Observation of Excess Benzene in Post-Fire Crude Oil and Risk Communication Implications

Abstract This investigation examines whether benzene in heat-affected crude oil declines as a volatility-only model would predict after tank-fire extinguishment with fluorine-free foam (F3). Among BTEX compounds, benzene has the highest vapor pressure and the lowest molecular weight and boiling point; it should therefore be depleted from the surface layer at least as fast as toluene, ethylbenzene, and the xylenes (TEX), and faster if evaporative loss dominates. Liquid samples were collected from the top 12 in. (30 cm) of a 2.4 m diameter × 2.19 m high test tank before burning and after repeated F3-extinguished burns. Aliquots were aerated in a closed vapor chamber and the headspace was analyzed as whole air. That assay characterizes an enriched source profile; the reported concentrations are not outdoor or breathing-zone exposures. Absolute benzene in the chamber headspace declined (81 ppmv in the single fresh-oil sample versus a post-burn mean of 31 ppmv), but TEX declined far more (for example, toluene from 680 ppmv to a post-burn mean of 26.5 ppmv). Benzene’s share of ΣBTEX vapor therefore rose from about 5% before burning to about 43% after burning (about 60% in overnight mixed-oil samples). A volatility-only account does not explain that pattern. A source term is required: conversion of precursors in the hot zone (thermal or steam-related cracking) and/or resupply of benzene from the cooler bulk liquid. Distinguishing those mechanisms is outside the scope of this observational study. The practical implication is that post-fire crude oil in deep tanks should not be assumed to have been stripped of benzene the way thin-film in situ burns are, and risk communication for responders should treat the residual liquid as a continuing benzene source.