OSHA's PSM standard lists acceptable PHA methodologies including HAZOP, What-If, Checklist, What-If/Checklist, FMEA, fault tree analysis, and an appropriate equivalent methodology, but does not mandate a specific method. The employer chooses. However, OSHA expects the chosen methodology to be appropriate for the process complexity and hazard severity. Applying a checklist to a highly reactive chemistry process would likely draw criticism. These letters address method selection criteria, combining methodologies, and what OSHA considers equivalent when a non-listed method is used.
What OSHA Has Said
Jul 26, 1995
Methodology Selection Criteria and Complexity and Hazard Severity
OSHA articulates the expectation that method selection should be commensurate with the complexity of the process and the severity of potential consequences. Simple checklists are inadequate for highly hazardous reactive processes.
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Nov 19, 2001
Combining PHA Methodologies for Complex Processes
OSHA addresses the use of combined methodologies such as a What-If/Checklist study supplemented by a fault tree for high-consequence scenarios. Combined approaches are permissible and often preferred for complex processes.
Read OSHA Letter ↗
Jun 3, 1993
Equivalent Methodologies and What OSHA Considers Acceptable
Early guidance on what OSHA considers an appropriate equivalent methodology for PHA. OSHA expects any non-listed method to be systematic, team-based, documented, and to produce findings at least as rigorous as the listed methods.
Read OSHA Letter ↗
⚡ Key Compliance Takeaway
Match the method to the hazard. HAZOP is the default choice for complex continuous processes; What-If/Checklist works well for batch operations with established playbooks. If you use a non-listed method, document why it is appropriate and ensure it produces findings equivalent in rigor to the listed alternatives.