Who is responsible for the loss caused by the water prevention in the bathroom on the first floor?

Legal analysis: you can find a property company to coordinate and find out the reasons among the residents in this unit. If the chemical pool is blocked, it shall be borne by the property; If the responsible person can't be found among the residents, even if the loss is small, other residents in this unit will be brought to court through judicial procedures to demand corresponding responsibilities.

Legal basis: Civil Code of People's Republic of China (PRC).

Article 83 If an obstruction or loss is caused to an adjacent person, the infringement shall be stopped, the obstruction shall be removed and the loss shall be compensated. If the neighboring party fails to perform the court's judgment, the injured party may ask the construction team to repair it downstairs, and the expenses incurred may be claimed by the upstairs owner to the court.