I have a planet that I want to have a greenish atmosphere. I've heard in a few places that chlorine gas has the potential to make the atmosphere greenish, but also to kill humans. So, the question is, what percent of Earth's atmosphere (in nitrogen) can I replace with chlorine gas in my atmosphere without seriously damaging humans? Humans would have to be able to inhabit this planet without dying because of the gas.

Note - A less important side question, what color would the atmosphere look like if I added the maximum amount?

  • $\begingroup$ "what percent of Earth's atmosphere (in hydrogen) can I replace" What do you mean, "in hydrogen"? $\endgroup$
    – Samuel
    Feb 17, 2016 at 23:24
  • $\begingroup$ Aaannnd, that now means what? You only want to replace the nitrogen portion of the atmosphere with chlorine? $\endgroup$
    – Samuel
    Feb 17, 2016 at 23:27
  • $\begingroup$ @Samuel Oops, I meant Nitrogen. In case it's still unclear, earth has along the lines of 78% N2, 21% O2, and 1% a lot of other things. So what I am saying is do something like change it to 60% N2, 18% Cl2, 21% O2, and 1% everything else. $\endgroup$ Feb 17, 2016 at 23:33
  • 3
    $\begingroup$ This seems like such a valid world building question, and yet the realist in me looks at your plan and thinks "stupid! stupid! stupid!" I'm really glad you creative types aren't in charge of the gas mixture I have to breath on a daily basis! The pollution is bad enough as is! =) $\endgroup$
    – Cort Ammon
    Feb 18, 2016 at 1:59
  • $\begingroup$ @CortAmmon I didn't really think about it until I posted the question. $\endgroup$ Feb 18, 2016 at 2:38

1 Answer 1


0.04% will kill you in about 30 minutes.

This paper on chlorine gas inhalation says:

Concentrations of about 400 ppm and beyond are generally fatal over 30 minutes, and at 1,000 ppm and above, fatality ensues within only a few minutes.

They cite safe workplace levels (6-hours of exposure) at 0.5 ppm (0.00005%).

It's unlikely you'll get the desired amount of green in the atmosphere using this method.

  • $\begingroup$ So the bottom line is that I can have 0.00005% of the total atmosphere. Hm, no wonder it's a trace gas. $\endgroup$ Feb 18, 2016 at 3:35

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