Abstract: The invention relates to a method of establishing and maintaining in sealed tents or the like mobile units, an environment which is independent of the surroundings with regard to temperature and contamination. The invention is characterized by taking air from the surroundings, extracting contaminated particles and gas from the air by filtration, and causing the air to pass into the mobile unit so as to establish a predetermined air pressure which is higher than the air pressure of the surroundings. The filtered air is mixed with air recycled from within the mobile unit when the aforesaid predetermined air pressure has been reached and mixing of the filtered air with recycled air is continued for as long as the internal air pressure is equal to or greater than the predetermined air pressure. The air mixture is heated or cooled to a predetermined temperature and is then distributed essentially uniformly in the mobile unit and filtered, in conjunction therewith, so as to extract any remaining particles and gas.