What are those small plastic things hanging around the campus buildings?

They're not too easily noticed, but once you see one, you'll notice the rest of them. The ones I first saw were hanging perpendicular to the counter at Cafe Q-- they're like these little lip balm sized clear plastic containers with some writing on the side and a blue dot on one side and are strung up by a thin string that resembles fishing line or something of the like. I also saw a bunch in Gilman (one right above the doorway of the main front entrance on the upper quad, two in the foyer-ish area where the roped off JHU seal is, and one on the stairs on the right side of the building between the 2nd and 1st floors)... What gives?!

EDIT: MYSTERY SOLVED! I saw two of them hanging by the class flags in the library on M level and decided to ask the security guard if they knew what they were-- turns out they're for measuring the amount of secondhand smoke in the buildings.