The features of the defeat of the human body by carbon monoxide are considered. It is shown that the percentage of carboxyhemoglobin in blood determines the degree of person in toxication. It is noted that the experimental study of the effect of CO on the person, as a rule, carried out in an environ¬ment where there were no other hazards of fire and the breath of man was calm at a flow rate of pulmonary ventilation of the order of 5-9 l/min. A mathematical model for calculating the percentage of carboxyhemoglobin in the blood when to CO is proposed. The model takes into account the concentration of CO in the breathing air, the mass of hemoglobin in the blood, the volume rate of ventilation, the volume of the “dead” space of the respiratory, lung diffusion capacity for CO and CO exposure time. A comparison of the estimated content of carboxyhemoglobin with the experimental data reported in the literature and obtained when exposed to human constant CO concentration during quiet breathing is made. It is shown that the calculated values match with the experimental values with an error not exceeding 27 %. The results of numerical experiments to determine the concentration of carboxyhemoglobin at an elevated rate of lung ventilation that is typical for fire conditions in the room are presented. It is shown that the critical value of the concentration of carbon monoxide (0.00116 kg/m3), adopted in the regulatory and scientific literature on fire safety, may make it impossible for the safe evacuation of people under certain conditions. In the context of the maximum possible volume of pulmonary ventilation rate, the corresponding lung diffusion capacity for CO, for an average adult human at a constant concentration of carbon monoxide, which is close to its critical value, some mild poisoning occurs within 1.06 min, moderately severe - by 2.64 min. When blood hemoglobin mini¬mum weight of an adult, mild poisoning occurs by 0.74 min, and moderately severe - after 1.84 min.