[roland35]

I am not a scientist so I can't pretend to understand it too well, but here is an example of a paper about it: https://www.academia.edu/17195945/O_2_pH_Multisensor_Based_o...

The dye was on a small paper disk, not in the air. It was measuring exhaled breath anyways so you weren't inhaling it.

CO2 was another interesting sensor! There is a specific IR frequency it absorbs, so we used that to sense it in real time.