In the docudrama Super Comet After the Impact, the impact is shown to have made the air within a few miles of it unbreathable, and humans need gas masks to breath in this area. How realistic is this?
I was told to come here after originally posting on the space stack.
That doesnt seem realistic since eventually all the dust from all the volcanic activity, as well as that caused by our doing, eventually makes its way back down to earth. Most dust particles end up being soaked up by earths cloud layer coming down in rain. I have to agree with Gimelist but volcanism can and has produced conditions you mentioned by expelling caustic and poisonous gases and for what I understand nuclear detonations and the radioactive dust they send into the air is far worse in the long run.
Disclaimer: I haven't watched the docudrama.
For a comet impact, I think the chemical effects (dumping poisonous gases into the atmosphere) would be completely overwhelmed by the thermal effects (lots and lots of kinetic energy converted to heat).
Those poisonous gases would surely be converted to plasma and/or blasted out in the shockwave from the impact. Any that were still solid at the point of ground impact would be splashed out in all directions, where they would quickly mix into the (suddenly superheated) atmosphere. Carbon monoxide, methane, and ammonia could burn once they COOLED enough to combine with atmospheric oxygen, but they'd already be diluted below toxic levels.
The heat from the impact would also cause all these products to rise into the stratosphere as a giant mushroom cloud, drawing in fresh air at its base.
So, no, I don't see how this could be realistic. I'm almost tempted to go look for the video to see just how bad it was... but not quite.
tl;dr while the docudrama is over-dramatised, yes, this is possible.
Simply put, we already have processes which do make air unbreathable in small or even large areas, from dust or poisonous gases.
Typically these are volcanic in origin, with fine dust from some volcanoes causing breathing difficulties across large areas, and from gases (often sulphur-based) burning lung surfaces or excluding oxygen.
Impact from a large body can definitely have outcomes in related areas - dust or gas output, and while the immediate impact has more of a destruction aspect, once that has passed, gas and dust can very much be an ongoing problem, possibly even a semi-permanent issue, if the impact opens up new volcanic activity along a fault.
Dust would not have to be chemically poisonous to render air unbreathable but a big impact would first of all release a blast of hot plasma that nothing nearby will survive with accompanying air blast. Dust would be mostly from Earth material and that will vary in chemistry according to what is locally present; the Chicxulub (dinosaur killer) impact hit where abundant calcium sulphate minerals were present and those were thought to have caused acid rain (as well as releasing a lot of CO2 that, along with massive wildfires, caused later global warming). I suspect those kinds of dusts at high levels would be poisonous but the heat alone from material thrown into the air and lower space was thought to cause spontaneous wildfires over large areas around the planet - making the dust superfluous for rendering air unbreathable.
