Porex says their filters are polyethylene, but don't designate polyethylene terephthalate (PET) so it may be another form of polyethylene. You might try a google search for "polyethylene "Material Safety Data Sheet"".
I couldn't find one specifically for Porex but I don't think I'd use it without more research. There have been many materials suggested here. The normal filler is PET (basically polyester) and it's found to be relatively safe if you don't burn it at high temps.
The Safety of Polyethylene Terephthalate (PET)
The Lipton Pyramid tea bags are also PET, as is the popular 'blue foam' and 'aquaclear'. But things that you might think are safe aren't - cotton for example, shouldn't be inhaled whereas wool is safe (unless you have an allegy to it of course). It appears that some forms of polyethylene (not PET) can become molten according to one MSDS I read.