Four core concepts behind every meter reading, radiation calculation, plume decision, and monitoring strategy. Generate unlimited practice problems — each visit produces something new.
Your multi-gas meter reports flammable vapors as %LEL — percent of the Lower Explosive Limit. The LEL is the minimum vapor concentration that can ignite if a spark occurs. But %LEL is not a safety percentage. It is a concentration measurement.
The insight most responders miss: many chemicals reach dangerous toxic concentrations at a small fraction of their LEL. At 15% LEL for hydrogen sulfide, you are already at 6,000 ppm — that is 600× the IDLH. The explosion hazard has not started. The health hazard started long ago.
Readings at multiple distances also tell you a story. A rising number as you approach maps the hazard gradient and gives you real-time data to set zone boundaries.
In the Chemical Identification section, search any chemical from these practice problems by name or UN number. The tool immediately displays the TWA and IDLH values — the exact thresholds that determine whether a given %LEL reading is already a health emergency. Look up each chemical you generate here and ask: at what %LEL does the reading cross the IDLH? The answer will be much lower than most responders expect.
The Advanced Tool includes a dedicated LEL to PPM Converter — enter any %LEL reading for any chemical and get the actual ppm concentration instantly, with a live comparison against the IDLH and TWA. It also flags when you have already crossed a toxic threshold at a reading most teams would consider routine. If you want to move from understanding the concept to running live calculations, that is the tool for it.
Radiation intensity does not decrease in a straight line as you back away from a source. It follows the Inverse Square Law: double your distance, and intensity drops to one quarter — not one half.
The formula: I₁ × D₁² = I₂ × D₂², where I is intensity (dose rate in mR/hr) and D is distance. This is the mathematical foundation behind the three protective actions: Time, Distance, Shielding.
Backing up a few feet from a high reading is not timid — it is a tactically significant action that can drop your dose rate dramatically.
The Chemical Identification section displays the PPE level and ERG isolation distances for radioactive materials. Use it to look up the radiological entries and review what standoff distances the ERG recommends — these distances are derived from the same distance-intensity relationship this concept teaches. Understanding why those numbers land where they do starts with understanding the Inverse Square Law.
The Advanced Tool includes a full Radiation: Inverse Square Law calculator with three operational modes: find safe distance given a dose limit, find the dose at a new distance, or calculate source strength from a field reading. It is the same math as these practice problems — run live with your actual field numbers. If you want to move from pencil-and-paper to a field-ready calculator that gives command-ready answers in seconds, that is where you go next.
When gas or vapor releases into open air it begins mixing with the atmosphere. As the cloud expands outward, concentration generally decreases with distance from the source — but the decrease is not uniform. Wind, temperature, terrain, and vapor density all shape the plume.
Meter readings at multiple distances are not just individual data points. They are a concentration map. A reading still above IDLH at 50 feet means the Hot Zone extends further than expected. A reading that drops below the TWA at 75 feet tells you exactly where the Cold Zone boundary belongs.
Meters don't just detect hazards — they define zones.
In the Chemical Identification section, look up any chemical from these scenarios. The tool displays the IDLH, TWA, and STEL — the exact boundary values used to define Hot, Warm, and Cold zones — plus ERG isolation distances and a live plume direction slider that shows how wind direction shifts the protection zone. Use it alongside these problems to understand why zone boundaries land where the math says they do.
The Advanced Tool adds GEBMO Behavioral Modeling — select the type of container stress (mechanical, thermal, or chemical) and the tool explains how the release will behave based on the chemical's physical properties. Paired with the Chemical ID data, it tells you not just what the concentration is at a given distance, but how the release is likely to evolve. That is the next step beyond reading a static gradient.
Not all vapors behave the same way. Vapor Density (VD) compares the weight of a vapor to air. Air = 1. VD greater than 1 means the vapor is heavier than air — it sinks. Less than 1 means it rises.
This is not just a data point — it is a tactical decision driver. Chlorine has a VD of 2.5. That means it is pooling in floor drains, crawl spaces, and ditches — not drifting at breathing height in open air. Your meters belong at ground level. If those drains connect to a basement a block away, the hazard travels there too.
VD determines where to put the probe. Period.
Every chemical in the Chemical Identification section displays a VD badge — SINKS or RISES — the moment you select it. Search any chemical from these practice problems and verify its behavior instantly. Then explore others: look up chemicals you commonly encounter on your response district and build the mental library of what goes down and what goes up before you need it on scene.
The Advanced Tool pairs VD data with GEBMO Behavioral Modeling and Containment Compatibility — so you can see not just where the vapor goes, but how the container failure mode affects the release pattern, and what materials are safe to use for mitigation based on the chemical type. VD is one piece of the picture; the Advanced Tool gives you the complete behavioral profile in one place.