Cleaning burnt-on grease with ammonia

(Apologies if this belongs elsewhere as it's only marginally related to cooking but I can't think of any other place that's a better fit.)

I recently ran into this (put your pans etc in a closed container {a garbage bag will do}, pour in a bit of household ammonia and let it sit for many hours) and love it. However, I also have some pans with those black plastic? handles, is it safe to do on them also? I also have a teflon pan that has some grease stuck on the outside. Since there's no way to treat only the outside {it's the fumes that do the work, you don't immerse things} is that safe?