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Toxic Chemicals
SINGLE COMBINED RELATIVE TOXICITY FACTOR DEVELOPMENT
What follows are the combination of the individual oral and inhalation TFs developed for cancer and noncancer risks to calculate a single, combined relative TF for
TRI chemicals. This chemical-specific single relative TF represents how many individuals could ingest or inhale each pound of chemical per year released to have
no adverse carcinogenic or noncarcinogenic impact from the chemical (i.e., the
threshold beyond which an adverse effect of some type could occur) in the unit of
doses/pound. As noted, the higher the measures of cancer risk (based on USEPA
cancer toxicity values) are, the higher the cancer toxicity associated with the chemical will be, whereas the higher the measures of noncancer risk (based on USEPA
noncancer toxicity values) are, the lower the noncancer toxicity will be. However, as
was described in detail, as part of developing the single relative chemical-specific
TFs, all four of the measures of chronic toxicity were converted into a dose relationship (doses/pound), which resulted in a direct relationship to toxicity (the higher
the number of doses per pound for a chemical, the higher the toxicity); hence, the
carcinogenic and noncarcinogenic effects become additive:
Single Chemical-Specific TF (doses/lb) = Σ (Ingestion Carcinogenic TF,
Inhalation Carcinogenic TF, Ingestion Noncarcinogenic TF,
Inhalation Noncarcinogenic TF)
For example, a chemical with a calculated TF of 57 doses/lb represents the fact that
1 lb of this chemical can be “shared” equally across 57 individuals (1/57 lb per person) without having an adverse carcinogenic/noncarcinogenic toxic effect, whereas
in comparison, another chemical with a higher calculated TF of 129 doses/lb represents the fact that 1 lb of this chemical can be shared equally across 129 individuals
(1/129 lb per person) without having an adverse carcinogenic/noncarcinogenic toxic
effect. In essence, the higher the TF is, the higher the toxicity will be because each
individual can only ingest or inhale a smaller fraction of the pound of chemical
released before reaching the toxicity threshold.
Subsequently, each chemical-specific TF was then divided by the current (as of
the time of writing this book) population of the United States (306 million) to determine the chemical-specific TF in units of doses/capita-pound:
Single Chemical-Specific TF (doses/capita-lb) = Single TF (doses/lb)
÷ 306,000,000 people in the United States
The TF in doses/capita-pound, as presented in Table 6.1, can be interpreted
as follows:
• A chemical-specific TF of 1 dose/capita-lb represents the fact that each of
the 306 million people in the United States has the potential to reach the
threshold beyond which an adverse effect can occur (carcinogenic or noncarcinogenic) from sharing 1 lb of that chemical, assuming exposure and
intake (ingestion or inhalation) are equal across the population.
Toxic Chemicals
SINGLE COMBINED RELATIVE TOXICITY FACTOR DEVELOPMENT
What follows are the combination of the individual oral and inhalation TFs developed for cancer and noncancer risks to calculate a single, combined relative TF for
TRI chemicals. This chemical-specific single relative TF represents how many individuals could ingest or inhale each pound of chemical per year released to have
no adverse carcinogenic or noncarcinogenic impact from the chemical (i.e., the
threshold beyond which an adverse effect of some type could occur) in the unit of
doses/pound. As noted, the higher the measures of cancer risk (based on USEPA
cancer toxicity values) are, the higher the cancer toxicity associated with the chemical will be, whereas the higher the measures of noncancer risk (based on USEPA
noncancer toxicity values) are, the lower the noncancer toxicity will be. However, as
was described in detail, as part of developing the single relative chemical-specific
TFs, all four of the measures of chronic toxicity were converted into a dose relationship (doses/pound), which resulted in a direct relationship to toxicity (the higher
the number of doses per pound for a chemical, the higher the toxicity); hence, the
carcinogenic and noncarcinogenic effects become additive:
Single Chemical-Specific TF (doses/lb) = Σ (Ingestion Carcinogenic TF,
Inhalation Carcinogenic TF, Ingestion Noncarcinogenic TF,
Inhalation Noncarcinogenic TF)
For example, a chemical with a calculated TF of 57 doses/lb represents the fact that
1 lb of this chemical can be “shared” equally across 57 individuals (1/57 lb per person) without having an adverse carcinogenic/noncarcinogenic toxic effect, whereas
in comparison, another chemical with a higher calculated TF of 129 doses/lb represents the fact that 1 lb of this chemical can be shared equally across 129 individuals
(1/129 lb per person) without having an adverse carcinogenic/noncarcinogenic toxic
effect. In essence, the higher the TF is, the higher the toxicity will be because each
individual can only ingest or inhale a smaller fraction of the pound of chemical
released before reaching the toxicity threshold.
Subsequently, each chemical-specific TF was then divided by the current (as of
the time of writing this book) population of the United States (306 million) to determine the chemical-specific TF in units of doses/capita-pound:
Single Chemical-Specific TF (doses/capita-lb) = Single TF (doses/lb)
÷ 306,000,000 people in the United States
The TF in doses/capita-pound, as presented in Table 6.1, can be interpreted
as follows:
• A chemical-specific TF of 1 dose/capita-lb represents the fact that each of
the 306 million people in the United States has the potential to reach the
threshold beyond which an adverse effect can occur (carcinogenic or noncarcinogenic) from sharing 1 lb of that chemical, assuming exposure and
intake (ingestion or inhalation) are equal across the population.