I have been reading more, with PVC plastics that greatest danger is NOT rigid plastic but soft. Like aquarium air lines, potentally leeching phthalates. So still not too much of a concern with stiff plastic tubs unless they are custom made from lexan.

Polyethylene and Polypropylene both seem fine unless you burn them so if you plastic racks are on fire your animals could inhale toxic gas... maybe you should save them from the fire.

I don't think that unless you work in firefighting this is a massive concern, just don't burn it. If you melt holes do that in a well ventilated area or with a respirator.

Still it is worth the 5 min to peek under the tub to find out what code it carrys and if it is a questionable one personally I'd invest the 8 bucks for a new one that is a safer plastic. No big deal.