No person shall enter or remain in any chamber, tank, vat, pit, pipe, flue or other confined space, or in any confined space in which dangerous fumes are likely to be present or the proportion of oxygen in the air is liable to have been substantially reduced, for any purpose unless he has been authorised to enter by a responsible person and either he is wearing a suitable breathing apparatus, or he is ensured of a supply of air adequate for respiration and to render harmless any fumes.