GHS, NFPA 704, DOT and the fire code, decoded.

Four systems describe the same chemical for four different readers, and their numbers run in different directions.

The GHS, as OSHA's Hazard Communication Standard adopts it, labels a chemical for the people who work with it: "Informs workers about the hazards of chemicals in workplace under normal conditions of use and foreseeable emergencies." NFPA 704 marks a building for the people who respond to it: "Provides basic information for emergency personnel responding to a fire or spill and those planning for emergency response." (OSHA QuickCard 3678-08 2013). DOT's hazard classes decide how a material is packaged, labelled and placarded for transport. The International Fire Code's hazard categories decide how much of it a building may hold and what it takes to keep it there.

The numbers are the trap. In the GHS a category runs "1-most severe hazard 4-least severe hazard"; in NFPA 704 "0-least hazardous 4-most hazardous" (OSHA QuickCard 3678-08 2013). NOAA puts it this way: "Section 2 on an SDS requires hazard category numbers that may look similar to NFPA values, but they are not the same. In fact, the HCS uses a hazard numbering system that is the reverse of the NFPA system; in the HCS system a 1 is the most hazardous number and a 4 is the least hazardous." A DOT class number names the kind of hazard, never its degree; the degree is the packing group ("Packing group (PG) is the degree of danger: I is great danger, II medium, III minor (49 CFR 171.8)."). And the fire code's classes carry their own numbers again.

So this page never converts one number into another. Where two systems draw lines on the same measurement, the flash point and boiling point of a liquid or the LD50 of a poison, it lays their lines side by side and says exactly where a category of one lands in the other, down to the tenth of a degree where a line drawn in Celsius meets one drawn in Fahrenheit. A liquid always flashes below the temperature it boils at, so a combination that would need the reverse is left out. Everywhere else the GHS's own allocation table names the transport class that goes with a category, and FireInspection360's fire code classifier, the same one its SDS reader runs, says what the fire code makes of a statement, a class or a rating on its own.

NFPA 704's criteria are NFPA's own ("Within the blue, red, and yellow quadrants a number from 0 to 4 indicates the degree of risk associated with the chemical. The higher the number, the higher the risk.", NOAA), and no public source prints them, so the page says which quadrant would rate a hazard of a given kind and never which number it would get. "The hazard category numbers found in section 2 of the HC2012 compliant SDSs are NOT to be used to fill in the NFPA 704 diamond." (OSHA QuickCard 3678-08 2013)