302. Ventilation: Except in cases where the safety of workers is ensured in compliance with paragraph 3 of section 303, no worker may enter or be present in an enclosed area unless the latter is ventilated either by natural or mechanical means such that the following atmospheric conditions are maintained:
(1) the concentration of oxygen shall be greater than or equal to 19.5% and less than or equal to 23%;
(2) the concentration of inflammable gases or vapours shall be less than or equal to 10% of the lower explosion limit;
(3) the concentration of one or more contaminants referred to under the sub- subparagraph of subparagraph 1 of the first paragraph of section 300 shall not exceed the standards provided in Schedule I for these contaminants;
If it proves impossible by ventilating the enclosed area to maintain an internal atmosphere in compliance with the standards provided under subparagraphs 1 and 3 of the first paragraph, a worker may only enter or be present in this area if he wears the respiratory protective equipment specified in section 45 and if the internal atmosphere of this enclosed area complies with subparagraph 2 of the first paragraph.