4.4 Estimating the Endangered Area

4.4 Estimating the Endangered Area

Quick Answer: NFPA 470 JPR 11.2.5 is estimate the endangered area. Initial isolation is a radius in all directions. Protective action is the downwind distance where you evacuate or shelter in place. Use orange isolation for the guide family. Use green Table 1 (and Table 3 when Table 1 sends you there) only for green-highlighted TIH or water-reactive materials when product is spilling. Small spill ≈ a single package up to about 55 U.S. gallons (208 L) or a small leak from a bulk container; large is more than that. Night protective-action distances are typically longer. A 1-ton chlorine leak is not a 55-gallon solvent drum. The ERG is the first-due number. Technicians refine it; they do not tattoo it on the map as the final product.

First-arriving engines open the ERG because that is the book written for the initial phase of a transportation incident. Hazardous materials technicians are the people who then have to say whether that number is still honest once you know it is a ton container, a liquid valve, a valley, and an inversion. JPR 11.2.5 is that refinement. If your only product is "the green page says 0.9 miles," you have not finished the technician job.

Initial isolation versus protective action

Orange guides give public-safety isolation (and fire-related evacuation) for a family of materials. White how-to pages still teach starting isolation on the order of 75 feet for solids, 150 feet for liquids, and 330 feet for gases (except explosives), then the numbered orange guide may raise those numbers.

Green Table 1 is a different tool: initial isolation plus downwind protective-action distances for highlighted toxic-inhalation-hazard (TIH / PIH) materials and specified water-reactives, split small versus large and day versus night. Green Table 3 further splits large spills of six common TIH gases by container type and wind speed. Those six are anhydrous ammonia (UN 1005), chlorine (UN 1017), ethylene oxide (UN 1040), hydrogen chloride (UN 1050 / 2186), hydrogen fluoride (UN 1052), and sulfur dioxide (UN 1079).

Trap from Chapter 3, applied here: green highlight plus a spill is the green-page trigger. Fire without a spill stays on orange isolation. Mixing orange fire distances with green PADs, or using Table 3 as a BLEVE table, produces the wrong endangered area.

Small versus large, day versus night

ERG 2024 (and the PHMSA technical basis document for the 2024 tables):

• Small spill: a single small package such as a drum of about 208 L (55 U.S. gallons), a small cylinder, or a small leak from a large package.
• Large spill: more than that — a spill from a large package, multiple small packages, or a bulk release. A chlorine ton container (~2,000 lb) is not a small spill.
• Day: after sunrise and before sunset. Unstable air mixes and dilutes the plume.
• Night: sunset to sunrise. More stable atmospheres and inversions mix less, so protective-action distances are typically longer for the same spill.

Table 1 distances are 90th-percentile modeled values for transportation releases, not a guarantee and not a precision instrument. They assume a statistical release, not your hole size.

ERG 2024 chlorine (UN 1017) — numbers technicians actually use:

• Table 1 small spill: isolate 200 ft in all directions; protect downwind 0.2 mi (day) / 0.9 mi (night).
• Table 1 large spill: refer to Table 3.
• Table 3 large spill, single ton cylinder / multiple small cylinders: isolate 500 ft; downwind PADs then depend on low / moderate / high wind and day / night (on the order of 0.3–0.9 mi by day and 0.4–1.5 mi by night for that container row — read the printed Table 3 rather than memorizing every cell).
• Table 3 highway tank truck: isolate 2,000 ft; day PADs about 1.6–3.5 mi by wind; night about 2.4–4.0 mi.
• Table 3 rail tank car: isolate 3,000 ft; day about 3.2–6.0 mi; night 4.1–7.0+ mi.

Container size is not a rounding error. That is why Table 3 exists.

Terrain, wind, vapor density, and model limits

The ERG plume is a simplified downwind box. Real land is not.

• Wind stretches and aims the protective-action zone. Low wind often lengthens toxic PADs for these gases (see Table 3 columns). High wind mixes faster but can still carry a dense cloud along a street canyon.
• Vapor density (Chapter 3): chlorine ~2.5 and propane ~1.5 follow low ground. A Table 3 distance drawn as a perfect downwind square under-protects the downslope neighborhood that is not "downwind" on the compass. Methane ~0.6 does the opposite and threatens overhead and peak spaces.
• Terrain and built environment: valleys, cuts, underpasses, and underground parking channel dense gases. High-rises create recirculation. A river valley at night is a chlorine special.
• Release rate beats inventory when the hole is small: a vapor valve on a ton container is not the same endangered area as a liquid valve on the same container. GEBMO "how" feeds 11.2.5.
• Limits: ERG distances are initial, transportation-oriented, statistical, and not IDLH isopleths, not PPE selection, and not a substitute for air monitoring. They do not know your inversion, your hillside, or that the "chlorine" tank is half empty. Facility releases, indoor releases, and long-duration leaks outgrow the green page.

The technician product is a refined estimate

First-due: open ERG, isolate, start protective actions. Technician:

1. Confirm identity (Chapter 3) and container (4.1) — Table 3 row depends on it.
2. Confirm spill versus fire-only, small versus large, day versus night, wind speed.
3. Adjust for liquid versus vapor leak, remaining inventory, and observed cloud.
4. Put meters on the expected path (low for dense gases). Expand the zone if the reading is still climbing at the book distance; you may contract slowly only with data and IC approval — never shrink because "the page feels big."
5. Brief what is ERG, what is measured, and what is still assumed. That briefing is JPR 11.2.5.

Do not treat ERG green distances as the final technician product. They are the authorized starting geometry.

Exam scene: 1-ton chlorine versus 55-gallon solvent drum

Scene A — leaking chlorine ton container (DOT 106A500X), night, low wind, residential downslope. Yellow index 1017, Guide 124, green highlight, product releasing. This is a large TIH spill from a single ton cylinder row on Table 3, not a 55-gallon Table 1 small-spill row and not orange-only isolation. Night and low wind push you toward the longer PAD columns. Chlorine vapor density ~2.5 means you also protect the creek bottom that is not on the wind arrow. Then monitor. If the cloud is still at the book line, the endangered area is bigger than Table 3, not smaller because you are a technician.

Scene B — 55-gallon tight-head drum of a non-highlighted flammable solvent on a loading dock, day, no fire, slow bung leak. This is a small liquid package. If the yellow/blue entry is not green-highlighted, you do not open Table 1 or Table 3. Use the orange guide's public-safety isolation (starting liquid isolation on the order of 150 feet, then the specific guide). Vapor may still be heavier than air and find a basement — that is property-driven adjustment, not a 7-mile chlorine PAD. Treating the drum like a rail car of chlorine evacuates the wrong county and abandons the dock. Treating the ton container like the drum under-protects the town.

Same JPR, two endangered areas. Container size, TIH status, day/night, and monitoring are what turn an ERG lookup into a technician estimate.