Fatal Combustible Dust Explosions
Didion Milling Company Fatal Combustible Dust Explosions
Didion Milling Inc.
📍 Cambria, WI
Incident: May 31, 2017  •  CSB Report: December 6, 2023
5
Fatalities
14
Injuries
Combustible Corn Dust
Chemical Involved
📋 Incident Summary

On the night of May 31, 2017, catastrophic combustible dust explosions and fire tore through the Didion Milling corn processing facility in Cambria, Wisconsin. Five of the 19 workers on the night shift were killed, and the remaining 14 sustained injuries. A smoldering fire in the dust collection system ignited a primary dust explosion that propagated through the interconnected pneumatic conveying and dust collector ductwork, triggering a series of secondary explosions that destroyed much of the facility.

The CSB found that Didion Milling had not conducted dust hazard analyses (DHAs), lacked explosion prevention and mitigation engineering controls (venting, suppression, isolation), and had inadequate dust collection system maintenance. The facility also had no combustible dust process safety management system, and the emergency response program was insufficient to address dust fire and explosion scenarios.

🔎 Key Findings
Finding 01
No Dust Hazard Analysis
Didion Milling had never conducted a formal Dust Hazard Analysis (DHA) to identify explosion risks in the corn milling process.
Finding 02
Inadequate Dust Collection System
The pneumatic conveying and dust collection ductwork were not designed or maintained to NFPA standards; transport velocities were insufficient, causing dust accumulation.
Finding 03
No Explosion Protection Engineering Controls
The facility lacked NFPA-compliant explosion venting, suppression systems, deflagration isolation, and pressure containment on vessels and ductwork.
Finding 04
No Combustible Dust PSM System
No formal process safety management system for combustible dust was in place, including no MOC, no PSI management, and no formal incident investigation process.
Finding 05
Inadequate Emergency Response
The emergency response plan did not address smoldering dust fires inside equipment or the risk of explosion propagation, and no evacuation signal system existed.
Finding 06
Process Safety Culture Deficiencies
Prior OSHA citations and industry warnings about combustible dust hazards at grain facilities were not acted upon to implement appropriate controls.
🔍 Root Causes
1
No Systematic Dust Hazard Management
Without a Dust Hazard Analysis, the facility never identified the specific risks posed by corn dust accumulation, ignition sources, or explosion propagation pathways.
2
Deficient Engineering Controls
The absence of explosion venting, deflagration suppression, and isolation systems meant that an initial smoldering fire could propagate into multiple devastating secondary explosions.
3
Inadequate Dust Collection Maintenance
Failure to maintain minimum transport velocities in ductwork allowed combustible dust to settle and accumulate inside conveying systems, providing fuel for the explosions.
4
Regulatory Gap
OSHA lacks a specific combustible dust standard; existing grain handling regulations did not require DHAs or explosion protection engineering controls at this facility.
☑ CSB Recommendations
→ Didion Milling
Develop a comprehensive combustible dust PSM system including MOC, PSI, DHAs, engineering controls, incident investigation, and emergency preparedness.
→ Didion Milling
Perform DHAs on all buildings handling combustible dust, revalidated every five years; implement explosion venting, suppression, and isolation per NFPA 61/68/69/652.
→ OSHA
Promulgate a combustible dust standard covering all industries, including hazard recognition, DHA, MOC, engineering controls, and emergency response.
→ OSHA
Develop a follow-up inspection program for facilities receiving hazard alert letters for unabated combustible dust hazards.
→ NFPA
Update NFPA 61 to unify DHA requirements, remove equipment exemptions, and incorporate MOC and incident investigation guidance.
💡 Lessons Learned
Dust Hazard Analysis (DHA) is the essential first step — without it, an operation has no systematic understanding of combustible dust explosion risks.
Explosion prevention (DHA, housekeeping, transport velocity) must be paired with explosion mitigation (venting, suppression, isolation) — either alone is insufficient.
Smoldering fires inside dust collection equipment must be treated as imminent explosion precursors, triggering immediate evacuation.
Combustible dust PSM — including MOC, PSI, incident investigation, and emergency response — is required to sustain safe operations over time.
The absence of an OSHA-specific combustible dust standard has left grain, milling, and food processing facilities without enforceable requirements.
PSM Element: Process Safety Culture & Continuous Improvement
🔨 Safety Meeting Toolbox Talk
▸Has a formal Dust Hazard Analysis (DHA) been conducted and documented for all areas handling combustible dust?
▸Are explosion venting, suppression, isolation, and deflagration containment systems installed per NFPA 68/69/652?
▸Is minimum transport velocity verified and monitored in all pneumatic conveying and dust collector ductwork?
▸Does the emergency response plan include procedures for smoldering fires in dust collection equipment and prompt evacuation?
▸Is a combustible dust MOC process in place so that ductwork or process changes trigger a new or updated DHA?
Immediate Action Items
✓Commission a third-party Dust Hazard Analysis (DHA) for all dust-handling operations per NFPA 652
✓Install explosion venting, suppression, and deflagration isolation on all at-risk vessels and ductwork
✓Verify and document minimum transport velocities; implement periodic ductwork inspection and cleaning program
✓Update emergency response plan to address smoldering fires and dust explosion scenarios
✓Establish combustible dust MOC process requiring re-evaluation of DHAs for any equipment or process change
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 1 PSM element (PHA). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Process Hazard Analysis (PHA)
A structured PHA or HAZOP study exists to identify exactly these scenarios before they occur. When PHA is absent, superficial, or overdue for revalidation, hazards operate unseen until they kill someone.
Supporting documents in our library →
Process Safety Management Consulting & Document Library
📂
PSM Document Library
32 ready-to-deploy PSM documents covering all 14 OSHA elements — procedures, checklists, and audit templates built for facilities operating under 29 CFR 1910.119.
Browse the Library →
📊
Free PSM Health Score
Find out where your PSM program stands across all 14 OSHA elements. Our free health score surfaces your biggest gaps in under 10 minutes — no account required.
Check Your Score →
📞
Consulting Services
PHA facilitation, PSM program builds, compliance audits, and OSHA inspection support. Transparent flat-fee pricing — no retainer required to get started.
View Pricing →
📋 Explore the full incident library: All 132 CSB Case Studies →