The U.S. Chemical Safety and Hazard Investigation Board (CSB) has released an update on its ongoing investigation into the fatal hydrogen sulfide gas release at the Woodland Pulp mill in Baileyville, Maine, that occurred on Jan. 27, 2026. The incident killed two employees and exposed 10 others to toxic hydrogen sulfide gas.

According to the CSB, investigators have determined that the two employees who died were not provided with personal hydrogen sulfide monitors, and there were no fixed hydrogen sulfide detectors or alarms installed in the Kraft Mill building where the release occurred. The agency also said Woodland Pulp did not have a system to account for personnel or track their locations during the mill shutdown, delaying the discovery of the two employees for several hours after the release ended.

The investigation update describes how the incident developed during a planned shutdown after Woodland Pulp reduced operations because of higher natural gas prices. As Bleach Plant equipment was drained, high-pH process fluids containing sulfur compounds entered the facility's acid sewer. A downstream pH probe detected elevated pH levels and automatically increased sulfuric acid addition to the system.

Because liquid accumulated in an upward section of the acid sewer, the pH probe continued calling for additional sulfuric acid. Investigators said the sulfuric acid reacted with sulfur compounds in the piping, generating hydrogen sulfide gas.

The CSB said the Bleach Plant scrubber fan was shut down at approximately 11:40 a.m. as part of the shutdown process. Without the scrubber operating, hydrogen sulfide gas traveled through connected piping into process vessels and escaped through openings in equipment inside the Kraft Mill.

The two employees who were fatally injured were working on an equipment drawing project unrelated to the shutdown on the second floor of the Kraft Mill, near several locations where the gas escaped. Both collapsed after being exposed to the hydrogen sulfide. Ten additional employees also were exposed, with some reporting symptoms including burning eyes, throat irritation, and headaches.

According to the investigation update, Woodland Pulp employees responded by closing the sulfuric acid supply valve and flushing the acid sewer with water. The hydrogen sulfide gas dissipated by approximately 3 p.m., but the two employees were not found until about 6:15 p.m.

The CSB identified several preliminary safety deficiencies at the facility, including:

• No system to control or account for personnel inside the Kraft Mill during the shutdown.
• No building ventilation system operating during normal operations or shutdowns.
• No stationary hydrogen sulfide detectors or alarms in the Bleach Plant area.
• No requirement for employees to wear personal hydrogen sulfide monitors.

The agency said Woodland Pulp was aware that hydrogen sulfide could form in the acid sewer piping but did not have sufficient systems in place to monitor or mitigate the hazard.

The investigation remains ongoing. The CSB said investigators continue to examine hydrogen sulfide detection and alarm systems, personnel access control, operating procedures, hazard identification, management of change, maintenance practices, and incident investigation processes. The agency's final report will include its complete findings, analysis, and any recommendations.