There would be no point in reaching for the respirator:
All the oxygen, nitrogen and other stuff in Earth's atmosphere has a whopping combined mass of 5 quadrillion tons, so a falling sky would mean that nearly 10 tons of molecules — roughly the heft of a school bus — would drop on every square meter of Earth's surface. Pancakes, everyone?
To keep things interesting, let's envision a less "crushing" scenario: What if the atmosphere suddenly disappeared — if an exceedingly rare quantum fluctuation caused all the atmospheric particles to unexpectedly jump to the other side of the galaxy, leaving Earth floating in a vacuum?
Vaclav Smil, distinguished professor in the faculty of environment at the University of Manitoba in Canada, said three things would kill us: oxygen deprivation, a severe drop in temperature, and exposure to a full dose of UV radiation from the sun, most of which the atmosphere currently blocks. … As all the birds and airborne insects plummeted to the ground around us like rocks, lacking an atmosphere to beat their wings against, and as the world fell eerily mute, lacking the air that normally carries sound, we would all die of oxygen deprivation in less than three minutes, Smil said.
(Photo by Flickr user NebraskaSC)