Combustible Dust Explosion
Didion Milling Grain Dust Explosion
Didion Milling, Inc.
📍 Cambria, WI
Incident: May 31, 2017 • CSB Report: 2019
5
Fatalities
11
Injuries / Affected
Corn Dust (Combustible Grain Dust — Deflagration and Explosion at Corn Milling F
Chemical Involved
3
CSB Recommendations
📋 Incident Summary

On May 31, 2017, five workers were killed and 11 were injured — some with life-altering injuries — when a series of corn dust explosions destroyed large portions of the Didion Milling corn milling facility in Cambria, Wisconsin. The explosions occurred in the early morning hours and were heard and felt for miles. Structural collapse from the blasts trapped several workers who were subsequently rescued.

Didion Milling produced corn flour and other milled corn products. The milling process generated significant quantities of corn dust throughout the facility. The CSB investigation found that corn dust had accumulated throughout the facility — in equipment, in overhead areas, and on structural surfaces — far in excess of what good housekeeping practices would have allowed.

The investigation identified a culture of normalized deviation at Didion Milling in which dust accumulations, equipment deficiencies, and safety violations had become accepted as normal operating conditions. Management was aware of the extensive dust accumulations and equipment issues but had not taken adequate corrective action. The facility had been cited by OSHA for grain dust violations prior to the fatal

🔎 Key Findings
Finding 01
Extensive Corn Dust Accumulations Throughout Facility — Culture of Normalized Deviation
Corn dust had accumulated throughout the Didion Milling facility in quantities far exceeding safe levels. The CSB found a culture of normalized deviation in which excessive dust accumulations, equipment deficiencies, and housekeeping failures had been accepted as normal by management and workers over time.
Finding 02
Series of Explosions Destroyed Facility — Five Workers Killed
The initial dust explosion triggered a series of secondary explosions that progressively destroyed large sections of the facility. Five workers were killed and 11 were injured. The scale of destruction was consistent with a major primary explosion followed by multiple secondary blasts powered by dust distributed throughout the facility.
Finding 03
OSHA Had Previously Cited Didion Milling for Grain Dust Violations
OSHA had cited Didion Milling for grain handling and dust housekeeping violations prior to the fatal explosion. The company had not adequately corrected the cited deficiencies, and dust accumulation conditions persisted and worsened until the fatal event.
Finding 04
Equipment Deficiencies Contributed to Dust Generation and Accumulation
Mechanical integrity deficiencies in the milling equipment contributed to abnormal dust generation and to the accumulation of dust in locations that would not have been reached under normal operating conditions. Equipment that was not maintained allowed excessive dust escape into the facility.
Finding 05
Didion Management Was Aware of Dust Conditions
CSB interviews and document review found that Didion management was aware of the extensive dust accumulations and equipment deficiencies at the facility. The failure to correct these conditions despite awareness is a defining characteristic of the normalized deviation that preceded the explosion.
🔍 Root Causes
1
Normalized Deviation — Dust Accumulations Accepted as Normal Operating Condition
The fundamental cause of the Didion explosion was the normalization of hazardous conditions: corn dust accumulations, equipment deficiencies, and housekeeping failures had been present for an extended period and had come to be regarded as normal by management and workers. Without a culture that treats hazardous deviations as unacceptable, corrective action does not occur.
2
Inadequate Grain Dust Housekeeping Program and Enforcement
Didion Milling did not have an adequate grain dust housekeeping program that established and enforced safe dust accumulation limits for each area of the facility. The absence of enforced housekeeping standards allowed dust to accumulate to explosive levels throughout the plant.
3
Mechanical Integrity Deficiencies Allowed Excessive Dust Escape
Equipment deficiencies contributed to dust generation and escape into the facility. A mechanical integrity program that maintained milling equipment in its designed condition would have reduced the rate of dust accumulation and the extent of the housekeeping challenge.
☑ CSB Recommendations
→ Didion Milling / Grain Milling Industry
Implement an aggressive grain dust housekeeping program with enforced dust accumulation limits for all facility areas; conduct combustible dust hazard assessments; address normalized deviation through safety culture programs that make hazardous conditions unacceptable at all levels of the organization.
→ OSHA
Strengthen enforcement of grain handling standard dust housekeeping requirements; implement follow-up inspection protocols when facilities have been cited for grain dust violations; consider enhanced penalties for facilities with repeat dust accumulation violations.
→ NFPA / Grain Industry Organizations
Update NFPA 61 (Agricultural and Food Processing Facilities) to include specific dust accumulation limits and enforcement triggers; develop grain industry guidance on combustible dust safety culture and normalized deviation prevention.
💡 Lessons Learned
Normalized deviation is one of the most dangerous conditions in industrial safety — more dangerous, in many respects, than an acute equipment failure, because it develops slowly and invisibly, eroding the boundary between hazardous and acceptable without any single obvious tipping point. At Didion Milling, dust accumulations that should have triggered immediate corrective action had instead become background conditions that no one acted on. The CSB described this as the central failure of the incident. Preventing normalized deviation requires active, enforced standards — not just policies on paper, but regular measurements against defined limits and mandatory corrective action when limits are exceeded.
Prior OSHA citations for the same hazard that ultimately caused the fatal explosion represent a missed opportunity to prevent the outcome. When a regulatory citation identifies a genuine hazard at a facility and the corrective action is inadequate, the facility is operating on borrowed time. Citation resolution must be treated as a process safety deliverable, not a compliance checkbox. Facilities should track citation closure the same way they track other process safety action items — with accountability, verification, and follow-up.
The Didion Milling explosion is a reminder that combustible dust is a hazard in agricultural and food processing facilities just as surely as it is in industrial chemical or plastics manufacturing. Corn, wheat, sugar, soy, and other agricultural commodities generate combustible dusts during milling, grinding, conveying, and storage operations. The NFPA agricultural facility standards and OSHA grain handling standard exist specifically because these facilities are not exempt from the laws of combustion physics. Grain dust has killed workers in milling facilities for over a century — the physics have not changed.
PSI: Process Safety InformationPHA: Process Hazard AnalysisSOP: Operating ProceduresMI: Mechanical IntegrityTRN: Training
🔨 Safety Meeting Toolbox Talk
►Has your facility established specific dust accumulation limits (in mm or fractions thereof) for each area where combustible dust is present? Are these limits enforced through regular measurement and mandatory corrective action?
►Do supervisors and managers at your facility treat visible dust accumulations on floors, elevated surfaces, and equipment as an unacceptable condition requiring immediate correction, or has dust accumulation become normalized as a background operating condition?
►Are open OSHA citations for dust hazards tracked as process safety action items with defined corrective actions, responsible persons, and target completion dates?
►Does your mechanical integrity program specifically address equipment components that, when worn or damaged, allow excess dust to escape into the facility? Are these components on an accelerated inspection schedule?
Immediate Action Items
✓Establish written dust accumulation limits (e.g., 1/32 inch maximum on flat surfaces) for each area of your facility where combustible dust is present; implement a regular inspection and documentation program.
✓Conduct a normalized deviation assessment for your facility: review housekeeping records, OSHA citations, near-miss reports, and maintenance records for evidence of recurring hazardous conditions that have not been corrected.
✓Verify that all open OSHA citations for dust hazards have documented corrective actions with completion dates, responsible owners, and verification steps.
✓Inspect all milling, conveying, and processing equipment for deficiencies that allow excess dust escape; prioritize repair of components with the highest dust generation contribution.
🔗 PSM Failures Behind This Incident

This incident traced to breakdowns across 5 PSM elements (PSI · PHA · SOP · MI · TRN). Each represents a documented gap that process safety documentation and consulting can close before a similar event occurs at your facility.

Process Safety Information (PSI)
Accurate, complete Process Safety Information is the foundation every other PSM element depends on. When PSI is missing or wrong — chemistry data, equipment specs, P&IDs — the entire hazard analysis is built on a flawed base.
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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.
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Operating Procedures (SOPs)
Operators cannot reliably hold safe operating limits without clear, current, enforced procedures. Deviation from acceptable operating conditions is a direct consequence of SOP failure.
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Mechanical Integrity (MI)
Equipment must be designed, inspected, and maintained to operate safely in its intended service. Mechanical integrity failures contributed to loss of containment here.
Supporting documents in our library →
Training & Operator Competency
Workers must understand process hazards — not just the steps on the page. Training records, refresher frequency, and verified competency are all OSHA PSM requirements.
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