what are the most harmful chemicals you have found in the water, and why are they harmful?

Question: what are the most harmful chemicals you have found in the water, and why are they harmful?
- Keywords:
  chemical,
  pollution,
  water
Asked by faux520shes to DrBecks, Mimi, Lauren G, Heather F, Emma T, artemiseales, Alex on 4 Mar 2025.
- Artemis Eales answered on 4 Mar 2025:
  
  The most harmful chemicals in the water in my opinion are metals such as lead and mercury. They can cause severe illness if ingested, which occurred in an incident in Japan where mercury poisoning from the environment was named Minamata Disease. It’s grim, which is why I consider it so harmful!
- Mimi Asogwa answered on 7 Mar 2025:
  
  There are so many harmful chemicals that gets washed into sea water eg medicines including antibiotics, metals, pesticides, radioactive substances and household chemicals. In my opinion, the most harmful is lead and can cause serious problems for children and pregnant women.

Question: what are the most harmful chemicals you have found in the water, and why are they harmful?