Phosgene was a chemical weapon in WW1 and was especially problematic because, drumroll please, it's heavier than air and doesn't dissipate readily. That's one reason they roundabout mention checking air in people's homes.

Air readings could be manipulated simply by taking them from the tops of hills, or upwind, or even a relatively low altitude via plane. People who don't know phosgene is heavier than air would not realize what was going on.