Not to mention that cars already contain many highly toxic materials, like sodium azide to generate gas for the air bags. From Wikipedia: "It may be fatal in contact with skin or if swallowed. Even minute amounts can cause symptoms. The toxicity of this compound is comparable to that of soluble alkali cyanides and the lethal dose for an adult human is about 0.7 grams."

And yet, it's still in pretty much every car. So I think we can probably handle some thorium, too.