[ipqk]

Tires are only about 25% natural rubber; the rest is synthetic rubber, heavy metals, plastics, and additives. These get emitted as fine particulates that stick around in the air. They may not technically be a gas, but we breathe it in just the same.

[jiggawatts]

I very much doubt that heavy metals such as mercury or lead are used to make tyres. I've only ever heard of steel belted tyres.

It used to be that wheels were balanced with small lead weights, but the use of lead for this has been banned in pretty much every western country, and wasn't contributing to road dust even when they were allowed.

[ipqk]

"Hundreds of other ingredients, including steel, fillers, and heavy metals — including copper, cadmium, lead, and zinc — make up the rest, many of them added to enhance performance, improve durability, and reduce the possibility of fires."

https://e360.yale.edu/features/tire-pollution-toxic-chemical...