The U.S. Chemical Safety and Hazard Investigation Board (CSB) released an investigation update July 14 identifying multiple safety deficiencies that contributed to the fatal hydrogen sulfide release at Woodland Pulp’s Baileyville, Maine, mill earlier this year.

The Jan. 27 incident killed a 20-year-old chemical engineering co-op student and a 26-year-old chemical engineer. Ten additional employees were exposed to hydrogen sulfide gas during the release. A few weeks after the incident, CSB announced it would examine the circumstances surrounding the release.

According to the CSB, the mill lacked fixed hydrogen sulfide detectors and alarms in the affected area and did not provide personal hydrogen sulfide monitors to the two employees who died. Investigators also found the company had no system to track personnel inside the facility during the shutdown, delaying rescue efforts. The two employees were not located until several hours after the release had ended.

The preliminary investigation found the release occurred during a planned mill shutdown driven by increased natural gas costs.

As operators drained bleach plant equipment, alkaline process fluids containing sulfur compounds entered the facility's acid sewer system. Automatic controls responded by adding sulfuric acid to reduce pH. Because liquids accumulated in an elevated section of piping, sulfuric acid continued to be injected longer than intended, reacting with sulfur compounds to generate hydrogen sulfide gas.

When operators shut down the bleach plant scrubber fan as part of the shutdown process, the toxic gas was no longer removed from the sewer system. Instead, it migrated through connected process equipment before escaping into the building through multiple openings.

The two engineers were believed to be working nearby on an equipment drawing project unrelated to the shutdown when they were overcome by the gas. Other employees reported symptoms including burning eyes, throat irritation and headaches.

Mill personnel eventually isolated the sulfuric acid supply and flushed the sewer system with water, allowing the gas to dissipate several hours later.

The CSB said Woodland Pulp was aware hydrogen sulfide could form within the acid sewer system but lacked adequate safeguards to detect or control the hazard.

The agency said investigators continue to examine hydrogen sulfide detection and emergency response systems, access control procedures and broader process safety management practices, including operating procedures, hazard identification, management of change, maintenance and incident investigations. Findings and any safety recommendations will be included in the board's final report.

The update comes as the CSB continues several major industrial accident investigations while facing potential funding reductions. Last month, lawmakers considered legislation that would reduce the agency's budget by roughly 40% as it investigates the fatal explosion at Nippon Dynawave Packaging's Longview, Washington, pulp mill and several other major chemical incidents.

Critics have warned the proposed cuts could limit the board's investigative capacity. However, earlier this month House representatives approved restoring the budget to its full $14 million in wake of the Longview incident. A final funding decision for 2027 fiscal year still awaits approval from the Senate.