On March 22, 2002, NIOSH received a request for a health hazard evaluation (HHE) from the Animal and Plant Health Inspection Service (APHIS), a division of the United States Department of Agriculture (USDA). The medical officer for APHIS was interested in determining if the current cholinesterase monitoring program for its animal health technicians in the tick eradication program was useful. In addition, APHIS had an interest in evaluating other methods for monitoring organophosphate (OP) exposure, including field test kit measurements of cholinesterase and urinary biomarkers. The primary OP used was coumaphos. NIOSH investigators observed various coumaphos application tasks and administered questionnaires concerning coumaphos application history, health symptoms, and general medical information. Blood was collected by venipuncture for laboratory cholinesterase measurements using the Ellman method, and by fingerstick for cholinesterase measurements by field test kit at the start of the workday (prior to use of coumaphos) and again at the end of the workday. Urine was collected for measurement of a possible urinary biomarker of coumaphos, 3-chloro-4-methyl coumarin (chlorferon), at the same times. An additional urine sample was obtained the following day. Environmental sampling included personal air sampling, patch sampling on the surface of employees' clothing, and skin wipe sampling of hands. These were conducted at the end of one or more coumaphos application tasks. All of the APHIS animal health technicians who met our criteria for participation agreed to participate in the evaluation, but only 15/59 met the criteria. Participants reported mixing coumaphos 8.5 days per month and spraying or dipping livestock 11 days per month, on average. Three reported being removed from working with coumaphos (one time each) in the past year due to low cholinesterase levels. Technicians were asked about symptoms consistent with OP poisoning experienced within 6 hours of coumaphos use in the past 3 months. Two each reported headache, weakness, and tearing eyes; and one each reported cough and nervousness. Reported personal protective equipment (PPE) use varied by task and type of PPE. None of the technicians had plasma cholinesterase (PChE) or acetylcholinesterase (AChE) levels outside the laboratory's range of normal. Neither PChE nor AChE changed significantly from pre- to post-shift with the Ellman method or the field test kit. A significant positive correlation was found between PChE levels measured with the field test kit and by the Ellman method (r=0.92, p<0.01). A significant positive correlation was also found between AChE levels measured by the two different methods (r=0.63, p<0.01). Chlorferon was detected in the urine of all animal health technicians after they used coumaphos. Urinary chlorferon levels were significantly higher 4-6 hours after use (p<0.01) and declined significantly by the next day (p=0.01). No coumaphos was detected in 8 of 9 personal breathing zone air samples. One showed trace amounts of coumaphos and represented a worst case scenario. The technician sprayed horses and dipped cattle for the entire day. Of the whole body patch samples, 58% detected measurable levels of coumaphos. Of handwipe sampling, 96% detected measurable amounts of coumaphos, indicating that the majority of exposures occur from hand contact with the pesticide. A formal PPE program was not in place. No standard guidance for what type of PPE should be worn during specific tasks was available and use of PPE varied by employee. PPE was poorly maintained and dirty. Employees were observed incorrectly wearing respiratory protection. A health hazard from dermal exposure to coumaphos existed for USDA APHIS animal health technicians. An inhalation hazard to the pesticide did not exist at the time of the NIOSH evaluation. Instituting a formal PPE program and requiring the use of PPE such as gloves and aprons during specific work tasks would lower dermal exposures.