This question is actually a three part question.
In the doctor who episode The Poison Sky
When the doctor activates the device that burns all the gas, why did it only burn the gas in the atmosphere if the gas had higher concentrations closer to the ground?
Why when he burnt all the gas did the cars not keep producing more?
If the gas in the atmosphere was the only gas to burn, why did the other gas on the ground disappear (not only that but it went away quickly).
Seems to me that if this were to ever happen really, either the gas would ignite over the whole planet and kill everything (or at the very least burn all the oxygen and everyone would suffocate), or the gas would linger and everyone would still suffocate.