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Old 01-22-19, 10:35 PM
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Spoonrobot 
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I wondered about this after a thread a while ago about the black powder that often ends up covering parts of a bike. Having noticed this myself and curious to determine if it was rubber from car tires breaking down (as argued) or something else I scraped a bunch off with a q-tip, washed into a solution and then dropped in a strong magnet. Turns out that my sample was almost entirely magnetic particles, so not rubber. Later I did the old masking tape/nickel air pollution test and it was equally enlightening. After only a day the masking tape was covered in a relatively thick layer of particles, probably a measurable amount in dozens of milligrams based on the pile I was able to scrape off.

I've always suspected there are a lot of metal/ceramic particles floating around from automotive brake rotors and pads and wouldn't be surprised if long term exposure did have an effect. Previously I had noticed when I ride gravel, or on the road during the pollen season it's not uncommon to have significant eye discharge (rheum, eye boogers) both during a ride and hours after that is tinted the same color as the gravel surface or the color of pollen. I didn't really think anything of it at the time.

So I don't have an answer but after my little experiment I started wearing clear glasses and will continue to do so. It seems to help when riding gravel and we'll see once we start getting pollen.
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