There is a limit, okay?

My anonymous UK friend who's entitled to have seen the following writes:

Dear Dr Who writers,

Say for the sake of argument I could suspend disbelief long enough to accept that 400 million cars could somehow spew out a sufficient volume of a toxic Sontaran gas such that within a day or so it could replace just less than 80% of the atmosphere. (If I can just repeat that for a second - eighty percent of the entire Earth's atmosphere.)

Please explain to me how the Doctor igniting 80% of the atmosphere could conceivably NOT kill most of the planet's population and destroy almost everything.

Yours sincerely,

Seriously That's Just Ridiculous, UK.