For decades, concerns about automobile pollution have focused on what comes out of the tailpipe. Now, researchers and regulators say, we need to pay more attention to toxic emissions from tires as vehicles roll down the road.

At the top of the list of worries is a chemical called 6PPD, which is added to rubber tires to help them last longer. When tires wear on pavement, 6PPD is released.

The trouble with tires doesn’t stop there. Tires are made primarily of natural rubber and synthetic rubber, but they contain hundreds of other ingredients, often including steel and heavy metals such as copper, lead, cadmium, and zinc.

Emissions Analytics, found that a car’s tires emit 1 trillion ultrafine particles per kilometer driven.

These particles end up in water and air and are often ingested. Ultrafine particles, even smaller than PM2.5, are also emitted by tires and can be inhaled and travel directly to the brain. New research suggests tire microparticles should be classified as a pollutant of “high concern.”

In a report issued last year, researchers at Imperial College London said the particles could affect the heart, lungs, and reproductive organs and cause cancer.