This pamphlet provides information on decompression for tunnel workers exposed to compressed air. During tunnel construction, the air had to be at a pressure great enough to keep out water and help support the tunnel structure. Topics included breathing compressed air, decompression sickness (the bends), prevention of decompression sickness, treatment, and guidelines for workers exposed to compressed air during tunnel construction. Workers in a compressed air environment took in more oxygen and nitrogen with each breath, and greater amounts of these gases were taken up by the body fluids in the compressed air environment. When a worker left the compressed air environment too rapidly, the body could not rid itself of the excess nitrogen dissolved in the blood fast enough to prevent it from collecting in the form of bubbles throughout the body. Treatment for a worker who has decompressed too quickly involved recompression and subsequent decompression over increased time periods. Workers are advised never to shorten the decompression period, not to sit in cramped positions during decompression or to sleep or rest in a cramped position soon after decompression has been completed, to avoid hot water showers up to 6 hours after decompressing, to be well rested and avoid use of alcohol the night before working in a compressed atmosphere, to be particularly careful during periods of colds and other illnesses, and never to use alcoholic beverages as a treatment for decompression pains.