Isobutanol is a high-value molecule offering several advantages over existing transportation biofuels. Isobutanol blended with gasoline at 16% is currently under development as a new automotive biofuel in the US. To evaluate the potential health effects from exposure to isobutanol gasoline blend when dispensed at the pump, a battery of GLP whole-body inhalation studies using Sprague-Dawley rats was conducted in accordance with US EPA fuel testing requirements, described in Section 211(b) of the Clean Air Act and its Alternative Tier 2 Rule. Testing included the following guideline studies: 90-day subchronic toxicity with neurotoxicity, neuropathology, glial fibrillary acidic protein (GFAP) measurements across 8 brain regions, and 4-weeks recovery; 28-day immunotoxicity; 28-day genotoxicity; one-generation reproductive toxicity; and embryo-fetal developmental toxicity. To approximate the hydrocarbon composition of fugitive emissions to which humans are potentially exposed during refuelling, light-end hydrocarbon emissions were generated from the whole fuel heated to 130 degrees F and condensed to form a vapor condensate (VC) test substance. Isobutanol levels in the VC were 2-3%. In all studies rats were exposed to re-vaporized test substance at target concentrations of 0 (air control); 2,000; 10,000; and 20,000 mg/m3, the latter being 50% of the lower explosive limit of the VC. Exposure frequency and duration were tailored to study design. The only treatment-related finding in the 90-day study was light hydrocarbon nephropathy in male rats, a known effect specific to male rats and not relevant to humans. No treatment-related findings were seen in neuropathology, GFAP, neurotoxicity, immunotoxicity, genotoxicity, or developmental toxicity. The no-observed-adverse-effect level (NOAEL) for these endpoints was 20,000 mg/m3. In the reproductive study transient changes in body weight gain and food consumption were seen in high-dose paternal and maternal animals. These effects occurred at hydrocarbon exposure levels several orders of magnitude above conditions typically experienced at dispensing pumps during refueling (<1 mg/m3, 5 min). Based on these results, inhalation exposure to light-end hydrocarbons emitted from 16% isobutanol gasoline during refueling is unlikely to present a unique health hazard to humans.
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