Chlorine exposure during a biological decontamination study in a mock subway tunnel

The Underground Transport Restoration (UTR) Operational Technology Demonstration (OTD) was a full-scale field study focused on remediation of a subway system after contamination with a Bacillus anthracis (Ba) surrogate (Bacillus atrophaeus, subspecies globigii [Bg]). The study involved all aspects of subway system remediation following contamination with a biological surrogate, including characterization, clearance sampling, and waste management.^{[ 1 ]} Personal exposure to chlorine gas was also monitored throughout the decontamination portion of the study. Process-based personal monitoring for chlorine was conducted using portable single gas monitors with chlorine sensors during Level A entry into the biologically-contaminated area (exclusion zone) during spraying operations. Additional monitoring was conducted during the mixing of pH-adjusted bleach solutions and waste item decontamination (immersion dunking). An analysis of variance was performed to compare process-based time-averaged chlorine exposure among the similar exposure groups. Chlorine exposure was highest for the Decon Sprayers, which was expected based on their proximity to the spray streams. Peak exposure levels (5-sec readings) ranged from 11 to at least 50 parts per million (ppm). It is likely that exposure exceeded 50 ppm, but this was the upper limit of measurement. Oversight personnel were farther away from the spray operation but still had significant peak chlorine exposures of 13-26 ppm. The rail cart operators had peak exposures of 13-19 ppm. Statistically significant differences were observed between time-weighted average exposure levels of Decon Sprayers and the other workers. Spraying of pH-adjusted bleach solution on subway tunnel surfaces for biological decontamination produced up to 50 ppm chlorine vapor in the air that far exceeded the occupational exposure limits of 0.5-1 ppm for chlorine, as well as the Immediately Dangerous to Life or Health limit of 10 ppm. Health and safety plans and operational activities must provide appropriate worker protection during such events where potential for chlorine overexposure has been demonstrated.