IICRC S500 defines three categories of water based on cleanliness and pathogenic risk. Category 3 — formerly 'black water' — includes sewage backflows, rising flood water, and any source that may contain pathogens, heavy metals, or toxigenic organisms.
Category 3 losses require removal of porous materials within the affected zone, antimicrobial application, and rigorous post-remediation verification — including surface total coliform and E. coli testing where appropriate.
The most common failure mode we see in the field is under-categorization: a contractor treats a Cat 3 event as Cat 2 to save scope. Months later, occupant illness and visible biofilm tell the truth.
An independent assessment establishes category and scope before remediation begins — protecting the occupants, the carrier, and the contractor.