It's the rubber tires that smell. I have a 2 wheel cart that I store in the basement occasionally, and it then smells like HF down there. It's not just Chinese rubber that smells that way. When my son was into Legos, the tires in those kits smell very similar.
BTW, did you know who makes the most tires in the world? Legos. They make more than twice as many as the next largest tire manufacturer. And they are not made in China. If you look at them closely, they are very much like full sized tires, just smaller versions.