The level of risk from a sanitary sewer overflow depends on its size, location, and weather conditions. Untreated wastewater can contain bacteria, viruses, and other harmful organisms, as well as debris or decaying organic material. During heavy rain, these contaminants are diluted, reducing environmental impact. In dry conditions, overflows are more concentrated and can pose greater environmental concerns. 

Sensitive groups—such as children, the elderly, and individuals with weakened immune systems—may face higher health risks if exposed. Public drinking water supplies are rarely affected, though private wells near an overflow site may be at risk. If an overflow presents a significant health concern, public notification may be issued.