primary component and the one responsible for the toxicity of Otto Fuel II is PGDN, a volatile liquid with a disagreeable odor. Because PGDN is the primary and most toxic component of Otto Fuel II and because only PGDN is relatively volatile compared with the other components, AEGLs have been derived in terms of PGDN with the notation that the values are appropriate for Otto Fuel II.

PGDN is a systemic toxicant with effects on the cardiovascular and central nervous systems. Its vasodilatory action results in headaches during human exposures. Dizziness, loss of balance, nasal congestion, eye irritation, palpitations, and chest pains have also been reported. Methemoglobinemia has been reported at the high concentrations used in studies with animals. The air-odor threshold in healthy subjects is 0.2 parts per million (ppm), but warning properties are poor inasmuch as olfactory fatigue sets in after as little as 5 minutes (min) (Stewart et al. 1974). Within 24 hours (h) of exposure, PGDN is rapidly and completely metabolized in vivo and eliminated primarily in the urine as inorganic nitrate.

Few data were available that met the definitions of AEGL end points. One inhalation study with 20 human subjects described headaches and slight loss of balance at exposure concentrations of 0.1 to 1.5 ppm for exposure durations of up to 8 h (Stewart et al. 1974). Acute exposure of monkeys for 6 h at concentrations ranging between 70 and 100 ppm resulted in severe signs of toxicity including convulsions but no deaths (Jones et al. 1972). In the same study, exposure of rats at a higher concentration, 189 ppm for 4 h, resulted in no toxic signs. Examination of the relationship between exposure duration and concentration for both mild and severe headaches in humans over periods of 1 to 8 h determined that the relationship is C1×t=k.

The AEGL-1 values were based on concentrations at 0.5 ppm and 0.1 ppm, which were the thresholds for mild headaches in healthy individuals at exposure durations of 1 and 6 h, respectively (Stewart et al. 1974). This effect can be considered the threshold for mild discomfort (only one subject was affected at each exposure), which falls within the definition of an AEGL-1. The 0.5-ppm concentration was used to derive the 30-min and 1-h AEGL-1 values, and the 0.1-ppm concentration was used for the 4- and 8-h values. Because the time and concentration values were based on the most susceptible subject, these concentrations were adjusted by an uncertainty factor (UF) of 3 to account for potential differences in human sensitivity and scaled to the appropriate time periods using the C1×t=k relationship. A UF of 3 was considered sufficient as no susceptible populations were identified (the headache effect is the same as that experienced by patients medicated with nitro-



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