Hazards in water distribution systems can be in the forms of microbial, chemical and physical.
Most microorganisms are harmless within water distribution systems. However, when infectious microorganisms enter the systems, they form biofilms  and create microbial hazards to the users. Biofilms are usually formed near the end of the distribution where the water circulation is low. This supports their growth and makes disinfection agents less effective. Common microbial hazards in distribution systems come from contamination of human faecal pathogens and parasites which enter the systems through cross-connections, breaks, water main works, and open storage tanks.
Chemical hazards are those of disinfection by-products, leaching of piping materials and fittings, and water treatment chemicals.
Physical hazards include turbidity of water, odors, colors, scales which are buildups of materials inside the pipes from corrosions, and sediment resuspension.
There are several bodies around the world that create standards to limit hazards in the distribution systems: NSF International in North America; European Committee for Standardization, British Standards Institution and Umweltbundesamt in Europe; Japanese Standards Association in Asia; Standards Australia in Australia; and Brazilian National Standards Organization in Brazil.