How far from a nuclear blast do you have to be to survive?

A nuclear detonation produces four effects with different reaches: thermal radiation, blast overpressure, prompt radiation and fallout. For a one-megaton airburst the fireball itself spans roughly a kilometre, overpressure sufficient to collapse ordinary buildings extends to around 5-7 kilometres, and thermal radiation intense enough to cause third-degree burns on exposed skin reaches roughly 10-11 kilometres in clear conditions. Prompt radiation is significant only close in, where blast has already been lethal. Fallout is the effect that reaches furthest and lasts longest, and its behaviour is dominated by wind and by whether the detonation touched the ground. Radioactivity from fallout decays very rapidly at first — roughly a factor of ten for every sevenfold increase in time — which is why civil defence guidance everywhere is to get inside a substantial building, stay in the most shielded part of it, and stay there for at least 24 to 48 hours.

What happens, in order

What changes the figure

• Distances scale roughly with the cube root of yield, so ten times the yield is about twice the radius
• Being inside any substantial structure removes a large fraction of both thermal and fallout dose
• Fallout decays fast; the first 24-48 hours account for most of the avoidable exposure

Effect radii compiled from published civil defence effect tables for a one-megaton airburst in clear conditions; real distances vary with yield, burst height and weather.

Physiology, not instruction. Every figure here is a population range under particular conditions, and individual variation is large — temperature, exertion, fitness and health move all of them substantially. Nothing on this page is medical advice.

Situations

• Can you survive a plane crash? Usually — across all US airline accidents studied, more than 95 per cent of occupants survived.
• How long do you have to escape a sinking car? Usually 30 seconds to a couple of minutes of floating — and the window is the way out, not the door.
• Can quicksand pull you under? No. Quicksand is roughly twice as dense as a human body, so you float — you sink to about waist depth and stop.
• How long can you survive in a hot car? Under an hour on a warm day for a child — the interior gains about 20 °C in the first ten minutes.
• How long does it take to drown? Under a minute to lose consciousness — and it is almost silent, which is why so many drownings happen in sight of others.
• What happens if a lift cable snaps? Essentially nothing — lifts hang from multiple ropes and have a speed-triggered brake that grips the rails.
• What height can you survive falling from? Survival is common below about 6 metres and rare above 20 — but people have survived falls from thousands.
• How likely are you to survive a shark attack? Very likely — of roughly 60 to 80 unprovoked bites worldwide each year, fewer than ten are fatal.
• What happens if you fall into lava? You would not sink — lava is about three times denser than a body — but you would not survive the heat either.
• What should you do if you are caught in a rip current? Rips pull you out, not under. Swim parallel to the beach until you are out of the flow, then come in.
• Could you survive inside a whale? You cannot be swallowed by almost any whale — their throats are too narrow — and a stomach offers no air.
• How long do you have to escape a burning house? Around three minutes from the alarm sounding in a modern furnished home.
• Can you survive a tsunami? Yes, by moving inland and uphill immediately — 30 centimetres of moving water can knock an adult off their feet.
• How long can you survive in a sealed container? Carbon dioxide becomes dangerous long before oxygen runs out — a few hours in a cubic metre of air.