If you use a water bottle as a bowl for smoking weed, and you accidentally burn the plastic and inhale the fumes, what can happen?

Burning plastic can: be carcinogenic, cancer causing. The small amount you inhaled is probably not an issue but over time it can be a problem. Please don't even microwave anything w/saran wrap on it and make sure water bottles are new and not out-dated because plastic goes into the water. Peace and good health. In general, plastic is dangerous if ingested.