In a store
The charming pharmacy in our small town has a low ceiling and is very long and narrow. The pharmacy counter itself is right at the back. I was picking up some medication at the counter one day and - shock, horror - look at the CO₂ reading: 958ppm in an almost empty store.
Luckily none of my children were with me - because they would have dropped to the floor and hyperventilated with embarrassment - as I showed the reading to the pharmacist. He was very concerned and said he would have a think about what could be done.
I returned a week later for something or other and had forgotten all about it, but as I walked into the pharmacy from the street I noticed a breeze blowing on my face and the reading had halved to 487ppm.
The pharmacist told me proudly that they now keep the back door open, had switched off the free-standing fan at the front door, which was blowing the air-conditioned air back in, and now everyone felt better.
Sometimes, very simple measures are all it takes.