I formerly had a colleague doing asthma research who made a direct correlation between frequency and severity of asthma in children with the microparticles of synthetic rubber that are the result of automotive tire wear. Huge study with multivariable analysis. Since smoking has diminished and coal mining hygiene has been improved this is considered the most common cause (pathologically proven) of black lung disease among city dwellers. Besides causing slow progressive fibrosis of lung lining membranes it can set off significant low level chronic allergic reactions that cause the spastic and inflammatory changes of reactive airways disease (aka asthma). One of the reasons asthmatics are so concerned about pollution alerts. Amazing how we never think about what happens to stuff that we wear out or burn or otherwise discard into our environment, just assuming it disappears miraculaously on its own.