It’s said that when the Trinity device was first tested, a semi-serious bet was made between the physicists involved on whether it would ignite the Earth’s atmosphere. On what basis was this thought an even remote possibility? Were they talking about a chemical ignition, even though it takes a net input of energy for nitrogen and oxygen to unite? Or were they talking about a hypothetical nuclear reaction involving nitrogen or oxygen?
And more importantly, how dumb are physicists anyway? How do you collect your winnings if the atmosphere does ignite and kill everyone? 
I think they were going for a ‘scare the straights’ reaction, to get as much of a rise as possible out of all the non-physicists on base. It’s still working 63 years later.
The possibility they were looking at was a nuclear reaction. It was a legitimate question to ask, but it was also a very easy question to answer: The calculations were quite clear that it wouldn’t happen. And, being physicists, they trusted the calculations.