Fallout is a complex mixture of fission products, or radioisotopes, created by splitting atoms. Many of the fission products decay rapidly and emit gamma radiation, an invisible yet highly energetic form of light. Exposure to too much of this radiation in a short time can damage the body's cells and its ability to fix itself — a condition called acute radiation sickness.
"It also affects the immune system and your ability to fight infections," Buddemeier said.
Only very dense and thick materials, like many feet of dirt or inches of lead, can reliably stop the fallout.
"The fireball from a 10-kiloton explosion is so hot, it actually shoots up into the atmosphere at over 100 miles per hour," Buddemeier said. "These fission products mix in with the dirt and debris that's drawn up into the atmosphere from the fireball."
Trapped in sand, dirt, cement, metal, and anything else in the immediate blast area, the gamma-shooting fission products can fly more than five miles into the air. The larger pieces drop back down, while lighter particles can be carried by the wind before raining over distant areas.
"Close into the [blast] site, they may be a bit larger than golf-ball-size, but really what we're talking about are things like salt- or sand-size particles," Buddemeier said. "It's the penetrating gamma radiation coming off of those particles that's the hazard."
That brings us back to why a car would be a terrible place to take shelter.
"Modern vehicles are made of glass and very light metals, and they offer almost no protection," he says. "You're just going to sit on a road someplace" and be exposed.
Buddemeier said he asked people what their knee-jerk response to a nuclear blast might be, and it wasn't comforting.
"There was actually a lot of folks who had this notion — and it may be a Hollywood notion — of 'Oh, jump in the car and try to skedaddle out of town if you see a mushroom cloud,'" he said.