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Section 2.4.1.1, pages 51589-51590 of the HRS rule

2.4.1.1:Toxicity factor. Evaluate toxicity for those hazardous substances at the site that are available to the pathway being scored. For all pathways and threats, except the surface water environmental threat, evaluate human toxicity as specified below. For the surface water environmental threat, evaluate ecosystem toxicity as specified in section 4.1.4.2.1.1.

Establish human toxicity factor values based on quantitative dose-response parameters for the following three types of toxicity:

  • Cancer-Use slope factors (also referred to as cancer potency factors) combined with weight-of-evidence ratings for carcinogenicity. If a slope factor is not available for a substance, use its ED10 value to estimate a slope factor as follows:

    • Slope factor = 1 / 6 (ED10)
  • Noncancer toxicological responses of chronic exposure-use reference dose (RfD) values.

  • Noncancer toxicological responses of acute exposure­ ­use acute toxicity parameters, such as the LD50.

Assign human toxicity factor values to a hazardous substance using Table 2-4, as follows:

  • If RfD and slope factor values are both available for the hazardous substance, assign the substance a value from Table 2-4 for each. Select the higher of the two values assigned and use it as the overall toxicity factor value for the hazardous substance.

  • If either an RfD or slope factor value is available, but not both, assign the hazardous substance an overall toxicity factor value from Table 2-4 based solely on the available value (RfD or slope factor).

  • If neither an RfD nor slope factor value is available, assign the hazardous substance an overall toxicity factor value from Table 2-4 based solely on acute toxicity. That is, consider acute toxicity in Table 2-4 only when both RfD and slope factor values are not available.

  • If neither an RfD, nor slope factor, nor acute toxicity value is available, assign the hazardous substance an overall toxicity factor value of 0 and use other hazardous substances for which information is available in evaluating the pathway.

Table 2-4. Toxicity Factor Evaluation

Chronic Toxicity (Human)
Reference dose (RfD) (mg/kg-day) Assigned value
 
RfD < 0.0005 10,000
0.0005 £ RfD < 0.005 1,000
0.005 £ RfD < 0.05 100
0.05 £ RfD < 0.5 10
0.5 RfD 1
RfD not available 0

Carcinogenicity (Human)
Weight-of-evidencea/slope factor
(mg/kg-day)-1
Assigned
value
A B C
 
0.5 £ SFb 5 £ SF 50 £ SF 10,000
0.05 £ SF < 0.5 0.5 £ SF < 5 5 £ SF < 50 1,000
SF < 0.05 0.05 £ SF < 0.5 0.5 £ SF < 5 100
­ ­ ­ SF < 0.05 SF < 0.5 10
Slope factor not available. Slope factor not available. Slope factor not available. 0

a A, B, and C refer to weight-of-evidence categories. Assign substances with a weight-of-evidence category of D (inadequate evidence of carcinogenicity) or E (evidence of lack of carcinogenicity) a value of 0 for carcinogenicity.

b SF = Slope factor.

Acute Toxicity (Human)
Oral LD50 (mg/kg) Dermal LD50 (mg/kg) Dust or mist LC50 (mg/l) Gas or vapor LC50 (ppm) Assigned value
 
LD50 < 5 LD50 < 2 LC50 < 0.2 LC50 < 20 1,000
5 £ LD50 < 50 2 £ LD50 < 20 0.2 £ LC50 < 2 20 £ LC50 < 200 100
50 £ LD50 < 500 20 £ LD50 < 200 2 £ LC50 < 20 200 £ LC50 < 2,000 10
500 £ LD50 200 £ LD50 20 £ LC50 2,000 £ LC50 1
LD50 not available LD50 not available LC50 not available LC50 not available 0

If a toxicity factor value of 0 is assigned to all hazardous substances available to a particular pathway (that is, insufficient toxicity data are available for evaluating all the substances), use a default value of 100 as the overall human toxicity factor value for all hazardous substances available to the pathway. For hazardous substances having usable toxicity data for multiple exposure routes (for example, inhalation and ingestion), consider all exposure routes and use the highest assigned value, regardless of exposure route, as the toxicity factor value.

For HRS purposes, assign both asbestos and lead (and its compounds) a human toxicity factor value of 10,000.

Separate criteria apply for assigning factor values for human toxicity and ecosystem toxicity for radionuclides (see sections 7.2.1 and 7.2.2).

 

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