PRINCIPLES OF TOXICOLOGY

19.4 PETROLEUM HYDROCARBONS: ASSESSING EXPOSURE AND RISK TO MIXTURES 483
Interpretation of Risk Assessment Results and Comment
For pregnant workers, this indicates that there is a 5 percent likelihood that the fetal blood lead
concentration may exceed 7.8 µg/dL in similarly exposed pregnant women. The calculated lead
concentration is below the CDC and USEPA level of concern of 10 µg/dL. A greater exposure
frequency, a higher dust lead concentration, or exposure to a highly soluble form of lead (such as lead
chloride or lead acetate) may result in a calculated PbB fetal,0.95 that could potentially exceed 10 µg/dL.
In practice, blood lead concentrations could also be measured in women of child-bearing age to provide
reassurance that they were not being overexposed.
Although the preceding equation does not evaluate inhalation exposures to lead, it could easily be
modified to do so. The Agency for Toxic Substances and Diseases Registry (ATSDR) has summarized
human inhalation studies of lead and determined biokinetic slope factors relating the air lead
concentration to increases in blood lead. For example, individuals exposed to lead concentrations in
air ranging from 3.2 to 11 µg/m 3 had average blood lead increases of 1.75 µg/dL for every µg/m 3 lead
in air. Individuals in the study reviewed by ATSDR were exposed for 23 h/day for 18 weeks. Given
that workers would not be exposed to the workplace atmosphere for 23 h/day, it would be reasonable
to assume that only half the air breathed in a day was from the affected workplace (i.e., a correction
factor of 0.5). If the above equation is modified to reflect exposure to lead in air at a concentration of
0.5 µg/m 3 , the equation would be revised as follows:
PbBfetal,0.95 = 1.8 1.645 ×
(1300 µg / g × 0.4 µg / dL ⋅ µg / day ⋅ 0.05 µg / day ⋅ 0.12) ⋅ 150 days / year
365 days / year
+ (0.5 µg / m3 × 1.75 µg / dL ⋅ µg / m 3 ⋅ 0.5) ⋅ 150 days / year
365 days / year
+ 2.0 µg / dL ⋅ 0.9
PbBfetal,0.95 = 8.2 µg/dL when inhalation exposure to lead is added to ingestion of lead
19.4 PETROLEUM HYDROCARBONS: ASSESSING EXPOSURE AND RISK TO
MIXTURES
Chemical mixtures present special problems to risk assessors. Mixtures may be made up of hundreds
of individual chemicals that are inadequately characterized with regard to their toxicity. Further, it is
often difficult or impractical to completely characterize the composition of the mixture. Such is the case
with 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 mixturespecific
toxicity information to evaluate the risks of complex chemical mixtures. Mixture-specific
toxicity information is preferred since the risk assessor does not have to make assumptions regarding
the toxicological interaction of the chemicals of the mixture. However, use of mixture-specific toxicity
information is only useful when the mixture in question is the same as the toxicologically characterized
mixture. This is an important caveat for risk assessments of petroleum hydrocarbon mixtures. After
being released to the environment, petroleum mixtures “weather” with time. Weathering causes the
loss of more volatile, water-soluble, and degradable petroleum hydrocarbons. As a result, weathered
petroleum fuel mixtures may no longer be chemically or toxicologically similar to the unweathered
fuel. Until toxicological data are available for weathered petroleum mixtures, risk assessments of
weathered 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 certain
compounds 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, indicator
chemicals typically used in risk assessments of unleaded gasoline include benzene, ethylbenzene,