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Toxicological Risks of Selected Flame-Retardant Chemicals
EXPOSURE ASSESSMENT AND RISK CHARACTERIZATION
Noncancer
Dermal Exposure
The assessment of noncancer risk by the dermal route of exposure is based on the scenario described in Chapter 3. This exposure scenario assumes that an adult spends 1/4th of his or her time sitting on furniture upholstery treated with zinc borate, that 1/4th of the upper torso is in contact with the upholstery, and that clothing presents no barrier. Zinc borate is considered to be ionic, and is essentially not absorbed through the skin. However, to be conservative, the subcommittee assumed that ionized zinc borate permeates the skin at the same rate as water, with a permeability rate of 10−3 cm/hr (EPA 1992). Using that permeability rate, the highest expected application rate for zinc borate (2 mg/cm2), and Equation 1 in Chapter 3, the subcommittee calculated a dermal exposure level of 6.3×10−3 mg/kg-d. The oral RfD for zinc borate (0.6 mg/kg-d; see Oral RfD in Quantitative Toxicity section) was used as the best estimate of the internal dose for dermal exposure. Dividing the exposure level by the oral RfD yields a hazard index of 1.0×10−2. Thus it was concluded that zinc borate used as a flame retardant in upholstery fabric is not likely to pose a noncancer risk by the dermal route.
Inhalation Exposure
Particulates
The assessment of the noncancer risk by the inhalation route of exposure is based on the scenario described Chapter 3. This scenario corresponds to a person spending 1/4th of his or her life in a room with low air-change rate (0.25/hr) and with a relatively large amount of fabric upholstery treated with zinc borate (30 m2 in a 30-m3 room), with this treatment gradually being worn away over 25% of its surface to 50% of its initial quantity over the 15-yr lifetime of the fabric. A small fraction, 1%, of the worn-off zinc borate is released into the indoor air as inhalable particles and may be breathed by the occupant. Equations 4 through 6 in Chapter 3 were used to estimate the average concentration of zinc borate present in the air. The highest expected application rate for zinc borate is about 2 mg/cm2. The estimated release rate for zinc borate is
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