Biofilms in household plumbing can cause chlorine to decay faster, leading to the formation of harmful chemicals in drinking water. Researchers in China studied PVC pipe loops exposed to treated water and found that biofilms inside the pipes consumed more chlorine than the water itself. This led to the creation of trihalomethanes, which are toxic byproducts. The study shows that biofilms play a significant role in reducing chlorine levels and increasing the presence of harmful substances in drinking water.