On the basis of known medical experiments we find out the correlation between the concentration of carbon monoxide (CO) in inhaling air and the concentration of carboxihemoglobyne (COHb) in human blood. All internal combustion engines produce exhaust gases containing noxious compounds: carbon monoxide, nitrogen oxides (NOx), carbon oxides (CxHy) and smoke. In a living room is important the smoke of cigarettes, smoke of furnaces, improper ventilation. In tunnel is most dangerous the carbon monoxide if it exceed an allowable level. In human blood the carbon monoxide causes increasing the concentration of carboxihemoglobyne and in this case the hypoxia of web. With help of mathematical model the concentrations of some dangerous substances at the end of tunnel were calculated. For this case a differential equation also was developed and it shows the correlation between concentration of carbon monoxide in the air and concentration of carboxihemoglobyne in the blood. The constructed mathematical model shows circumstances in the tunnel (velocity of air moving as effect of induction. concentration of noxious substances and criterial number). Also a corresponding computer program was developed, which makes possible a quick and simple calculations. All the results are proved by experiments. Finally the differential equation was done, which shows a temporal connection between both parameters as a function of tunnel characteristics. (C) 2003 Published by Elsevier Science B.V.