(1) Supply, return and exhaust air openings in rooms or spaces shall be protected by grilles having openings of a size that will not allow the passage of a 15 mm diameter sphere.
(2) Outdoor air intakes and exhaust outlets at the building exterior shall be designed or located so that the air entering the building system will not contain more contaminants than the normal exterior air.
(3) Exterior openings for outdoor air intakes and exhaust outlets shall be shielded from the entry of snow and rain and shall be fitted with corrosion-resistant screens of mesh having openings not larger than 15 mm, except where climatic conditions may require larger openings.
(4) Screens required in Sentence (3) shall be accessible for maintenance.
(5) Combustible grilles, diffusers and other devices for the supply and return air openings installed in walls and ceilings shall have a flame-spread rating of,
(a) not more than 200 in bathrooms, and
(b) not more than 150 in rooms or spaces other than bathrooms.