API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

3-104 API RECOMMENDED PRACTICE 581
The IDLH is a 30-minute exposure limit. The cloud dispersion model should determine areas in the cloud that
have time-weighted average concentrations exceeding the IDLH for a period of 30 minutes or longer.
5.9.2.4 Emergency Response Planning Guidelines—ERPG-3
ERPGs have been developed for toxic chemicals by the American Industrial Hygiene Association (AIHA), for
three levels of increasing danger to exposed personnel. The ERPG-3 criteria is used and represents the
maximum concentration (ppm) below which it is believed nearly all individuals could be exposed for up to 1
hour without experiencing or developing life-threatening effects.
The cloud dispersion model should determine areas in the cloud that have time-weighted average
concentrations exceeding the ERPG-3 limit for a period of 1 hour or longer.
5.9.2.5 Acute Exposure Guideline Limit 3—AEGL-3
AEGLs represent ceiling exposure values for the general public and are published for emergency periods of
10 minutes, 30 minutes, 1 hour, 4 hours, and 8 hours. The concentration in the toxic cloud is checked against
exposure durations of 10 minutes, 30 minutes, and 1 hour, since it is assumed that the release will be detected
and mitigated within that time frame.
AEGLs are published for three levels of exposure—AEGL-1, AEGL-2, and AEGL-3—each one representing
increasing levels of danger to the exposed personnel. The most life-threatening level, AEGL-3, is used when
comparing it against the concentrations as calculated by the cloud dispersion model. The AEGL-3 limit is the
airborne concentration (ppm) of a substance at or above which it is predicted that the general population,
including susceptible but excluding hypersusceptible individuals, could experience life-threatening effects or
even death. Airborne concentrations below AEGL-3, but at or above AEGL-2, represent exposure levels that
may cause irreversible or other serious, long-lasting effects or impaired ability to escape.
5.9.2.6 Lethal Concentration—LC50
The median lethal concentration of a toxic substance is the atmospheric concentration (typically in ppm)
causing one half of a tested population to die. These tests are often done on rats or mice. Although these
values cannot be directly extrapolated from one species to another, they are generally used as an indicator of
a substance’s acute toxicity. The exposure time is indicated with the test and can vary between 10 minutes
and 8 hours. The formula to determine an LC50 is found in 49 CFR 173.133(b)(1)(i).
5.9.2.7 Temporary Emergency Exposure Limit 3—TEEL-3
Temporary emergency exposure limits (TEELs) were developed for the purpose of conducting consequence
assessments for chemicals for which no AEGL or ERPG values existed. They have been developed by the
U.S. Department of Energy Subcommittee on Consequence Assessment and Protective Actions, with four
levels of increasing danger to exposed personnel. Consequence analysis uses the TEEL-3, which is the
maximum concentration in air below which nearly all individuals could be exposed for a 15 minutes without
experiencing or developing life-threatening health effects. The TEEL value is meant to be a temporary value
that will be replaced by an ERPG or AEGL.
The cloud dispersion model should determine areas in the cloud that have time-weighted average
concentrations exceeding the TEEL-3 limit for a period of 15 minutes or longer.
5.9.3 Release Duration
The potential toxic consequence is estimated using both the release duration and release rate (see Section
tox
4.9.10 for a discussion of determination of the duration). In general, the toxic leak duration, ld
n
, should be
calculated per Equation (3.186) for each release hole size as the minimum of: