Rule 232.
(1) The
initial threshold screening level (ITSL) for each toxic air contaminant shall
be determined as follows:
(a) If an inhalation
reference concentration (RfC) can be determined from best available information
sources, then the ITSL equals the inhalation RfC.
(b) If an ITSL cannot be determined under the
provisions of subdivision (a) of this subrule and an oral reference dose (RfD)
can be determined through best available information and data are not available
to indicate that oral route to inhalation route extrapolation is inappropriate,
then the ITSL is determined as follows:
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(c) If an ITSL cannot be determined under the
provisions of subdivision (a) or (b) of this subrule and an occupational
exposure level (OEL) exists for the toxic air contaminant, then the ITSL is
determined as follows:
ITSL = OEL divided by 100
Where the OEL is the lowest value of either the national
institute of occupational safety and health (NIOSH) recommended exposure level
listed in the NIOSH pocket guide to chemical hazards or the time-weighted
average or ceiling threshold limit value listed in the TLVs and BEIs. The NIOSH
Pocket Guide to Chemical Hazards is adopted by reference in
R
336.1902. TLVs and BEIs. Threshold Limit Values for
Chemical Substances and Physical Agents, and Biological Exposure Indices, is
adopted by reference in
R
336.1902.
(d) If an ITSL cannot be determined under the
provisions of subdivision (a), (b), or (c) of this subrule, then the ITSL may
be determined from a 7-day, inhalation no observed adverse effect level (NOAEL)
or lowest observable adverse effect level (LOAEL) as follows:
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Where:
UF = A value from 1 to 10 determined on a case-by-case basis,
considering type and severity of effect.
The ITSL may be determined on a case-by-case basis using
NOAELs or LOAELs from repeated dose studies other than 7-day
studies.
(e) If an ITSL
cannot be determined under the provisions of subdivision (a), (b), (c), or (d)
of this subrule, then the ITSL may be determined from a 7-day, oral, NOAEL or
LOAEL as follows:
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Where:
WA = Body weight of experimental
animal in kilograms (kg).
IA = Daily inhalation rate of
experimental animal in cubic meters/day.
b = Absorption efficiency by the oral route of
exposure.
a = Absorption efficiency by the inhalation route of
exposure.
UF = A value from 1 to 10 determined on a case-by-case basis,
considering type and severity of effect.
The ITSL may be determined on a case-by-case basis using
NOAELs or LOAELs from repeated dose studies other than 7-day
studies.
(f) If an ITSL
cannot be determined under the provisions of subdivision (a), (b), (c), (d), or
(e) of this subrule, then the ITSL may be determined from an inhalation LC50
that is 4 or more hours in duration as follows:
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Where:
LC50 = A calculated concentration of a chemical in air to
which exposure for a specific length of time is expected to cause death in 50%
of a defined experimental animal population.
(g) If an ITSL cannot be determined under the
provisions of subdivision (a), (b), (c), (d), (e), or (f) of this subrule, then
the ITSL may be determined from a 1-hour inhalation LC50 as follows:
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(h) If an ITSL cannot be determined under the
provisions of subdivision (a), (b), (c), (d), (e) (f), or (g) of this subrule,
then the ITSL may be determined from an animal oral LD50 as follows:
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Where:
LD50 = The dose of a chemical that has been calculated to
cause death in 50% of a defined animal population.
WA = Body weight of experimental
animal in kilograms (kg).
IA = Daily inhalation rate of
experimental animal in cubic meters/day.
(i) If an initial threshold screening level
cannot be determined under the provisions of subdivision (a), (b), (c), (d),
(e), (f), (g), or (h) of this subrule, then the initial threshold screening
level = 0.1 ug/m3.
(2) The averaging times to be used for ITSLs
are as follows:
(a) If the ITSL is derived
from an OEL as in subrule (1)(c) of this rule, then the averaging time is 8
hours for ITSLs based on time-weighted average threshold limit values or
recommended exposure levels and 1 hour for ITSLs based on ceiling threshold
limit values or recommended exposure levels.
(b) If the ITSL is derived as in subrule
(1)(a) or (b) of this rule, then the averaging time is annual.
(c) If the ITSL is derived as in subrule
(1)(d), (e), (f), (g), (h), or (i) of this rule, then the averaging time is
annual.
(d) The department may
require shorter averaging times if necessary to provide adequate protection
from the acute effects of a toxic air contaminant.
