The green-bordered section of the Emergency Response Guidebook (ERG) gives first responders specific evacuation and isolation distances for materials that pose a toxic inhalation hazard. While the orange section covers general response procedures, the green section provides the actual numbers: how far to initially isolate a spill and how far downwind to protect the public. These distances are organized across three tables and broken down by spill size and time of day.
What the Green Section Covers
The green section focuses exclusively on materials that release toxic vapors people could breathe in. These are classified as Toxic Inhalation Hazard (TIH) materials, sometimes called Poison Inhalation Hazard (PIH) materials. You’ll know to consult the green section when a material’s entry in the yellow or blue pages is highlighted, indicating it requires the additional distance information found here.
The section contains three tables, each serving a different purpose. Table 1 is the core of the green section and the one responders use most. Tables 2 and 3 provide supporting information about specific gases.
Table 1: Initial Isolation and Protective Action Distances
Table 1 is where responders look up a specific TIH material by its UN identification number or name and find two critical pieces of information. The first is the initial isolation distance, which defines a circular zone around the spill where everyone should be moved away immediately. The second is the protective action distance, which tells responders how far downwind they need to either evacuate people or have them shelter in place.
Each material’s entry is further divided into four columns of protective action distances based on two variables: spill size (small or large) and time of day (day or night). A small spill generally involves a single small package, small cylinder, or a small leak from a large container. A large spill involves a large container or multiple small containers. This distinction matters because a bigger release means more toxic vapor spreading over a greater area.
The day and night distinction exists because of how the atmosphere behaves. During daytime, the sun heats the ground and creates vertical air mixing that helps break up and disperse toxic vapor clouds. At night, the atmosphere becomes more stable, and vapor plumes hold together and travel farther downwind without dispersing. The ERG defines “day” as sunrise to sunset and “night” as sunset to sunrise.
In the 2024 edition, chemical warfare agents were removed from Table 1 and moved into the separate section on criminal or terrorist use of chemical, biological, and radiological agents. Distances for existing materials were also revised.
When Distances May Need to Increase
The distances in Table 1 are starting points, not guarantees. Several real-world conditions can push the actual danger zone farther than the table suggests. If a vapor plume is channeled through a valley or between tall buildings, it disperses more slowly and can travel greater distances. Snow cover and strong temperature inversions, where a layer of warm air traps cooler air near the ground, also reduce atmospheric mixing and keep the plume concentrated longer.
Spills that happen near sunset deserve extra caution. The atmosphere is transitioning from daytime mixing to nighttime stability, so using the nighttime protective action distances is more appropriate even though it’s technically still daylight. For materials with a listed protective action distance of 11.0 kilometers (7.0 miles) or more, the actual distance could be significantly larger under poor atmospheric conditions.
Table 2: Water-Reactive Materials
Table 2 lists materials that produce large amounts of toxic gas when they come into contact with water. For each material, the table identifies which specific toxic gas is generated. This is important for two reasons: it tells responders what kind of vapor hazard they’re dealing with if the spill reaches a body of water or gets rained on, and it signals that water should not be used as an extinguishing agent.
The gases listed in Table 2 are provided for informational purposes only. The initial isolation and protective action distances in Table 1 already account for the toxic gases these materials produce when they contact water, so responders don’t need to look up separate distances for the generated gas. When a product in Table 1 includes the note “when spilled in water,” that’s the cue to check Table 2 for details on which gases form.
Table 3: Common TIH Gases
Table 3 provides isolation and protective action distances for the most commonly encountered toxic inhalation hazard gases. This table is useful when responders know the general type of gas involved but may not have a specific UN number to look up in Table 1. The 2024 edition includes revised distances for the gases in this table.
How Responders Use the Green Section in Practice
The green section is designed for the earliest moments of a hazmat response, before specialized monitoring equipment arrives. A responder identifies the material using its UN number, placard, or shipping papers, then turns to Table 1 to get immediate distance recommendations. They establish the initial isolation zone as a circle around the spill, then create a protective action zone extending downwind. The protective action zone is not a circle. It stretches in the downwind direction based on the distance listed in the table, forming an elongated area that accounts for vapor drift.
Within the protective action zone, the two options are evacuation or shelter-in-place, depending on conditions. The distances give incident commanders a concrete starting point for these decisions rather than relying on guesswork during a high-pressure situation. As more information becomes available, such as actual wind speed, container integrity, or the exact volume spilled, responders can adjust the zones accordingly.