Scientists at the Defence Institute of Physiology and Allied Sciences dipas), New Delhi, have recently developed a gas mask containing five per cent carbon dioxide ( CO 2 ) and 95 per cent oxygen ( O 2 ), that reduces hearing loss among industrial workers. dipas director, W Selwamurthy said that noise constricts blood vessels, including those leading to the ear, thus affecting hearing. the CO 2 in the mask reverses this effect by dilating the vessels. Similarly, O 2 meets the additional energy demand of specialised receptor cells in the ear that pick up noise signals. In a noisy environment, these cells are under strain as they are contantly responding to the stimulus.