What you should do

“The good news is, if you make it through the blast and shock wave, you are now in a survivable situation,” Schlegelmilch says. If you’re close to the blast you need to get to a shelter. “At best, you have 15 to 20 minutes before the fallout starts to come back down,” he says. “If you’re further out, you could have more time but the reality is, you aren’t going to have time to evaluate the situation. If you see a nuclear flash, the first thing to do is get behind a barrier in case the shock wave comes. Then get to the inner part of a building. Ideally you’d be protected by thick concrete, underground if possible. Glass and most metals won’t provide you much protection.” (See right.) If the explosion was ground-based, you can also protect yourself by getting above the blast, usually higher than the ninth floor of a building. Be sure to stay near the centre of the building in a room with substantial walls. (See below.)

If you somehow get stuck outside and think fallout is falling around you, cover your nose and mouth with a rag and close your eyes. Get to shelter immediately, where you should remove your outer layer of clothing, including that rag you just held over your face, and double seal it in plastic bags. Take a shower as soon was possible.

The danger of fallout is relatively short-lived. The Department of Homeland Security says that radioactivity is reduced by 90 per cent after seven hours. Two days later, only a single per cent of the original radiation remains. Still, maybe stay in that shelter for one more day. Just to be extra safe.