61
Quantifying Toxicity
The RfDo is expressed as milligrams of chemical per kilogram of body weight per
day. It can be thought of as the daily dose that would have no impact or the dose
above which there would be a toxic impact. The higher the RfDo is, the lower the
noncarcinogenic toxicity from a particular chemical will be (i.e., a higher RfDo represents a higher “acceptable” threshold) relative to a chemical that has a lower RfDo.
For example, a chemical with an RfDo of 2 mg/kg-day represents the fact that
2 mg/kg can be ingested per day and have no noncarcinogenic impact, whereas a
dose above 2 mg/kg ingested per day could pose a noncarcinogenic toxic impact.
INHALATION NONCARCINOGENIC TOXICITY
We based our assessment of inhalation noncarcinogenic toxicity (noncancer risk
due to inhalation of a chemical) on the inhalation reference concentration (RfCi)
of the USEPA, which is an estimate (with uncertainty spanning perhaps an order of
magnitude) of a continuous inhalation exposure to the human population (including
sensitive subgroups) that is likely to be without an appreciable risk of deleterious
effects during a lifetime. The RfCi is expressed as milligrams of chemical per cubic
meters of air. It can be thought of as the concentration of a chemical in air that can
be breathed over a lifetime without experiencing a toxic impact. The RfCi is not
expressed relative to body weight because an individual’s breathing rate is proportional to his or her body weight. The higher the RfCi is, the lower the noncarcinogenic toxicity from a particular chemical will be (i.e., a higher RfCi represents a
higher “acceptable” threshold) relative to a chemical that has a lower RfCi.
For example, a chemical with an RfCi of 3 mg/m 3 represents the fact that 3 mg/m 3
can be inhaled per day and have no noncarcinogenic impact, whereas a dose inhaled
above 3 mg/m 3 can pose a noncarcinogenic toxic impact.
DEVELOPMENT OF A SINGLE TOXICITY FACTOR
These four individual chemical-specific measures of chronic toxicity were used to
develop a relative single TF for TRI chemicals. The TF is based on the concept of
how many individuals could equally share (ingest or inhale) a pound of chemical
released per year to have no adverse impact, either from excess cancers or from
noncarcinogenic effects (toxicity threshold).
The analysis of specific toxicity of any given chemical does require a more
detailed analysis of exposure pathways and individuals affected. We have simplified
the analysis to come up with a single, relative chemical-specific TF to rank chemicals
relatively by their overall toxic effect. In addition, the measures of toxicity obtained
from the USEPA Web site do get updated periodically; therefore, our calculations of
TFs as presented in this book are indeed intended solely for relative ranking versus
representing absolute numbers.
We welcome suggestions on how to improve our analysis or come up with additional toxicity information to better rank chemicals. Not to rank chemicals by
some toxicity measure and therefore not prioritize which ones to focus on eliminating is unacceptable. Without some type of toxicity ranking, all chemicals will be
considered “bad,” the undertaking to reduce or eliminate use of all (vs. targeted)
Quantifying Toxicity
The RfDo is expressed as milligrams of chemical per kilogram of body weight per
day. It can be thought of as the daily dose that would have no impact or the dose
above which there would be a toxic impact. The higher the RfDo is, the lower the
noncarcinogenic toxicity from a particular chemical will be (i.e., a higher RfDo represents a higher “acceptable” threshold) relative to a chemical that has a lower RfDo.
For example, a chemical with an RfDo of 2 mg/kg-day represents the fact that
2 mg/kg can be ingested per day and have no noncarcinogenic impact, whereas a
dose above 2 mg/kg ingested per day could pose a noncarcinogenic toxic impact.
INHALATION NONCARCINOGENIC TOXICITY
We based our assessment of inhalation noncarcinogenic toxicity (noncancer risk
due to inhalation of a chemical) on the inhalation reference concentration (RfCi)
of the USEPA, which is an estimate (with uncertainty spanning perhaps an order of
magnitude) of a continuous inhalation exposure to the human population (including
sensitive subgroups) that is likely to be without an appreciable risk of deleterious
effects during a lifetime. The RfCi is expressed as milligrams of chemical per cubic
meters of air. It can be thought of as the concentration of a chemical in air that can
be breathed over a lifetime without experiencing a toxic impact. The RfCi is not
expressed relative to body weight because an individual’s breathing rate is proportional to his or her body weight. The higher the RfCi is, the lower the noncarcinogenic toxicity from a particular chemical will be (i.e., a higher RfCi represents a
higher “acceptable” threshold) relative to a chemical that has a lower RfCi.
For example, a chemical with an RfCi of 3 mg/m 3 represents the fact that 3 mg/m 3
can be inhaled per day and have no noncarcinogenic impact, whereas a dose inhaled
above 3 mg/m 3 can pose a noncarcinogenic toxic impact.
DEVELOPMENT OF A SINGLE TOXICITY FACTOR
These four individual chemical-specific measures of chronic toxicity were used to
develop a relative single TF for TRI chemicals. The TF is based on the concept of
how many individuals could equally share (ingest or inhale) a pound of chemical
released per year to have no adverse impact, either from excess cancers or from
noncarcinogenic effects (toxicity threshold).
The analysis of specific toxicity of any given chemical does require a more
detailed analysis of exposure pathways and individuals affected. We have simplified
the analysis to come up with a single, relative chemical-specific TF to rank chemicals
relatively by their overall toxic effect. In addition, the measures of toxicity obtained
from the USEPA Web site do get updated periodically; therefore, our calculations of
TFs as presented in this book are indeed intended solely for relative ranking versus
representing absolute numbers.
We welcome suggestions on how to improve our analysis or come up with additional toxicity information to better rank chemicals. Not to rank chemicals by
some toxicity measure and therefore not prioritize which ones to focus on eliminating is unacceptable. Without some type of toxicity ranking, all chemicals will be
considered “bad,” the undertaking to reduce or eliminate use of all (vs. targeted)