rubber
- Organic compounds, such as BPA, phthalates and polycyclic aromatic hydrocarbons
- Antioxidants, such as 6PPD, to extend tire life
- Heavy metals, including zinc, cadmium, copper and lead
But that’s not all. TWPs act like pollution sponges, soaking up and re-releasing harmful substances in the environment, like pesticides, metals, antibiotics and even bacteria. This makes it harder for scientists to ascertain just how dangerous TWPs are.
Where TWPs end up
TWPs are just about everywhere around the globe, including the Arctic.
In the air
Some remain suspended in the air – smaller particles can stay airborne for up weeks and travel thousands of miles by wind.
TWPs are more concentrated near roads, so people living in urban areas with a higher density of car traffic may be more exposed. One estimate found that city residents breathe in 3,200 TWPs a day. TWPs disproportionately affect low-income communities and communities of color, who are more likely to live near highways.
In the soil
Most TWPs shed over land eventually settle in the soil, where they can alter its properties. Contaminated soil is more compact, less effective at retaining water and home to fewer essential soil organisms, like microbes and worms. Worms’ ingestion of these particles can affect their gut biodiversity, body weight, reproduction and survival.
These soil changes go on to harm plant life. TWPs have been found to reduce the root and shoot growth of crops like mung beans, soybeans, leeks and wheat.
In our water
Rain can carry TWPs from soil into our waterways. An estimated 5% to 10% of global plastics in our oceans is tire wear. These particles threaten aquatic life, causing cell damage, hormone disruption, weakened immunity and even death.
Researchers found that zebrafish exposed to TWPs suffered from abnormal eye development that affected their swimming. Salmonids are another well-known victim.
A common tire additive, 6PPD, prevents degradation. Sunlight transforms it into the highly toxic 6PPD-quinone, or 6PPD-Q. 6PPD-Q from TWP runoff kills a significant amount – between 40% and 90% – of the coho salmon population that spawns in West Coast waterways.
How TWPs may harm humans
TWPs have infiltrated ecosystems and threatened their inhabitants. What about human health? Researchers are just beginning to answer that question, but what we already know raises concerns.
TWPs are not just a potential threat; there’s real evidence of exposure. A study of 150 children, adults and pregnant women in China found 6PPD and 6PPD-Q in their urine. Alarmingly, pregnant women had the highest concentrations of both chemicals.
Already, researchers have linked TWPs and their additives to cell damage, lung inflammation, heart problems, reproductive risks and digestive issues in animal and human cell studies.
How we’re exposed
There are three main ways we can be exposed to TWPs:
Inhalation
Breathing in TWPs is the most common pathway. The particles can settle deep in the lungs and cause damage. In one study, mice developed lung scarring, better known as pulmonary fibrosis, when exposed to TWPs.
Consuming food and water
TWPs also end up in what we eat and drink. They can accumulate in fish and be passed on to us when we eat seafood.
Leafy greens are another concern. A recent study found that 6PPD and 6PPD-Q were absorbed by the roots of hydroponic lettuce and transferred to the leaves we eat.
This is alarming because TWPs may contaminate the water and sewage sludge used for agriculture. In fact, one study estimated up to 35% of these particles pass straight through wastewater treatment, often ending up in water for farming – and even drinking.
Contact with skin
Suspended TWPs may interact with your skin. More research is needed on whether these particles can pierce the outermost layer of skin, but various tire additives combined with oxygen, such as DPA and TMQ, cause skin irritation. The European Union even designated 6PPD as a skin sensitizer.
How to protect yourself
To reduce your exposure to plastic particles, including TWPs, in drinking water, consider using a reverse osmosis water or activated carbon filter.
For airborne plastic particles in your home or car, a purifier with a HEPA filter and activated carbon may help. It won’t eliminate all microplastics but may lower your exposure.
Limiting the time you spend near roads can also make a difference. When walking outside, if possible, seek out residential streets or paths that are further from busy roads, where there may be less tire pollution.
How to reduce TWPs where we can
It’s hard for individuals to completely avoid TWPs, but there are a few ways to reduce how many are shed from driving. These changes will not fix the problem on their own but can help decrease emissions while we work towards systemic change.
- Avoid aggressive driving, such as speeding, sudden braking and sharp turns. Cars that travel faster or stop and start suddenly generate more TWPs.
- Replace your tires when appropriate and ensure they are properly inflated. Old and under-inflated tires wear more quickly.
- Use public transportation or carpool whenever possible to reduce the number of vehicles on the roads.
Steering toward systemic change
TWPs are just one of many consequences of our car dependence. So what does systemic change look like?
It starts with addressing TWPs by reimagining how our communities are built. This wouldn’t just solve one problem, it would also unlock a wider network of benefits, from cleaner air to safer, more equitable cities.
That culture shift won’t happen overnight. In the meantime, governments must hold industries accountable. Current regulations on what tire manufacturers must disclose about their products are too lax. But we are beginning to see change.
Under a new California law, tire companies must explore replacements for harmful additives such as 6PPD. Next year, the EU will set pioneering abrasion limits and test procedures for all tires sold in its member countries.
Stronger oversight and innovation in the tire industry are needed to protect both human health and the environment. While we cannot eliminate TWPs entirely, through better awareness, industry pressure and a fundamental rethinking of how we move through the world, we can reduce their impact. Get behind the wheel; it’s time to steer towards a healthier future for all.
