# HRS Guidance Manual, Page 158

**Equations for Calculating the I and J
Indices**

**(4)**:**Calculate
the I and J indices for all hazardous substances for this well that meet
the observed release criteria.** Make two lists of substances that meet
the observed release criteria: hazardous substances with screening
concentrations for cancer risk; and hazardous substances with screening
concentrations for non-cancer effects. Each hazardous substance may be on
one, neither, or both of the lists. If more than one sample has been taken
from a well and these samples are comparable (e.g., taken in the same time
frame, collected using the same field techniques, analyzed by the same
methods), for each hazardous substance, select the *highest*
concentration to use in the calculations below.

- Calculate the I index for all hazardous substances with screening concentrations for cancer risk that meet the observed release criteria, using the following equation:

where:

C_{i} = concentration of substance i in well;

SC_{i} = screening concentration for cancer risk, which is the
concentration corresponding to 10^{-6} individual cancer risk for
oral exposure for hazardous substance i; and

n = number of hazardous substances that meet observed release criteria and for which an SC is available.

- Calculate the J index for all hazardous substances with screening concentrations for non-cancer effects that meet the observed release criteria, using the following equation:

where:

C_{j} = concentration of substance j in well;

CR_{j} = screening concentration for non-cancer effects, which
is the concentration corresponding to the reference dose for oral exposure
for hazardous substance j; and

m = number of hazardous substances (1) that meet observed release criteria and (2) for which a CR is available.

- If
either the I or J index is greater than or equal to 1, score the well
as Level I. If both the I and J indices
are less than one, score the well at Level
II. An example of calculating the I and J indices is presented in
.*Highlight 7-28*

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