Nuclear threat
A nuclear weapons attack would involve multiple explosions causing massive damage, including fires and radiation exposure that can lead to cancer, organ failure, or death. Even a single weapon could instantly kill tens of thousands in a large city, though this depends on the size and location of the bomb.
The most immediate concern after a nuclear strike would be an explosion, followed by a period of sandlike radioactive particles (nuclear fallout) contaminating the ground and air. The most effective way to protect yourself during this time is to find a well-built, multistory building with few windows and a basement. Underground parking garages and subway stations are also good shelter options. A basement is preferable because the shock wave created by a nuclear explosion will shatter windows above it, allowing radiation to enter.
When a nuclear device explodes, it creates an orange fireball and bright flash of light that can cause temporary blindness up to 10 km away. You can prepare for this by putting on protective eyewear, covering your head with a hat, and hunkering down under a blanket or sleeping bag.
If a nuclear device was aimed at the US, residents would have about 30 minutes to find shelter before the blast and subsequent fallout hit their cities, depending on the weapons’ yields. With submarine-launched missiles, this time could be less than three minutes.
If a warning was issued by the government before the nuclear strike, people in affected areas would expect to hear sirens on TV and radio and on cellphones. However, if the warning is mistakenly sent as an erroneous push alert on cellphones, like in Hawaii in 2018, or is delayed, it might be too late for people to take action.