Some years ago, I lived in an overheated apartment, and desperately wanted to stop getting shocks every time I walked across the room. So I placed a small metal bowl full of water atop one of the heaters. By the end of the winter, a dirty white solid coated the bottom of the bowl that was impossible to remove--the remains of the salts originally dissolved, invisibly, in the tap water. That's what would most likely happen to green dye--a mixture of FD& C Yellow 5, FD& C Blue 1, and some propylene glycol--when added to the humidifier. It would be left behind.

Most humidifiers work by heating up the water enough so that some of the water molecules get enough energy to leap out of the container, and join the air--mostly nitrogen and oxygen--which normally surrounds us. Just a little bit of water takes some of the crackle out of the air; too much, of course, and the air will get humid and uncomfortable. Fortunately for kids, it takes less energy for a water molecule to escape than for the dyes, the propylene glycol, or any salts dissolved in our water supply. So while the water molecules are escaping, dye molecules--and any other molecular trespassers-are left behind, and make a mess only in the water reservoir.

Some humidifiers, however, don't work in quite the same way. Cool air humidifiers pulverize the water and shoot it out to the world as a fine liquid mist; in these systems, everything else travels with the water. So green food dye might be expected to paint an entire room. To foil just such a possibility, most such systems come with a water filter-a disposable piece designed to attract and hold tight to most foreign matter--so that the liquid spray is close to pure.