When a disaster strikes, one of the most important variables to consider is distance. Understanding how far you are from the epicenter of the danger will allow you to make immediate decisions about next steps, whether that's to stay where you are or to evacuate immediately to a safer location. This is especially critical in incidents where hazardous materials may be involved — for example, an industrial accident or a terrorist attack. These events may spread dangerous chemical, biological, or radiological contamination that's invisible to the naked eye, and if you're too close, you could be exposed. The HAZMAT rule of thumb can help you quickly gauge if it's time to bug out.

Photo: Koalabaerchen / Wikimedia Commons (CC BY-SA 3.0)

In The First Responder's Field Guide to Hazmat & Terrorism Emergency Response, author Jill Levy explains how this rule of thumb can serve as a gauge of evacuation distance:

“If you're not sure of the appropriate distance, back out and err on the side of safety. Consider, too, the old “rule of thumb.” If you've backed out to the recommended distance but can see the incident beyond your outstretched thumb, you may still be too close.”

Photo: Flickr.com/DieselDemon | CC BY 2.0

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Although this applies to many HAZMAT incidents, it's not universal. Internet rumors have spread claiming that this rule applies to the mushroom cloud from a nuclear explosion, but that's false. Multiple experts have attested that the heat, blinding light, pressure blast, and fallout from an actual nuclear explosion can still be dangerous even if the cloud appears smaller than your outstretched thumb. If it's visible at all, you're in big trouble. Unlike dirty bombs, which use conventional explosives to scatter radioactive materials over a smaller radius, the danger radius of an A-bomb or H-bomb can reach for dozens of miles.

Despite rumors on social media, Vault Boy wasn't using the rule of thumb to gauge a nuclear threat.

Part of the alleged nuclear application of this rule traces back to a 2013 Reddit comment. The comment author implied that Vault Boy, the mascot for the popular Fallout video game series, was giving the thumbs-up gesture to check the size of a mushroom cloud. However, producer Brian Fargo quashed these rumors in a Twitter post, explaining that Vault Boy isn't using the thumbs-up test — he “simply has a positive attitude.”

Although the rule of thumb is not relevant to nuclear blasts, it can help you gauge your distance from more conventional HAZMAT threats. To maximize its effectiveness, use it in tandem with another memorable HAZMAT rule, the three U's:

• Upwind
• Upstream
• Uphill

If you can cover your view of an incident with your thumb, and wind, water, and gravity aren't carrying contaminants towards you, it's probably safe enough to take a moment to consider what to do next. In any other case, it might be wise to start moving away ASAP.

Written by Patrick McCarthy