BACKGROUND: Dogs have been used by law enforcement and military personnel for 30 years to detect narcotics and explosives because of their keen sense of smell. The Minnesota Protection Control Agency (MPCA) is now using specially trained dogs to check schools and other facilities for mercury contamination. The MPCA estimates that there is around two pounds of mercury "hidden" in most schools. The U.S. Environmental Protecton Agency is also conducting research on how to use dogs for detection of indoor air pollutants such as toxic molds, illegal pesticides, and gasoline vapors from contaminated ground water.
HOW DOGS SMELL: Dogs' noses are such sensitive chemical detectors that they can detect a target compound in the presence of other odors at much higher concentrations; they can even identify odors concentrated in a small object or piece of ground as small as a dime. They can even discriminate between a target odor and one that is closely related. Scent comes from an object in a plume that swirls and eddies so there are patches of dense odor and areas of faint odor. A dog will scan back and forth with its nose along those varying densities to try and locate the source of a smell.
INSIDE THE NOSE: When the dog inhales, a fold just inside its nostrils opens to allow air to flow through the upper part of the nose where mucus-covered scent receptors grow. Once inside the nose, chemical vapors dissolve in those receptors, and the chemical interactions are converted into electrical signals that travel along the olfactory nerve to the olfactory bulb and then to the dog's brain, which processes the data according to recognized patterns of odor signals. Dogs have around 220 million such receptors ý 40 times more than humans -- which can become sensitive to many different unrelated chemicals.
ABOUT MERCURY: Mercury, also sometimes called quicksilver, is an element that is just one of five metals that are liquid at room temperature. Mercury is used in thermometers and barometers, though recently alcohol has become more popular for oral thermometers. It is poisonous to humans when absorbed through the skin or in the stomach and intestines, causing brain damage, but it is still sometimes used in dental fillings because the amount absorbed may be low enough for humans to tolerate.