How to read the green pages of the Emergency Response Guidebook. IID, PAD, Table 1, Table 3, and the 30-minute rule — translated from federal-document English into plain English for responders, planners, and curious shippers.
When a hazmat incident involves a toxic-by-inhalation (TIH) material, the first responder's first question is: how far back do I need to be? The Emergency Response Guidebook (ERG) answers that question with two numbers per material: an Initial Isolation Distance (IID) — the radius in every direction — and a Protective Action Distance (PAD) — the downwind zone where people should shelter or evacuate.
The numbers come from PHMSA, in partnership with Transport Canada (SCT) and Mexico's SCT. They live in the ERG's green-bordered pages. Table 1 covers about 272 substances; Table 3 provides container-by-container detail for six materials where the answer changes drastically based on what's leaking. Both tables are calibrated for the first 30 minutes only — long enough for site-specific atmospheric modeling to take over.
A circle around the spill — clear in every direction
Measured in feet · The same number day or night · All people, all directions
The Initial Isolation Distance is the radius around the incident from which all unprotected people must be kept clear, in every direction. It is the smallest of the ERG's evacuation circles, but it is the only one that does not care about wind direction. People to the upwind side are not safe from a release: gas can shift direction, wind can swirl around buildings, and an unprotected bystander breathing a high concentration even briefly can be incapacitated.
IID is published in feet. Small-spill IIDs typically range from 100 ft to 200 ft for most TIH materials. Large-spill IIDs for the most dangerous materials can extend to 3,000 ft (over half a mile) for a rail tank car release of chlorine — and that is just the all-directions isolate zone, before any PAD is applied downwind.
What "isolate" means in practice
No bystanders within the radius. Move everyone — pedestrians, drivers, photographers, the curious — outside the circle.
No entry without PPE. Responders entering the radius need full respiratory and chemical protection rated for the material.
Approach from upwind. Even though the isolate circle covers all directions, response operations always set up upwind of the release.
Larger if fire is involved. IID is for inhalation hazard only. If fire is involved, the orange-page ERG guide for that material specifies a larger isolation radius — often 1 mile in all directions for tank cars.
PADProtective Action
Protective Action Distance
A downwind zone — shelter or evacuate
Measured in miles · Different by day vs night · Downwind only
The Protective Action Distance is the downwind zone where the toxic cloud is likely to reach hazardous concentrations. Unlike the IID circle, the PAD is shaped like a rectangle extending downwind from the spill. It assumes the wind is blowing in one general direction and the cloud is being carried that way. Inside the PAD zone, people should either shelter in place (close windows, shut off HVAC, stay indoors) or evacuate perpendicular to the wind direction — moving sideways to get out of the cloud's path is faster than running downwind ahead of it.
PAD is published as two numbers per material: a day value and a night value. Night PADs are always larger — sometimes 2 to 5 times larger. Why? Atmospheric stability. At night, the ground cools faster than the air above, creating a stable surface inversion that traps the toxic cloud near the ground and lets it travel much farther horizontally before diluting. Daytime sunlight drives convective mixing that lofts the cloud upward, diluting it faster.
For UN1017 Chlorine, a small-spill nighttime PAD is 0.9 miles. A large-spill night PAD from a rail tank car? 7.0+ miles. The "plus" in 7.0+ means the ERG ran out of confidence in its modeling at that distance; treat it as "everyone in a 7-mile downwind corridor needs protective action."
Day vs night — why the difference?
Daytime: sunlight heats the ground → rising thermals mix the gas cloud upward → faster dilution → shorter downwind reach.
Nighttime: ground cools fast → cool surface layer is stable → gas cloud hugs the ground and travels horizontally → far longer reach.
Transition periods: dawn and dusk inversions can be the worst — use the night value to be safe.
Spill Size
Small spill vs large spill — the 55-gallon threshold
Every Table 1 entry has two scenarios: a small spill and a large spill. The distinction is operational, not theoretical.
Small spill is a release from a package or container with a capacity of 200 liters (about 55 gallons) or less, or a leak from a larger container that is currently releasing at a low rate. Think: a 55-gallon hazmat drum that has tipped over, or a cylinder that is hissing through a damaged valve. The 55-gallon figure aligns with a standard hazmat drum size.
Large spill is a release from a container with capacity greater than 200 liters, or simultaneous releases from multiple small packages at the same scene. Think: a punctured highway tank truck, a derailed tank car, or a leaking storage vessel.
The numerical gap between the two is real: large-spill PADs are commonly 5 to 10 times larger than small-spill PADs. For UN1005 Anhydrous Ammonia, the small-spill IID is 100 ft and the small-spill night PAD is 0.1 mi. For a railcar release (Table 3), the IID jumps to 1,000 ft and the night PAD pushes past 2.5 miles.
If in doubt about which scenario applies, use the large-spill number. The cost of over-evacuating is hassle. The cost of under-evacuating is bodies.
T3Detailed Tables
ERG Table 3
Container-by-container distances for six materials
Used when container type and wind speed change the answer materially
Table 1 gives you a one-size-fits-most pair of distances for each TIH material. That works for the vast majority of incidents. But for six specific materials, the difference between a 100-pound cylinder release and a 90-ton rail tank car release is so vast that one set of numbers would be either useless or absurd. For these six, ERG publishes Table 3 — a detailed breakdown by container type and wind speed.
Each container row in Table 3 has three PAD values for day and three for night, corresponding to low wind (under 6 mph), moderate wind (6–12 mph), and high wind (over 12 mph). Counterintuitively, low wind is usually the worst case for PAD distance: the cloud doesn't disperse, it just creeps along the ground. High wind dilutes faster but the cloud travels farther horizontally before dilution finishes.
On a UN page like UN1017 Chlorine, the "View Table 3 detail" button reveals the full 4-container × 8-distance matrix. Use it when you know the container type and wind speed. If you don't know either, default to the worst-case row (rail tank car) and worst-case wind (low).
Water-Reactive Materials
When non-TIH materials still get evacuation distances
Roughly 88 entries in our IID/PAD dataset are materials that are not themselves toxic-by-inhalation, but become a TIH problem on contact with water. They are water-reactive: when spilled into water (or hit with rain, or sprayed by responders), they release a toxic gas.
The IID/PAD numbers shown for these materials are calibrated for the water-contact scenario. The released TIH gas — usually hydrogen chloride (HCl), hydrogen fluoride (HF), ammonia (NH3), or sulfur dioxide (SO2) — is what the distances are protecting against. If the material is in a dry environment with no water available, the IID/PAD does not directly apply; the orange-page ERG guide for the material covers other hazards (fire, corrosion, contact toxicity).
Examples on the site include UN1162 Dimethyldichlorosilane (releases HCl in water), UN1295 Trichlorosilane (releases HCl), and many chlorosilanes, acid chlorides, and metal hydrides. On their UN pages, a water-reactive callout appears at the top of the IID/PAD card naming the specific gas emitted.
Critical Caveat
These distances do not account for fire
If fire is involved, do not use these distances.
ERG Table 1 and Table 3 are calibrated only for the inhalation hazard from a toxic gas release. They do not account for:
Fire and thermal radiation
BLEVE (boiling liquid expanding vapor explosion) from pressurized containers
Container rupture and projectile hazards
Contact and corrosion hazards from direct exposure to the material
When fire is involved, switch to the orange-bordered ERG guide for the material. Those guides specify explicit fire and BLEVE evacuation distances — typically 1 mile in all directions for tank cars involved in fire. Many fire-scenario distances are also "all directions," not just downwind.
The 30-Minute Rule
These are first-30-minute numbers
The ERG green pages are explicitly designed for the first 30 minutes of an incident — long enough for a first responder to establish an initial perimeter, isolate the scene, and stand up the next layer of response. They are not intended for sustained operations or extended incident management.
After 30 minutes, the response should transition to:
Site-specific atmospheric dispersion modeling. Tools like ALOHA (Areal Locations of Hazardous Atmospheres) — also from NOAA/EPA — use real-time wind, terrain, and source-strength data to refine zones.
Professional hazmat teams with monitoring instruments, on-site weather data, and engineering controls.
Toxic exposure thresholds like AEGLs, ERPGs, or PACs — published per material and per exposure duration (10 min, 30 min, 1 hr, 4 hr, 8 hr) — that give a more precise picture of where the line is between "shelter in place" and "evacuate."
The 30-minute rule is one of the most-violated principles in hazmat response. ERG distances are not a perimeter setting; they are a starting point. Plan the handoff to the next layer of response from the moment you arrive.
On UNLookup
Where the numbers live
Every TIH material in our dataset has an IID/PAD card on its UN page. Try:
Or use the homepage search to look up any UN number. If the material is TIH or water-reactive, its IID/PAD card appears below the Shipping Exceptions card.
Where the Rules Come From
Regulatory anchors
PHMSA Emergency Response Guidebook 2024. The authoritative source. The green pages contain Table 1 (about 272 substances) and Table 3 (six materials with detailed breakdowns). The book is updated every four years; the next edition is ERG 2028. Free PDF from PHMSA (phmsa.dot.gov).
49 CFR §172.602. Federal regulation that requires emergency response information to accompany hazmat shipments. The ERG itself is not a federal regulation, but it is the de facto standard incorporated by reference into incident command and HAZWOPER training.
NOAA CAMEO Chemicals. The same IID/PAD dataset is available in the NOAA CAMEO Chemicals desktop application and online database (cameochemicals.noaa.gov). CAMEO is a free federal tool from NOAA, EPA, and DOT for emergency planners and first responders.
ALOHA. The follow-on modeling tool for post-30-minute incident response. Free from NOAA/EPA, used to refine evacuation zones based on real-time atmospheric conditions, source-term modeling, and terrain.
FAQ
Common questions
What's the difference between IID and PAD?
IID (Initial Isolation Distance) is the radius around an incident from which all unprotected people must be kept clear in every direction — measured in feet. PAD (Protective Action Distance) is the downwind distance over which protective actions like shelter-in-place or evacuation should be considered — measured in miles. IID is a circle; PAD is a rectangle extending downwind. Both come from PHMSA ERG Table 1 for toxic-by-inhalation materials.
Why are nighttime evacuation distances larger than daytime?
At night, the air near the ground cools faster than the air above, creating a temperature inversion. The stable surface layer resists vertical mixing, so a toxic gas cloud stays low and travels much farther horizontally before diluting. Daytime sunlight drives convection that mixes the cloud upward, diluting it faster. ERG night PAD values are typically 2 to 5 times larger than day values.
When do I use ERG Table 1 vs Table 3?
Table 1 covers most TIH materials with two preset scenarios — small spill (≤55 gal) and large spill (>55 gal) — each with day and night PADs. Table 3 provides finer container-by-container distances for six materials: anhydrous ammonia (UN1005), chlorine (UN1017), ethylene oxide (UN1040), anhydrous hydrogen chloride (UN1050), anhydrous hydrogen fluoride (UN1052), and sulfur dioxide (UN1079). Use Table 3 when you know the container type and current wind speed; otherwise default to Table 1.
How is the ≤55 gallon small spill threshold defined?
A small spill is a release from a package or container with capacity of 200 liters (about 55 gallons) or less, or a leak from a larger container that is currently releasing at a low rate. A large spill is from a container greater than 200 L or multiple small packages releasing simultaneously. The 55-gallon figure aligns with a standard hazmat drum. Large-spill PADs are commonly 5 to 10 times larger than small-spill PADs.
Do ERG evacuation distances account for fire or explosion?
No. Table 1 and Table 3 distances are calibrated only for the inhalation hazard from a toxic gas release. They do not cover fire, BLEVE, pressurized container rupture, or thermal radiation. If fire is involved, switch to the orange-page ERG guide for the material — those guides include explicit fire and BLEVE distances, often 1 mile or more in all directions for tank cars. The Table 1 distances are intended for the first 30 minutes only.
UNLookup is a reference utility. The evacuation distance information above summarizes the PHMSA Emergency Response Guidebook 2024 green pages and is not a substitute for the current edition of the ERG, on-scene atmospheric monitoring, or professional incident command judgment. The ERG is updated every four years; this page reflects the 2024 edition data and will be updated when ERG 2028 is released. For active incidents, always consult the actual ERG and qualified hazmat personnel.