R 336.1232 Methodology for determining initial threshold screening level.

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: 3 20 70 m kg x RfD Oral ITSL  (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: ITSL NOAEL x x hours perday hoursperday 35 100 24 exposed ITSL LOAEL x xUF x hours perday hoursperday 35 100 24 exposed 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: ITSL = NOAEL (mg / kg / day) 35 x 100 x W I x b a A A ITSL = LOAEL 35 x 100 xUF x W I x b a A A 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: ITSL LC x  50 500 100 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: ITSL LC x x  50 500 100 40 (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: ITSL = 1 500 x 1 40 x 1 100 x LD50 (mg / kg) x W 0.167 x I A A 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. Editor's Note: An obvious error in R 336.1232 was corrected at the request of the promulgating agency, pursuant to Section 56 of 1969 PA 306, as amended by 2000 PA 262, MCL 24.256. The rule containing the error was published in Annual Administrative Code Supplement, 2016. The memorandum requesting the correction was published in Michigan Register, 2020 MR 24.