in my room. And there are two meters on it (this is a true story by the way).
One meter measures the amount of dust in the air, and the other measures offensive smell. I'm not really sure how.
When the dust/ smell quantities are where they should be, the lights are green, when they are not perfect, the light is yellow, and when they get bad, the light turns red.
The lights are pretty much always green.
One time Danny walked into my room, I looked over, and the smell light was red.
That's my story.