What hazards does stagnant water in plumbing systems pose?

Stagnant water in plumbing systems presents significant hazards primarily related to bacterial growth and vector breeding. When water remains still for extended periods, it creates an ideal environment for harmful bacteria, mold, and algae to thrive. These microorganisms can contaminate the water supply, posing health risks to those who consume or come into contact with the water.

Moreover, stagnant water can attract pests such as mosquitoes, which can breed in stagnant pools. This not only poses a nuisance but can also lead to the spread of diseases such as West Nile virus or Zika virus. The combination of these health hazards makes the presence of stagnant water a serious concern in plumbing systems.

While stagnant water may contribute to factors such as increased water temperature and decreased water quality, the most critical and direct health risks arise from the growth of bacteria and the potential for pest breeding, making this the correct choice.