Researchers have genetically engineered a bacterium that can sniff out toxic chemicals at parts-per-billion concentrations. That doesn't sound like a lot, but it's the level at which human health effects are often first seen, say the system's developers, Michael Simpson of Oak Ridge National Laboratory and Gary Sayler of the University of Tennessee. Unlike other bacterial sensors, which require large and expensive instrumentation, this device is small and potentially inexpensive. Bacteria engineered to detect specific chemicals are deposited on chips less than two millimeters long. Upon exposure to the target chemical, the bacteria glow with an intensity proportional to the concentration of the chemical. The sensors can detect environmental pollutants such as compounds that mimic estrogen, residue from jet fuel in ground water, and chemicals that indicate food spoilage. The researchers have built a working prototype and hope an upcoming alliance with Perkin-Elmer will bring the device to market within three years.