Twenty-Five Years After the Mailed Anthrax Attacks: Remediation and Environmental Clearance of the Trenton Processing and Distribution Center.

After the 2001 mailed anthrax attacks, the US Postal Service faced the unprecedented challenge of decontaminating large-scale mail processing facilities contaminated with Bacillus anthracis. This case study documents the environmental characterization, decontamination, and clearance protocol for reoccupancy of the Trenton Processing and Distribution Center, one of the most extensively contaminated sites. Environmental sampling demonstrated widespread contamination and the need for full-scale gaseous fumigation using chlorine dioxide. Clearance for reoccupancy was guided by an independent, multidisciplinary Environmental Clearance Committee, which adopted a stringent "no growth" standard based on tiered postfumigation testing. Evaluation included biological indicators, followed by surface sampling and aggressive air sampling. Results were negative for viable B. anthracis and the Environmental Clearance Committee concluded that the facility was safe for reoccupancy. This experience highlights the technical, operational, and policy challenges of large-scale biological remediation and underscores the importance of independent scientific oversight in decision making under conditions of uncertainty.