do water thermos cause mercury poisoning?

Answer 1

No. They don't have mercury in them so they can not cause mercury poisoning. Only glass touches the contents in a thermos.

Answer 2

No. Mercury is not used in any modern manufacturing process that contacts food.

However, some old "thermos" used glass rather than stainless steel or plastic, like virtually all insulated mugs and containers today. There were some old glass manufacturing processes 30-40 years ago that involved heavy metal use, so if you're worried, just avoid any really old thermos with a glass interior, and just keep or sell them as antiques.