Solvent Evaporation in an Isolated Subsurface Structure: An Unrecognized and Underappreciated Risk

Isolated subsurface structures readily collect solvents spilled onto surrounding surfaces or poured into opening(s) in the manhole cover. Fatal overexposures and fires/explosions have occurred following these events. This work documents evaporation of 10 mL of lacquer thinner from a paper towel positioned near the base of a vertically oriented precast concrete chamber (volume = 2.5 m³) and exchange through opening(s) in the manhole cover monitored using a Photoionization Device (PID) sensor. A sixth order polynomial fitted by Microsoft Excel best describes the process of evaporation and dispersion in the airspace and exchange with the external atmosphere. Restoration of the uncontaminated atmosphere can require 48 hours or more under these conditions. A manhole cover containing a single opening is most likely to retain vapor for the longest period, and one with two circumferential openings opposite each other is least likely. Results presented here argue for the involvement of individuals made knowledgeable by education, experience, and training in confined spaces to address this unrecognized and underappreciated risk. Optimizing ventilation induced by natural forces in isolated subsurface structures is a natural application of the NIOSH (National Institute for Occupational Safety and Health) Prevention through Design initiative.

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