In the absence of reliable data, the present work was performed to estimate the dose effect function of carboxyhemoglobin (COHb) on behavior in humans. By meta analysis, a COHb-behavior dose-effects functions was estimated for rats and corrected for effects of hypothermia (which accompanies COHb increases in rats but not in humans). Using pulmonary function models and blood-gas equations, equivalent COHb values were calculated for data in the literature on hypoxic hypoxia (HH) and behavior. Another meta analysis was performed to fit a dose-effects function to the equivalent-COHb data and to correct for the behavioral effects of hypocapnia (which usually occurs during HH but not with COHb elevation). The two extrapolations agreed closely and indicated that for healthy, sedentary persons, it would require 18-25% COHb to produce a 10% decrement in behavior. Confidence intervals were computed to characterize the uncertainty. Frequent reports of lower-level effects were discussed.