Paired total and respirable particulate samples were collected in the breathing zone of 186 painters at five Air Force bases. Using the composition of the starting material to estimate total reactive isocyanate group (TRIG), exposure levels were 0.87 mg/M3 (geometric mean) in the total fraction and 0.12 mg/M3 in the respirable fraction with geometric standard deviations of 2.7 and 3.3, respectively. Dichotomous samplers specific for TRIG were used during the painting of a military vehicle and indicated 0.39 ± 0.15 mg/M3 TRIG. This was 20 times the Health and Safety Executive's recommended time weighted average for TRIG. The TRIG levels seen in these studies indicate the need for personal protective equipment or additional engineering controls. Breakthrough studies with 1,6 hexamethylene diisocyanate (HDI) and HDI-biuret were conducted on a series of negative pressure air purifying respirator (APR) cartridges. Initial penetration of TRIG through the APR cartridges ranged from greater than 70% to less than 3% giving apparent protection factors of >1.5 and >33. These samples also showed that HDI was present in the aerosol fraction at levels significantly below its saturated vapor pressure.