Every UN page on this site shows an ERG guide number. This is what that number means — how the guidebook is built, how responders use it, and how to go from a placard on a truck to a page of response actions in under a minute.
The Emergency Response Guidebook (ERG) is the single most-used reference at a hazardous materials incident. It is published by the U.S. Department of Transportation's Pipeline and Hazardous Materials Safety Administration (PHMSA), jointly with Transport Canada and Mexico's SICT, and it is distributed free to fire, police, and EMS agencies across North America. A copy sits in the cab of most fire apparatus on the continent.
Its job is narrow and important: get a first responder through the first 30 minutes of an incident safely. It is not a chemistry textbook and not a regulation. It is a decision aid — look up the material, get a guide number, follow the actions, set a perimeter, and hand off to a hazmat team with the right information already gathered.
The current edition is ERG 2024. A new edition is released every four years. This page explains how the book is structured and how to use it; the actual guide number for any given material is on its UN page — for example UN1203 Gasoline (Guide 128) or UN1017 Chlorine (Guide 124).
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How It's Organized
The color-coded pages
The ERG is organized by the color of its page borders. You can flip straight to a section by its color without reading a table of contents. There are six functional sections:
White pages — front
Instructions for using the book, examples of shipping papers, and the identification charts. The placard and label ID chart lets you match a diamond you can see on a vehicle to a guide number. There are also rail car, road trailer, and pipeline identification charts. This is where you start when you do not have a UN number.
Yellow pages — index by number
Every listed material in ascending UN/ID number order. Three columns: ID number, guide number, material name. If you have the four-digit number from a placard or shipping paper, this is the fastest path. A highlighted entry means the material is a toxic inhalation hazard — go to the green pages too.
Blue pages — index by name
The same materials, sorted alphabetically by name instead of by number. Use this when a witness or shipping document gives you a chemical name but no UN number. Same three columns, same toxic-inhalation highlighting convention as the yellow pages.
Orange pages — the guides
The heart of the book. Around 60 numbered guides, 111 through 174, each a two-page spread. A single guide covers a whole family of materials that share similar hazards. Every guide has the same three-part structure: Potential Hazards, Public Safety, Emergency Response. This is where the actual instructions live.
Green pages — evacuation distances
Initial isolation and protective action distances for toxic-by-inhalation materials. Table 1 has small-spill and large-spill distances for day and night. Table 2 lists water-reactive materials and the toxic gases they release. Table 3 gives detailed distances by container type for six common materials. UNLookup covers this in depth on the evacuation distances page.
White pages — back
Reference material that supports the guides: protective clothing types, fire and spill control notes, an explanation of BLEVE (boiling liquid expanding vapor explosion), criminal and terrorist incident guidance with safe standoff distances, the glossary, and emergency contact numbers.
1-4Workflow
The Lookup
Placard to response actions in four steps
The workflow every hazmat awareness course teaches
The ERG is built around a single repeatable lookup. Whether you arrive at a highway crash or a warehouse leak, the path is the same:
Step 1 — Identify the material
From a safe distance, with binoculars if needed, read the four-digit UN/ID number from the placard or orange panel, or get it from the shipping papers. No number visible? Use the placard and rail car identification charts in the front white pages to narrow it down by shape and color.
Step 2 — Find the guide number
Have a number? Look it up in the yellow pages. Have only a name? Look it up in the blue pages. Either way you land on a three-digit guide number. If the entry is highlighted, flag it — that material is a toxic inhalation hazard.
Step 3 — Read the orange guide
Turn to that numbered guide in the orange pages. Read Potential Hazards first to understand what you are dealing with, then Public Safety for isolation distances and protective equipment, then Emergency Response for fire, spill, and first-aid actions.
Step 4 — Check the green pages if highlighted
If the material was highlighted in step 2, go to the green pages and pull the initial isolation and protective action distances. Set your perimeter from those numbers, then hand off to the responding hazmat team with the guide number, the distances, and the wind direction already noted.
3Sections
Anatomy of an Orange Guide
What's on every guide page
Potential Hazards · Public Safety · Emergency Response
Every one of the orange guides follows the same three-part layout, so a responder who has used one guide can use any of them.
Potential Hazards
Lists fire/explosion hazards and health hazards. Critically, this section tells you which hazard is worse for that material — for some, fire is the dominant threat; for others, the health hazard is. That ordering shapes every decision that follows.
Public Safety
The immediate isolation distance, recommended protective clothing, and general evacuation guidance. For a spill, it gives an isolate-and-deny-entry distance in all directions. For materials with a green-page entry, it points you there for precise downwind distances.
Emergency Response
Split into Fire, Spill or Leak, and First Aid. Fire covers small-fire and large-fire tactics plus tank-fire and BLEVE warnings. Spill or Leak covers containment and what not to do (for example, never apply water to certain water-reactive materials). First Aid covers exposure response until EMS takes over.
111+Guide #s
Guide Numbers 111–174
Grouped by hazard, not by material
Why two unrelated chemicals can share one guide
Guide numbers are not assigned per material — they are assigned per hazard family. Materials that behave the same way in an emergency share a guide, even if their chemistry is unrelated. That is why one guide can cover dozens of UN numbers.
The numbers run roughly 111 to 174 and are loosely clustered by hazard type. A few real examples from materials on this site:
Loosely: the 110s and 120s cover gases and flammable liquids, the 130s flammable and combustible liquids and solids, the 140s oxidizers and peroxides, the 150s toxic and infectious substances, the 160s radioactive materials, and the 170s metals and miscellaneous hazards. The grouping is approximate — always confirm the exact guide from the index, never from the range.
Reading the Number
What the "P" suffix means
Some guide numbers carry a letter P — for example 116P, 128P, or 130P. The P stands for polymerization.
Polymerization is a chemical reaction in which small molecules link into long chains. For some materials this reaction can run away — it generates its own heat, which speeds the reaction, which generates more heat. In a sealed container, runaway polymerization builds pressure fast and can rupture the container violently, even without an external fire.
When a guide number ends in P, responders treat the heat-and-rupture risk as elevated from the start. Heat, contamination, or loss of the chemical inhibitor that normally keeps the material stable can all trigger it. A P-suffixed guide is a signal to think hard about container cooling and about how long that container has been exposed.
The Highlight
Why some index entries are highlighted
Scan the yellow or blue index pages and you will see that some entries are highlighted while most are not. The highlight is not decoration. It means the material is a toxic inhalation hazard (TIH) — a gas, or a material that releases a gas, dangerous to breathe even at low concentration.
For a highlighted material, the orange guide alone is not enough. You must also turn to the green pages and pull the initial isolation distance and the day or night protective action distance. The orange guide gives you a generic isolation distance; the green pages give you the material-specific, time-of-day-specific evacuation zone.
UNLookup carries the same green-page data. On a TIH material's UN page, an evacuation distances card appears with the isolation and protective action distances built right in. The full explanation of how those numbers work is on the evacuation distances page.
Editions
ERG 2024 and the four-year cycle
The ERG is revised every four years. The lineage runs ERG 2012, ERG 2016, ERG 2020, and the current ERG 2024. The next edition is ERG 2028.
Each revision can change guide assignments, adjust evacuation distances as toxicology data improves, and add UN numbers for newly regulated materials. It is not safe to assume a guide number from a 2016 book still holds in 2024. Responders are trained to confirm they are working from the current edition, and dispatch centers replace the books on the four-year cycle.
UNLookup's data reflects ERG 2024. When ERG 2028 is published, the guide numbers and evacuation distances on this site will be refreshed against it.
On UNLookup
Finding the guide number here
Every UN page on this site shows the assigned ERG guide number alongside the hazard class and packing group. It is, in effect, a digital version of the yellow-page lookup — type a UN number or a chemical name into the search and you get the guide number instantly.
UNLookup is a planning and reference aid. During an actual incident, responders should work from the physical ERG in the apparatus, the official ERG mobile app, or dispatch — not from a website that may be offline when it is needed most.
Get a Copy
Where to get the ERG
Free PDF. PHMSA publishes the complete ERG 2024 as a free download at phmsa.dot.gov. There is no cost and no registration.
Official mobile app. PHMSA also publishes a free ERG mobile app for iOS and Android with the full guide content, searchable by UN number or name. Useful as a backup, though a charged phone is not a guarantee at a scene.
Printed copies. Emergency response agencies receive printed copies through state emergency management channels. The print edition is the primary tool — it works with no battery, no signal, and gloved hands.
FAQ
Common questions
What is the Emergency Response Guidebook (ERG)?
A field reference published by PHMSA, with Transport Canada and Mexico's SICT, to help first responders through the first 30 minutes of a hazmat incident. Responders look up the UN/ID number from a placard or shipping paper, find the assigned guide number, and follow standardized response actions. The current edition is ERG 2024.
What do the colored pages in the ERG mean?
White pages (front) cover how to use the book plus identification charts. Yellow pages index materials by UN/ID number. Blue pages index the same materials by name. Orange pages hold the numbered response guides (111-174). Green pages hold the evacuation distance tables for toxic-inhalation materials. White pages (back) cover protective clothing, fire and spill control, and the glossary.
How do I find the right ERG guide for a material?
If you have the four-digit UN/ID number, look it up in the yellow pages. If you only have the name, use the blue pages. Both give you a three-digit guide number; turn to that guide in the orange pages. If the index entry is highlighted, the material is a toxic inhalation hazard and you must also consult the green pages for evacuation distances.
What does the "P" after a guide number mean?
The P indicates the material can undergo violent polymerization — a runaway, self-heating reaction — if exposed to heat or contamination. Polymerization can build pressure and rupture a container even without an external fire. A P-suffixed guide tells responders to treat the heat and container-rupture risk as elevated.
How often is the ERG updated?
Every four years. The current edition is ERG 2024, following 2020, 2016, and 2012; the next is ERG 2028. Each revision can change guide assignments and evacuation distances, so responders should always work from the most recent edition rather than an older copy.
UNLookup is a reference utility. The information above summarizes how the PHMSA Emergency Response Guidebook 2024 is organized and used; it is not a substitute for the guidebook itself, for hazmat responder training, or for the judgment of qualified emergency personnel. During an actual incident, work from the current physical ERG, the official PHMSA ERG app, or dispatch. Guide numbers and evacuation distances change between editions — always confirm against the current edition.