The atmosphere's main components are similar to Earth's; 71% nitrogen, and 23% oxygen. However, there is a high level of carbon dioxide, which constitutes 3% of the atmosphere. This is more than enough to be fatal to humans in a very short exposure. It also contains small amounts of argon, as detected by the spaceship Prometheus.
So I checked Carbon Dioxide Toxicity on Wikipedia. Here an excerpt (emphasis mine) :
Adaptation to increased levels of CO2 occurs in humans. Continuous inhalation of CO2 can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week.
However the LV-223 article on the alien anthology wikia sightly differ (emphasis mine) :
The atmosphere is composed of 71% of nitrogen, 3% carbon monoxide, 23% oxygen and traces of argon gas. It also contains small amounts of argon, as detected by the spaceship Prometheus. The air on LV-223 is mostly identical to Earth's atmosphere, but contains poisonous levels of carbon dioxide (CO2) making it impossible for humans to breathe outside. (Earth levels average about .0389% for comparison)
So I checked Carbon monoxide poisoning on Wikipedia
12,800 ppm (1.28%) Unconsciousness after 2–3 breaths. Death in less than three minutes.
I did not find any official source to say which one is right. While the plot suppose monoxide, I just want to know if they goof in the movie.