Particular attention was directed at chemical and biological detectors during the 1960s. The first devices were primitive field alarms to detect chemicals. Although the development of sensitive biological warfare agent detectors was at a standstill, two systems were, nonetheless, investigated. The first was a monitor that detected increases in the number of particles sized 1 to 5 µm in diameter, based on the assumption that a biological agent attack would include airborne particles of this size. The second system involved the selective staining of particles collected from the air. Both systems lacked enough specificity and sensitivity to be of any practical use.Hersh SM. Chemical and Biological Warfare: America's Hidden Arsenal. Indianapolis, Ind: Bobbs-Merrill; 1968.
