Revision of Proposed Specific Regulatory Level and Augmentation of Record for Proposed Regulation: Title 27, California Code of Regulations, Section 25805, Specific Regulatory Levels: Chemicals Causing Reproductive Toxicity Sulfur Dioxide

The Office of Environmental Health Hazard Assessment (OEHHA) is revising the Maximum Allowable Dose Level (MADL) for sulfur dioxide proposed for adoption into regulation (Title 27, California Code of Regulations, section 25805).  The MADL was first proposed as 220 micrograms per day in July, 2012.  The new proposed MADL is 10,000 micrograms per day.  OEHHA is also augmenting the administrative record for the proposed regulation.  The augmentation consists of data submitted to OEHHA in comments on the proposed regulation1.  As required by Government Code section 11346.8(c), and Title 1, Section 44 of the California Code of Regulations, OEHHA is giving notice of this revision and augmentation.  The proposed regulatory action is being taken pursuant to OEHHA’s authority under Proposition 65.2

The Notice of Proposed Rulemaking for the proposed sulfur dioxide MADL was published on July 6, 2012, in the California Regulatory Notice Register (Register 2012, No 27-Z, page 910), which initiated a public comment period.  Written comments from the public were received during the comment period, which ended August 20, 2012.  One comment stated that there was an error in a statistic presented in the papers published in the scientific literature that provided the lowest observable effect level (LOEL) which formed the basis for the MADL3.  The comment stated that a rounding error in the statistical analysis resulted in a reduction in fetal weight being incorrectly reported as statistically significant (p<0.05).  The commenter subsequently provided the original data records for the study to OEHHA.  Reanalysis of the data by OEHHA confirmed that the commenter’s statement was correct, and that the change in fetal weight at this exposure level was not statistically significant. 

The study in question, by Murray et al., reported a developmental LOEL of 23.9 parts per million (ppm), based on the measured time-weighted sulfur dioxide concentration. OEHHA now considers this level as the no observable effect level (NOEL) for that study.   

As noted in the Initial Statement of Reasons for the proposed regulation, another inhalation study by Singh (1989)4 demonstrated reduced birth weight after prenatal exposure to sulfur dioxide.  This effect was statistically significant for mice exposed to sulfur dioxide at 65 ppm for 24 hours/day.  At 32 ppm, a reduction in birth weight was not statistically significant.  Thus, the study by Singh (1989) provided a NOEL of 32 ppm for mice exposed for 24 hours/day and, for purposes of Proposition 65, is now the most sensitive study deemed to be of sufficient quality (Section 25803(a)(4)).  Consequently, OEHHA has revised the proposed MADL.

The following calculations were performed in accordance with Section 25803 to derive the MADL for sulfur dioxide using data and exposure parameters from Singh (1989):

  • Conversion of air concentration in ppm to milligrams per cubic meter (mg/m3) using a conversion factor of 2.64 mg/m3 per ppm5
    (32 ppm ´ 2.64 [mg/m3 per ppm]) = 84.48 mg/m3
  • Calculation of the NOEL dose for a 30 gram mouse (0.030 kilograms [kg]) with an inhalation rate of 0.063 m3/day6, 7
    (84.48 mg/m3 ´ 0.063 m3/day) ¸
    (0.030 kg) = 177.41 mg/kg/day
  • Calculation of the NOEL dose for a 58 kg woman
    177.41 mg/kg/ day ´ 58 kg =  10289.78 mg/day,
    or 10,000 mg/day after rounding
  • The MADL is derived by dividing the NOEL by one thousand (Section 25801(b)(1)).  Thus, the adjusted NOEL was divided by 1,000 to obtain the MADL:
                         MADL = 10,000 mg/day ¸ 1000 = 10,000 micrograms/day

OEHHA will accept written comments on this amendment to the proposed regulation and augmentation of the record through May 6, 2013.  All written comments must be submitted to OEHHA no later than 5:00 p.m. on May 6, 2013.

The public is encouraged to submit written information via e-mail, rather than in paper form.  Send e-mail comments to  Please include “SULFUR DIOXIDE MADL” in the subject line.  Hard-copy comments may be mailed, faxed, or delivered to:

Mailing Address:  Sam Delson
Office of Environmental Health Hazard Assessment
P.O. Box 4010, MS-19B
Sacramento, California 95812-4010
Fax: (916) 324-6511
Street Address:  1001 I Street
Sacramento, California 95814

A public hearing on this proposed regulatory amendment will be scheduled upon request.  To request a hearing send an e-mail to Sam Delson at or the address listed above by no later April 19, 2013, which is 15 days before the close of the comment period.  OEHHA will mail a notice of the hearing to the requester and interested parties on the Proposition 65 mailing list for regulatory public hearings.  The notice will also be posted on the OEHHA web site at least ten days before the public hearing date.  The notice will provide the date, time, and location of the hearing. 

Please direct inquiries concerning the proposed regulatory action described in this notice to Sam Delson, or by calling (916) 327-3015.  Monet Vela is a back-up contact person for inquiries concerning processing of this action and is available at: or by telephone at (916) 323-2517

Public Comments

Comment period closed on May 6, 2013. One comment was received.

Monday, May 6, 2013

Footnotes and References

1Available on our site here.

2 The Safe Drinking Water and Toxic Enforcement Act of 1986, codified at Health and Safety Code section 25249.5 et seq.  

3Murray FJ, Schwetz BA, Crawford AA, Henck JW, Quast JF, Staples RE (1979). Embryotoxicity of inhaled sulfur dioxide and carbon monoxide in mice and rabbits. J Environ Sci Health C 13(3):233-50.
Murray FJ, Schwetz BA, Crawford AA, Henck JW, Staples RE (1977). Teratogenic potential of sulfur dioxide and carbon monoxide in mice and rabbits. Doe Symp Ser 47: 469-478.

4Singh J (1989). Neonatal development altered by maternal sulfur dioxide exposure. Neurotoxicology 10(3): 523-7.

5Office of Environmental Health Hazard Assessment (OEHHA), Evidence on the Developmental and Reproductive Toxicity of Sulfur Dioxide, OEHHA, California Environmental Protection Agency, Sacramento, California, February 2011

6Bond JA, Dahl AR, Henderson RF, Dutcher JS, Mauderly JL, Birnbaum LS (1986). Species differences in the disposition of inhaled butadiene. Toxicol Appl Pharmacol 84: 617-627.

7Depledge MH (1985). Respiration and lung function in the mouse, Mus musculus (with a note on mass exponents and respiratory variables). Respir Physiol 60: 83-94.