PRINCIPLES OF TOXICOLOGY - Biology East Borneo

19.4 PETROLEUM HYDROCARBONS: ASSESSING EXPOSURE AND RISK TO MIXTURES 483Interpretation of Risk Assessment Results and CommentFor pregnant workers, this indicates that there is a 5 percent likelihood that the fetal blood leadconcentration may exceed 7.8 µg/dL in similarly exposed pregnant women. The calculated leadconcentration is below the CDC and USEPA level of concern of 10 µg/dL. A greater exposurefrequency, a higher dust lead concentration, or exposure to a highly soluble form of lead (such as leadchloride or lead acetate) may result in a calculated PbB fetal,0.95that could potentially exceed 10 µg/dL.In practice, blood lead concentrations could also be measured in women of child-bearing age to providereassurance that they were not being overexposed.Although the preceding equation does not evaluate inhalation exposures to lead, it could easily bemodified to do so. The Agency for Toxic Substances and Diseases Registry (ATSDR) has summarizedhuman inhalation studies of lead and determined biokinetic slope factors relating the air leadconcentration to increases in blood lead. For example, individuals exposed to lead concentrations inair ranging from 3.2 to 11 µg/m 3 had average blood lead increases of 1.75 µg/dL for every µg/m 3 leadin air. Individuals in the study reviewed by ATSDR were exposed for 23 h/day for 18 weeks. Giventhat workers would not be exposed to the workplace atmosphere for 23 h/day, it would be reasonableto assume that only half the air breathed in a day was from the affected workplace (i.e., a correctionfactor of 0.5). If the above equation is modified to reflect exposure to lead in air at a concentration of0.5 µg/m 3 , the equation would be revised as follows:PbB fetal,0.95 = 1.8 1.645 ×(1300 µg / g × 0.4 µg / dL ⋅ µg / day ⋅ 0.05 µg / day ⋅ 0.12) ⋅ 150 days / year365 days / year+ (0.5 µg / m3 × 1.75 µg / dL ⋅ µg / m 3 ⋅ 0.5) ⋅ 150 days / year365 days / year+ 2.0 µg / dL ⋅ 0.9PbB fetal,0.95 = 8.2 µg/dL when inhalation exposure to lead is added to ingestion of lead19.4 PETROLEUM HYDROCARBONS: ASSESSING EXPOSURE AND RISK TOMIXTURESChemical mixtures present special problems to risk assessors. Mixtures may be made up of hundredsof individual chemicals that are inadequately characterized with regard to their toxicity. Further, it isoften difficult or impractical to completely characterize the composition of the mixture. Such is the casewith petroleum fuels such as gasoline and diesel fuel that contain hundreds of organic compounds.The USEPA indicates that when adequate information is available, it is preferable to use mixturespecifictoxicity information to evaluate the risks of complex chemical mixtures. Mixture-specifictoxicity information is preferred since the risk assessor does not have to make assumptions regardingthe toxicological interaction of the chemicals of the mixture. However, use of mixture-specific toxicityinformation is only useful when the mixture in question is the same as the toxicologically characterizedmixture. This is an important caveat for risk assessments of petroleum hydrocarbon mixtures. Afterbeing released to the environment, petroleum mixtures “weather” with time. Weathering causes theloss of more volatile, water-soluble, and degradable petroleum hydrocarbons. As a result, weatheredpetroleum fuel mixtures may no longer be chemically or toxicologically similar to the unweatheredfuel. Until toxicological data are available for weathered petroleum mixtures, risk assessments ofweathered petroleum mixtures are typically performed using either an “indicator chemical” or a“surrogate” chemical approach.The indicator chemical approach to petroleum hydrocarbon risk assessment assumes that certaincompounds in a petroleum hydrocarbon mixture can be used to represent the environmental mobility,exposure potential, and toxicological properties of the entire petroleum mixture. For example, indicatorchemicals typically used in risk assessments of unleaded gasoline include benzene, ethylbenzene,