Hi SPS ... Some people make it their sole aim in life to frighten the pants off other people. Yes, there's cadmium in tyres. There's also cadmium on a lot of screws and bolts used to hold together pans, ovens, washing machines etc. etc. There's arsenic in light-emitting diodes. There's sodium and chlorine in table salt. Most parts of the potato plant contain solanin - an extremely effective alkaloid poison. Nettles contain loads of nasty substances. Apple pips contain strychnine.
The rubber used in tyres contains piles of substances which, in large enough doses and assuming that they managed to free themselves from the chemical bonds in which they're held, would quite effectively make a mess of you. But the amounts are usually piddling and they are in stable compounds anyway.
People have been using tyres as planters for decades, and there has been not a single case of anyone reporting harmful effects. The potato tubers do NOT pick up the cadmium from the rubber, just as they do not pick up the solanin from the rest of the plant (unless you expose them to light and they go green - that's solanin, and that's why you shouldn't eat green potatoes. Having said that, no one has ever died from green potato poisoning.
Take no notice of scaremongers. Yes, sometimes there's cause for concern but, believe me, the world will shout soon enough and loudly enough about that. But cadmium in rubber belongs with the green cheese on the moon.
As far as I can ascertain (and I look on a regular basis) there is no evidence anywhere suggesting that ANYTHING leeches from tyre rubber under normal circumstances. Hope that puts your mind at rest.
The secret of life is to aim below the head (With thanks to MMM)