NIOSH Report on Blastomycosis Outbreak at Paper Mill Discusses Potential Exposure Sources

A new report published by NIOSH discusses potential sources of exposure to the fungus Blastomyces at a pulp and paper mill in Michigan that caused a large outbreak of blastomycosis in 2022–2023. Blastomycosis is a rare fungal disease caused by inhalation of Blastomyces spores. The outbreak at the mill was the largest documented in the United States and the first associated with an industrial work site. Between Nov. 1, 2022, and May 15, 2023, there were 162 cases of blastomycosis among workers. Eighteen were hospitalized for the illness, and one worker died. Mill management requested that NIOSH conduct a health hazard evaluation (HHE) to investigate sources of Blastomyces exposure.

In April and August 2023, the agency visited the mill collected more than 500 environmental samples both indoors and outdoors to analyze for the presence of Blastomyces, which is found in moist soil or decaying wood and leaves in the midwestern and southern United States, according to a CDC report published earlier this year. The mill where the outbreak occurred was located on a river in a wooded area consistent with Blastomyces’ habitat. NIOSH staff took samples of soil, organic material, surface dust, and filter materials from the mill’s HVAC systems and conducted a ventilation assessment focused on the air-handling and makeup air units that introduced outside air into the mill.

The agency’s samples were negative for Blastomyces, but the results “do not necessarily indicate that Blastomyces was not present, given the limits of detection of our test methods and historical difficulties in identifying it in environmental samples,” the NIOSH report explains. The original source of Blastomyces is likely outside the mill, perhaps from the nearby riverbanks. Agency staff considered the possibility of Blastomyces spores being transported by wind from the original source. But if this occurred, the report says that the source is probably still near the mill since other individuals in the surrounding community were not affected by blastomycosis.

“Blastomyces creates an airborne respiratory hazard by releasing spores into the environment, so the original source(s) of Blastomyces spores may have been different from workers’ locations,” the report states. “This is particularly true if spores were spread around the mill by air currents or wind indoors.”

NIOSH staff also found working conditions inside the mill that could have increased workers’ blastomycosis risk, including indoor pooling water and visible mold.

“While our findings do not exclude potential outdoor exposure to Blastomyces at some point during the outbreak, our findings suggest there was possible indoor exposure either from spores entering from outside or Blastomyces surviving or growing inside the mill,” the report says.

To learn more about NIOSH’s evaluation and the prevention and control measures it recommended to mill management, see the full HHE report (PDF).