> Why would tires contain cadmium or lead? I guess because of the chemical additives used in the rubber manufacturing process to improve the rubber's material

> Why would tires contain cadmium or lead? I guess because of the chemical additives used in the rubber manufacturing process to improve the rubber's material properties. E.g. https://www.sciencedirect.com/science/article/pii/S1021949816301259 https://www.sciencedirect.com/science/article/pii/S102194981... (a random article warning about heavy metal contamination of rubber stoppers used in drug manufacturing). I remember 1493 talking about how natural rubber, while it has many otherwise useful properties, responds poorly to heat and cold. So vulcanization and other chemical transformations helped improve that and make rubber appropriate for a much wider range of applications. It looks like some of the vulcanization processes use metal oxides, including lead oxides, as an additive. The examples on the Wikipedia page using lead oxides are for neoprene (which is unlikely to be in tires, I guess) but it seems that butadiene, which is in tires, is made with "metal oxide catalysts" https://en.wikipedia.org/wiki/Butadiene https://en.wikipedia.org/wiki/Butadiene I wonder if the catalysts contaminate the resulting product or something. (Clearly the butadiene itself isn't supposed to contain metals and the catalyst, being a catalyst, isn't supposed to be absorbed into the reaction.) Could there be rubber manufacturers who just throw some heavy metal oxides directly into their rubber because they're cheap and they help make the end product more durable, even though there are in principle better and safer ways to achieve the same results? Maybe they're thinking "this particular rubber isn't going to be eaten or come in contact with food, and it's not going to come in contact with skin very much, so it doesn't really matter if it's a bit toxic". I've seen Prop 65 warnings for lead on all sorts of electrical cables -- maybe it was used to manufacture the rubbers and vinyls in the insulators?