If you have a chemical that is classified (Health, Fire, Reactivity) 2, 1, 2. And another chemical that is 1, 3, 2. If these two chemicals are stored together, what is put on the storage container/building? What is the rule for this; the largest in each category is displayed, 2, 3, 2. And the specific hazards are listed?
According to Section 18.104.22.168 of NFPA 704:
Where more than one chemical is present in a building or specific area, professional judgment shall be exercised to indicate ratings using the following methods:
(1) Composite Method. Where many chemicals are present, a single sign shall summarize the maximum ratings contributed by the material(s) in each category and the special hazard category for the building and/or the area.
(2) Individual Method. Where only a few chemicals are present or where only a few chemicals are of concern to emergency responders (taking into account factors including physical form, hazard rating, and quantity), individual signs shall be displayed. The chemical name shall be displayed below each sign.
(3) Composite–Individual Combined Method. A single sign shall be used to summarize the ratings via the Composite Method for buildings or other areas containing numerous chemicals. Signs based on the Individual Method shall be used for rooms or smaller areas within the building containing small numbers of chemicals.
"Section 22.214.171.124 of NFPA 704 provides three different methods to rate multiple chemicals. In addition, professional judgment must still be used to increase or decrease the rating to more accurately assess the degree of hazard, perhaps due to quantities, or synergistic effects of the chemicals, etc. The ratings for a chemical that contains multiple ingredients should be obtained from data for the mixture as presented on the SDS." ~From this NFPA FAQ