The health issues with the Polyurethanes are because of the isocynates in the hardener - isocyanates = cyanide poison. It effects your cell's ability to transport electrons and oxygen - in other words your cells cant breath. This basically kills all of you at once, effecting the organs first, and yes it's cumulative - once it's inside you it either takes a long time to leave your body, or never leaves. As the cells die the isocyanates simply move to the neighboring cell and prevent it working etc. You can get them into your body from the spray mist, or by your skin contacting the mixed paint or hardener alone. Exposure to cyanide even at low airborne concerntrations (a few hundred PPM) can kill you in minutes. My father had a license to use cyanide powder for poisoning/trapping vermin back in the day when these things were allowed... it's seriously nasty stuff. Burning anything covered with or made of polyurethane also releases isocyanates (cyanide) so they should not be used inside the aircraft cabin, in case of fire.