S520 is an ANSI-approved consensus standard published by the IICRC. It defines three conditions: Condition 1 (normal fungal ecology), Condition 2 (settled spores from a Condition 3 source), and Condition 3 (actual indoor mold growth).
Remediation under S520 is goal-oriented: return all materials in the work area to Condition 1. The standard specifies containment, engineering controls, PPE, and the role of the independent assessor in writing the protocol and performing PRV.
Critically, S520 contemplates an independent investigator distinct from the remediation contractor. That separation is the standard's structural safeguard against conflicts of interest.
Every Hurtler protocol references S520 by section so the contractor's deliverables are unambiguous.