This study presents a method for calculating the concentration of hydrocarbon releases in enclosed areas using empirical equations for the evaporation rate. The approach of the method is to estimate the hydrocarbon exposure concentration in the air under conditions assumed. A methodology for assessing risk to individual exposed workers or others using probit expressions is suggested. The toxicity criteria and available human exposure data were examined and guidelines for risk assessment suggested for benzene-air and toluene-air systems. The values of probit constants with mole fractions of lethal concentrations in a mixture of hydrocarbons and a non-toxic substance were predicted. The probit values calculated with mole fractions can be used to estimate guidelines to prevent toxicity within enclosed working areas.