A tagging technique used by the KGB’s Third Chief Directorate during the Cold War to monitor the movements of individuals who were difficult to keep under direct observation. “Spy dust” was the general term for nitrophenyl pentadien (NPPD), luminal, and other chemicals. A sample was provided by the Central Intelligence Agency’s source COWL, and a defector, Vitali Yurchenko, confirmed the procedure for its use.
Under a secret research program code-named METKA, the compound was applied to the clothing, shoes, or person of the target, thus allowing the target to be followed from a safe distance by watchers equipped with the appropriate detection devices. Although invisible to the naked eye, the chemicals could be tracked passively by detectors at strategically located choke points or could be illuminated by infrared beams. Fear that NPPD was mutagenic and possibly carcinogenic resulted in a formal protest from the U.S. State Department in 1985.