Federal investigators say two firefighters were killed and 13 firefighters and EMS providers critically injured after water was directed into an oxygen-limiting silo containing combustible wood dust.
Two volunteer firefighters were killed and 13 firefighters and EMS providers were critically injured when a silo at a Maine lumber mill exploded during a fire in May, according to a new investigation by the National Institute for Occupational Safety and Health (NIOSH).
The agency has warned that firefighters responding to silo fires must understand the differences between silo types and their contents, and specifically says they should never direct water or foam into an oxygen-limiting silo.
The explosion happened on May 15, 2026, at a 40-acre lumber mill producing white pine products. The facility used multiple mills, kilns and a biomass-fired cogeneration plant, while wood shavings and sawdust generated during production were collected and processed through silo systems.
According to NIOSH’s report, the facility’s automatic notification system first reported a fire in the silo. Employees subsequently found smoke and firefighters arrived at the site, where they deployed a hoseline and directed water into an opening at the base of the silo.
Facility owners and employees remained nearby to answer questions and assist firefighters.
After some time, firefighters believed the fire was largely under control. The silo then suddenly launched into the air, followed by a large plume of fire.
The resulting blast threw firefighters, EMS providers and mill employees away from the silo. Around two hours later, a personnel accountability check identified one firefighter as missing. He was subsequently declared dead. A second firefighter died four weeks later from injuries sustained in the explosion.
NIOSH’s investigation found that both facility employees and responding firefighters were unaware that the silo was an oxygen-limiting silo, or that the wood shavings and residual sawdust stored inside represented a combustible dust hazard.
The incident silo, located at the facility’s second bagger system, was a 20-foot by 60-foot oxygen-limiting Harvestore silo. Wood shavings and sawdust were deposited into the silo before being removed through an auger system at its base.
Although the facility’s emergency evacuation and fire prevention plan identified wood dust as a potential hazard, it focused on dust accumulating around electrical panels and equipment. Employees interviewed by NIOSH did not understand that the material stored inside the silos constituted a combustible dust hazard.
As a result, firefighters were not informed of the combustible dust risk before or during the incident.
The State Fire Marshal’s Office and the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) classified the original fire as accidental, with the area of origin identified as the unloader mechanism at the base of the silo.
Their preliminary investigation identified combustible dust as the cause of the subsequent explosion.
NIOSH found that previous incidents at the lumber mill may also have influenced the tactics used during the May 2026 fire.
The six fire departments involved in the response had attended several silo fires at the facility during the previous 20 years. However, those fires had occurred in the silo serving the facility’s first bagger system, which was a conventional silo.
Firefighters told NIOSH that previous fires had behaved like “Roman candles,” venting through the top of the silo. They had successfully brought those incidents under control using a deluge system installed in the conventional silo.
The incident silo was different.
NIOSH concluded that the responders’ previous experience with fires at the facility contributed to their decision to use water on the oxygen-limiting silo.
The agency says water entering an oxygen-limiting silo can introduce oxygen and disturb combustible material, potentially creating the conditions for an explosion. It notes that, over the past 40 years, it has investigated several incidents in which firefighters directing water into oxygen-limiting silos unintentionally caused explosions.
The report therefore calls for fire officers and firefighters to receive training on different types of silos and the hazards associated with their contents.
It also states that firefighters should choose appropriate tactics for silo fires and never direct water or foam into an oxygen-limiting silo.
The investigation identifies several measures intended to reduce the risk of similar incidents.
NIOSH recommends that fire departments develop pre-incident plans for high-hazard occupancies such as lumber mills and establish and maintain unified command with representatives from those facilities throughout an incident.
It also calls for the use of personnel accountability systems capable of identifying the location and function of all operating personnel.
For lumber mills, NIOSH recommends that emergency and fire safety plans clearly identify combustible dust explosion hazards and ensure that this information is shared with responding fire departments.
The agency further recommends that facilities properly operate and maintain silos and implement applicable requirements of NFPA 660, Standard for Combustible Dusts and Particulate Solids.
Finally, NIOSH says relevant codes and standards should be updated to require hazard labeling on bulk storage enclosures containing combustible dust.
The report also found no evidence that structural firefighting turnout gear or self-contained breathing apparatus contributed to the fatality of the firefighter examined in its investigation.
The findings underline the importance of understanding the construction and contents of industrial storage systems before firefighting tactics are deployed, particularly where combustible dusts and oxygen-limiting equipment are involved.
For NIOSH, the Maine incident demonstrates that tactics that have previously proved effective at a facility can carry very different risks when the equipment, material or fire conditions are not the same.
