All internal combustion engines produce exhaust gases containing noxious compounds: carbon monoxide (CO), nitrogen oxides (NO,), carbon oxides (CxHy) and smoke. With the help of a mathematical model the concentrations of some dangerous substances at the end of the tunnel were calculated, and were replaced by a criterial number. A corresponding computer program was also developed thus enabling quick and simple calculations of some concentrations and the criterial number. Finally, a criteria for the border between natural and forced ventilation is presented as a correlation between the criterial number and other parameters.