I don't think a regular stovetop can get a pan to 450°C, my gas stove gets an empty pan to about 300°C maximum. It doesn't happen in normal situations, if it happens it probably means you forgot your pan on the stove. Heating Teflon at 300°C for several hours is bad, but personally, in that situation, I would worry more about causing a house fire.

Teflon flu is a thing, but it is relatively rare, especially considering how widespread Teflon pans are. That's a few hundred cases per year in the US, by comparison, there are about 1000x more house fire, with cooking equipment being a leading cause.