Where do most particulate emissions attributed to cars come from? California speaks as if their primary source is the tailpipe. That was true in the past. But today most vehicle-related particulate matter comes from tire wear. Cars are heavy, and as their tires rub against the road, they degrade and release tiny, often toxic particles. According to
measurements by an emission-analytics firm, in gasoline cars equipped with a particle filter, airborne tire-wear emissions are more than 400 times as great as direct exhaust particulate emissions.