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Toxicity Hazards

Estimation of an equivalent toxic dose is based on exposure duration divided by test duration raised to the 1/n exponent.
The concentration providing a one-hour equivalent dose to a 4-hour test would be twice the test concentration for an
exponent of 2. An example for how the “2” exponent is determined through AEGL data follows.

It should be noted that there are some toxic effects that may be related to concentration rather than dose or is nearly
independent of exposure duration.

3.11.1 Example Determination of n using AEGL values

If AEGLs are available at different exposure durations, a value for “n” may be determined from the dose equation,
Equation 3-1. For example, the AEGL-3 for chlorine (CAMEO Chemicals) is listed at 50 ppm for 10-minute and 20 ppm for
60-minute exposure. As these values and exposure duration represent a similar response and dose,
50 (10 min) = 20 (60 min) or (50/20) = 60/10
n
n
n
n ln(50/20) = ln(60/10) and n = 1.96 or roughly 2

3.12 Inhalation Toxic Mixtures

The ERPG values for toxic chemicals may be orders of magnitude different from each other.
 Assuming the entire mixture at a toxicity equivalent to the “worst component” may be grossly conservative.
 Assuming the toxicity of each chemical is independent of other toxic materials in the mixture may underestimate
the hazard

The US Department of Energy (US DOE), for example, has recommended an “additive” approach (which is similar to
Le Chatelier’s rule), as is shown in Equation 3-3 [31]:

ERPGMixture = 1 /  (yi /ERPGi) Equation 3-3

This additive approach is also used for classifying the acute toxicity of mixtures under the United Nations Globally
Harmonized System (GHS).

3.12.1 Example Estimation of Equivalent ERPG-3 for Vapor Mixtures

As an example; the estimated ERPG-3 for a mixture of 0.5 mole fraction (or 0.457 wt. fraction) sulfur dioxide (ERPG-3
of 25 ppm) and 0.5 mole fraction (0.543 wt. fraction) carbon disulfide (ERPG-3 of 500 ppm) is:

ERPG-3mix = 1 /  yi / ERPG-3i = 1 / [ 0.5 / 25 + 0.5 / 500 ] = 1 / 0.021 = 47.6 ppm

Note that ERPG-3 is based on a one-hour exposure duration. Any correction for exposure duration should be made
for each individual chemical prior to determination of the mixture equivalent value.

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