Screening assessment on MBMBP phenols: chapter 11

Potential to Cause Harm to Human Health

Exposure Assessment

Quantitative data upon which to base upper-bounding estimates of intake of MBMBP were not available for any environmental media in Canada or elsewhere. Estimated environmental concentrations were modelled for air, water and soil based on the information provided in the section 71 survey (Canada 2001). Children aged 0.5-4 years appear to be the subgroup (of the general population) most highly exposed to MBMBP in Canada. Their maximum upper-bounding daily intake is 3.4 × 10-4 µg/kg-bw per day--an estimate based on modelled environmental concentrations (see Appendix 1). Elevated exposure from drinking water may occur in populations close to point-source releases of MBMBP to surface water. Due to the absence of monitoring data, upper-bounding estimates of daily intake associated with potential point-source releases of MBMBP to surface water were estimated based on conservative modelled concentrations in drinking water.Footnote 1 Estimated intakes ranged from 7.1 × 10-3 µg/kg-bw per day (12-19 years) to a maximum of 3.7 × 10-2 µg/kg-bw per day for the 0-6 months age group.

Confidence in the exposure database is considered to be very low to low, as it is based solely on modelled concentrations of MBMBP in air, soil and water, and there is no indication of whether MBMBP is present in food. In view of MBMBP’s high octanol-water partition coefficient, exposures through food and breast milk could occur. However, given the low concentrations predicted in water and soil and the fate of MBMBP in the environment, it is unlikely that exposures through foodstuffs and breast milk would exceed the conservative estimate presented here. MBMBP may also be present in residual amounts in consumer products, but no data were available as a basis for quantifying this exposure, although it is expected to contribute minimally to total intake compared with soil.

Health Effects Assessment

Based on a screening-level evaluation of available toxicological data on MBMBP (see Appendix 2), the lowest lowest-observed-effect level (LOEL) identified was 6 mg/kg-bw per day in dogs exposed to MBMBP via the diet for 90 days (ACC 1965b). At this dose, there was a significant difference in plasma alkaline phosphatase activity from pre-exposure levels to those measured at weeks 12 and 17 in dogs exposed to MBMBP compared with controls. Histopathological changes in the liver were also observed at the higher exposure levels (i.e., 10 mg/kg-bw per day or more). Results of the one limited chronic bioassay and the results of quantitative structure-activity relationship and structure-activity relationship modelling do not indicate that MBMBP is carcinogenic; similarly, the available limited data and model predictions do not suggest that the substance has a high potential for genotoxicity.

The confidence in the database on health effects is considered to be moderate, based on the number of available toxicity studies addressing acute, repeated-dose and long-term genetic, reproductive and developmental toxicity endpoints.

Characterization of Risk to Human Health and Uncertainties

Comparison of a conservative critical effect level (i.e., 6 mg/kg-bw per day) for slight changes in biochemical parameters in a 90-day study in dogs with the highest of the upper-bounding estimates of exposure for all age groups in the population living in the vicinity of a point source (i.e., 0.037 µg/kg-bw per day) for the 0- to 6-month age group (based on conservative modelling of potential releases to the environment) resulted in a margin of exposure of approximately 160 000.

Based on the level of confidence in the available database and the conservative nature of this evaluation, including the use of an upper-bounding exposure estimate based on modelled predictions and lowest reported effect level, the margin between estimated exposure levels of MBMBP and those causing health effects in experimental animals is considered adequate to address elements of uncertainty associated with limitations of the database for population exposure and health effects (including intraspecies and interspecies variations in sensitivity, as well as the biological adversity or severity of the effects deemed critical), in which confidence is low and moderate, respectively.

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