In the absence of reliable data, this work was performed to estimate the dose-effects function of carboxyhemoglobin (HbCO) on behavior in humans. Meta analysis is the quantitative analysis of the combined findings of a number of research reports. By meta analysis, an HbCO-behavior dose-effects function was estimated for rats and corrected for effects of hypothermia (which accompanies acute HbCO increases in rats but not in humans). By use of pulmonary function models and blood gas equations, equivalent HbCO values were calculated for data in the literature on hypoxic hypoxia and behavior. Another meta analysis was performed to fit a dose-effects function to the equivalent HbCO data and to correct for the behavioral effects of hypocapnia (which usually occurs during hypoxic hypoxia but not with HbCO elevation). The two extrapolations agreed closely and indicated that, for healthy sedentary persons, 18-25% HbCO would be required to produce a 10% decrement in behavior. Confidence intervals are computed to characterize the uncertainty. Frequent reports of lower-level effects are discussed.