Fungal Fallout: CDC Report Exposes Blastomycosis Risks for Indoor Air Investigators
/A 2025 CDC report published in the Morbidity and Mortality Weekly Report details the largest documented U.S. outbreak of blastomycosis — a fungal lung disease — among paper mill workers in Michigan between November 2022 and May 2023. The investigation identified 162 cases out of 645 workers, with illness peaking in March 2023. Despite extensive sampling of soil, dust, and HVAC systems, no environmental source of Blastomyces spores was detected. For indoor air quality (IAQ) consultants, the findings underscore the complexity of detecting biologic contaminants in industrial environments where mechanical ventilation, organic debris, and moisture converge to create potential reservoirs for microbial exposure.
From an exposure-assessment standpoint, the outbreak highlights key diagnostic and control challenges. Though 12% of affected workers required hospitalization and one death occurred, the report stresses that Blastomyces likely infiltrated the mill through unfiltered ventilation systems or open bays rather than proliferating indoors. Figures and tables throughout the report — including Figure 1 (page 2) showing the sharp spike in cases and Figure 2 (page 4) mapping infections by work area — illustrate how airborne pathogens can disperse across departments, from maintenance zones to administrative offices. For consultants conducting IAQ investigations, this case study reinforces the value of integrated epidemiologic surveillance and building-system forensics to pinpoint elusive bioaerosol pathways.
For those engaged in IAQ remediation and consulting, the 2025 CDC findings serve as a call to elevate biological hazard awareness beyond molds and bacteria. The paper mill’s mitigation response — which included upgrading HVAC filtration, cleaning ductwork, and instituting respiratory protection protocols — mirrors best practices familiar to industrial hygienists. Yet the inability to detect the fungus even after 533 environmental samples also illustrates a persistent gap in field diagnostics for fungal spores in complex environments. The report concludes that rigorous ventilation maintenance, moisture control, and worker training remain essential tools in preventing airborne fungal outbreaks in industrial and commercial indoor settings.