Abstract: A system, in accordance with the invention, relates to ventilating a tunnel 1 in the event of fire or emission of gases C or aerosols. The system comprises a tunnel cover 20 and a mobile fan 21. The tunnel cover 20 has an opening 29 through which the fan 21 blows air. This increases the static pressure at the cover 20, which change the direction of air at a desired direction. One of the advantages of the invention is that the tunnel cover 20 makes it possible to utilise considerably smaller mobile fans 21 to ventilate a tunnel 1 in the event of fire or emission of gases than when using earlier known techniques.