Section 111. Water used for humidifying purposes in a textile factory shall be taken either from a public supply of drinking water, or from some other source of pure water, or from a supply of water which, although in the opinion of the attorney general not suitable for drinking purposes, is sufficiently free from impurities to be not dangerous to the health of employees when used for humidifying purposes; and all ducts for the introduction or distribution of humidified air shall be kept clean.