I carried it around with me for a few days in London, commuting in by train from the suburbs. There were two places that had the highest particulate levels. The first was in my kitchen when I was cooking (no comment!), and the second was on the London Underground. Particulates were far higher on the tube than on the street in central London. So high I would have thought it was a serious health risk for tube drivers. Presumably this mostly comes from the steel wheels on steel track. I don't know whether ingesting steel particulates is more or less healthy than other particulates though. Has anyone done any studies into whether tube drivers have higher rates of health problems than overground drivers?

Edit: table 1 of this paper confirms that the particulates on the underground are mostly iron: https://assets.publishing.service.gov.uk/government/uploads/...