2.3 UN/NA Numbers and NFPA 704

Class diamonds tell you what kind of hazard. Four-digit identification numbers tell you which listed substance or entry you are looking up. NFPA 704 diamonds, which look deceptively similar as a square-on-point, tell you how severe a facility's health, flammability, and instability hazards are rated — they do not identify a UN listing. Mixing those three languages is one of the highest-yield technician traps in hazard identification.

UN numbers: the international ID

A UN number is a four-digit number assigned to a dangerous-goods entry in the UN Model Regulations and carried into the U.S. HMR, IMDG, and ICAO/IATA. The prefix UN means the number is internationally recognized. Examples you will actually see: UN1203 (gasoline), UN1978 (propane), UN1830 (sulfuric acid), UN1017 (chlorine), UN3480 (lithium ion batteries). The same UN number can cover a family of similar materials under a generic or n.o.s. proper shipping name; it is still a listed identity, not a laboratory assay.

On scene, the UN number is displayed:

• On the orange panel or on the placard of bulk tanks and vehicles
• As a package mark near the DOT labels (UN followed by the four digits)
• In the basic description on shipping papers (covered in the next section)

Write it down exactly. UN1203 and UN1230 are different entries. Dropping a digit or reversing two digits sends you to the wrong ERG guide.

NA numbers: North America only

The prefix NA means the identification number is authorized for North American transportation and is not a UN listing. NA numbers exist because some domestic materials or U.S./Canada-only descriptions are not assigned a UN number, or because U.S. domestic practice (especially combustible liquids and some pesticides) needs an ID number that international rules do not use the same way.

Classic teaching examples:

• NA1993 — Combustible liquid, n.o.s. (the workhorse domestic combustible entry)
• Other NA entries for certain domestic pesticides and other North America-only descriptions

UN = use it as an internationally recognized ID. NA = treat it as a valid U.S./Canada transport ID, but do not expect every foreign guidebook, vessel, or aircraft document to treat it as a UN number. Both UN and NA numbers still appear on orange panels, placards, and shipping papers in North American highway and rail service. Both still have four digits. The prefix is the distinction.

There is also an ID prefix for certain consumer-commodity / ID8000 air entries; do not confuse ID, UN, and NA on a shipping paper. If the paper says NA1993, you are not looking up UN1993 as if they were the same listing.

How technicians use the number with the ERG (preview only)

The Emergency Response Guidebook is the next chapter. For identification, you only need the lookup path:

1. Read the four-digit number from the orange panel, placard, package mark, or shipping paper.
2. Open the ERG yellow-bordered pages (ID-number index).
3. Find the number, read the name and the guide number, and note any green highlight that points you to Table 1 isolation distances for TIH materials.
4. Go to the orange guide for generic fire/spill/first-aid actions for that guide number — generic, not SDS-level product chemistry.

The blue pages index by name if you have a name and no number. Yellow is the number path. Do not invent a UN number from an NFPA 704 rating or from a GHS pictogram.

2024 ERG change you must not overclaim: starting with the 2024 edition, chemical and biological warfare agents were removed from the yellow and blue ID listings. Those agents are collected in the criminal or terrorist use / chemical and biological warfare section and are listed by type and symptoms rather than as ordinary commercial UN entries in the yellow pages. If the problem in front of you looks like a chemical-warfare release, do not assume you will find it by flipping yellow pages the way you find UN1203.

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NFPA 704: the facility diamond

NFPA 704 is the Standard System for the Identification of the Hazards of Materials for Emergency Response. You will see it on fixed facilities: tanks, storage rooms, outdoor tank farms, and many building entrances. It is a square-on-point divided into four quadrants:

Ratings in the blue, red, and yellow quadrants run 0 through 4. That scale is severity at the facility, under NFPA's criteria, often for the material as used or stored. It is not a DOT packing group, not a class number, and not a four-digit UN number.

White special hazards

The white quadrant does not use 0–4. Standard special-hazard marks technicians are expected to recognize:

• W with a slash (W̄) — unusual reactivity with water. This is a tactic warning, not a DOT 4.3 placard. A facility can be water-reactive without displaying a blue 4.3 transport diamond.
• OX — oxidizer
• SA — simple asphyxiant (limited in the standard to specified inert gases such as nitrogen, helium, neon, argon, krypton, and xenon)

Older signs sometimes still show ACID, ALK, or COR in white. Current 704 practice puts corrosivity into the health rating rather than a COR special hazard. If you see a legacy COR, treat the product as corrosive until you prove otherwise, but do not teach COR as the current white-quadrant code.

704 is not a transport placard

A tank farm can display a 704 diamond and, if the tank is a transport tank or the facility offers hazardous materials, DOT-style information may also be present. Rules of engagement:

• 704 = fixed-facility severity communication for responders walking up to a building or tank dike.
• DOT placards / UN-NA numbers = transport classification and identification.
• A red 4 in the 704 flammability quadrant means the material is rated extremely flammable under NFPA 704. It does not mean UN0004, Class 4, Packing Group IV, or Radioactive Yellow-IV. There is no Packing Group IV, and 704 ratings stop at 4.

Exam trap: 704 "4" is not a UN number

If a stem shows a wall diamond with blue 3, red 4, yellow 2, and a slashed W, the correct read is: serious health hazard, extreme flammability, moderate instability, water-reactive special hazard. The wrong read is: look up UN number 4, or 342, or 3420, by concatenating the digits. Never concatenate 704 ratings into a UN/NA number. Never use a 704 rating as a class number in the lower corner of a DOT diamond.

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Putting UN/NA and 704 on the same incident

Highway cargo tank in a ditch: hunt placards, orange panel, shipping papers. The 704 diamond on the nearby warehouse is about that warehouse, not the cargo tank, unless the tank is a facility tank that also happens to be marked 704.

Warehouse fire with a 704 on the door and drums inside: the 704 is your immediate severity snapshot; the drum labels, UN marks, and SDS identify products. If drums were delivered this morning, the 704 on the building may lag the inventory. Confirmation still belongs to monitoring and to documents — which is the next section.

Yellow-page lookup works for commercial UN/NA numbers. It is the wrong first reflex for a 704 wall sign and, as of the 2024 ERG, the wrong first reflex for chemical warfare agents that no longer sit in the yellow section as ordinary UN listings.