when i say plastic, i mean examples like drinking water from a water bottle that has been sitting in the car and is warm, or microwaving food in a plastic container
Cigarettes would be more poisonous.
Plastic has to melt or burn to release any sort of toxins, so cigarettes are definitely more poisonous. But it takes an awful lot of cigarettes an awful long time to do any more harm than inhaling any other type of smoke into your lungs would do.
