How can a hologram tell if there are poisonous gases in the atmosphere?

Question: How can a hologram tell if there are poisonous gases in the atmosphere?
- Keywords:
  atmosphere,
  gas,
  hologram,
  poisonous
Asked by Tia to Dervil on 11 Nov 2016. This question was also asked by 395ghtg42.
- Dervil Cody answered on 11 Nov 2016:
  
  The holograms I make are a specific colour, usually I make green coloured holograms. Then if there is a dangerous gas present, it changes the properties of the hologram so that it’s colour changes to red. The colour change is quite fast and easy to see!

Question: How can a hologram tell if there are poisonous gases in the atmosphere?