The cholinesterase-monitoring program was mandated by the Washington State Supreme Court in 2002 as a system to warn handlers and employers about pesticide exposure before handlers become ill. Presently, only the cholinesterase-inhibiting pesticides (organophosphates and carbamates) offer a window on early exposure through a simple blood test. This test identifies handlers who may not be adequately protected in the workplace. Relative to previous years, this past year had the lowest proportion of handlers with inhibited cholinesterase that triggered a worksite evaluation, with 7 percent, or a work removal, which was less than 1 percent.

Speculated causes of exposure

The Washington State Department of Labor and Industries (L&I) had evaluated the workplaces and practices of handlers with inhibited cholinesterase activity in an effort to identify potential sources of exposure. These investigations generated a list of problems that, based on sound work hygiene principles, were likely to increase exposure. Some examples included use of ball caps under personal protective equipment and inadequate cleanup procedures (see "Potential exposure routes identified," page 44).

But only by showing that these problems occurred predominantly among those who had an inhibition can the case for true causation be made. For instance, while L&I found that handlers with cholinesterase inhibition often wore ball caps, that practice could also be quite common among handlers without inhibition. By comparing the work practices and conditions of handlers who had inhibited cholinesterase activity to those who did not, we can begin to confirm which factors are truly associated with pesticide overexposure.


The Pacific Northwest Agricultural Safety and Health Center at the University of Washington explored whether there were any particular work activities or practices associated with inhibited serum cholinesterase activity. Although L&I uses the term "cholinesterase depression," the study authors prefer to use the synonymous term "inhibition" to avoid confusion with the psychological condition. A number of pesticide handlers thought they were being tested for emotional depression.

During the 2006 and 2007 seasons when handlers came to the clinic for their follow-up test, information was gathered about their work habits during the previous 30-day period. This was before their cholinesterase status was known. Using an icon-based touch screen with audio recorded questions in Spanish or English nearly 155 handlers provided information on their pesticide handling activities and other nonoccupational factors, such as age, smoking, alcohol use, and medical conditions (see "Information collected").

Comparing those with cholinesterase inhibition to those with normal cholinesterase, two work activities were significantly associated with inhibited cholinesterase activity:

1. Mixing and loading—Handlers reporting mixing and loading in the 30 days prior to their follow-up test were twice as likely to have inhibition as those who did not. Mixer/loaders had on average 5.25 percent greater serum cholinesterase inhibition.

2. Cleaning spray equipment—Handlers reporting cleaning spray equipment had 4.4 percent greater serum cholinesterase inhibition on average and were nine times as likely to show inhibition. The L&I investigations found that some handlers did not wear adequate personal protective equipment for this activity, which may explain (at least in part) the association between cleaning spray equipment and cholinesterase inhibition.

Three personal protective equipment practices were found to be protective:

1. Use of full-face respirators was significantly more protective than half-face models. Relative to the full-face varieties, those using half-face respirators had 7 percent greater cholinesterase-inhibited activity and were seven times as likely to have a value showing overexposure.

2. Wearing chemical-resistant footwear offered greater protection. Handlers who did not wear chemical-resistant boots had 11 percent greater inhibition and were 7.6 times as likely to have an abnormal test value compared to those who wore protective boots.

Using a locker to store personal protective equipment was also protective, likely because clothes were changed, reducing exposure time. It also may have reflected general workplace hygiene. Handlers who did not store personal protective equipment in a locker had 7.6 percent more inhibition and were almost six times as likely to have inhibition requiring a worksite evaluation.

What was not associated?

Several problems uncovered by the L&I site visits were not associated with serum cholinesterase inhibition in the survey, including wearing a hooded sweatshirt, baseball cap, or bandana underneath personal protective equipment. However, other practices such as hand washing and adequate change out of respirator cartridges, while not identified as risk factors in the study, may still be a concern and should remain a focus of training.

Cholinesterase inhibition is also believed to be related to several nonoccupational exposures, including solanaceous alkaloids in certain foods and some illicit drugs. There are also some prescription medicines that are cholinesterase inhibitors, but they are generally used to treat debilitating neurologic diseases like Alzheimer’s disease. A few health conditions—liver diseases and malignancies—can also interfere with cholinesterase activity.

For these other factors to be responsible for cholinesterase inhibition among handlers, they would have to be in place exclusively and only just before the follow-up blood test, but not during the baseline testing. It would be unusual for the medical conditions linked to inhibited cholinesterase activity to pop up only during the follow-up testing and not at baseline, and furthermore, most are so disabling that it is unlikely an affected individual would be able to meet the heavy demands of agricultural work. With the exception of Tylenol, the medications that could influence the test results would probably be taken by a worker at baseline as well as at follow-up (thus negating any evidence of inhibition) because they all treat chronic conditions.

We looked at some of these issues and found no associations. Few people had the medical conditions or used the medications thought to interfere with cholinesterase. We also asked about smoking, alcohol consumption, and use of Tylenol in the last 30 days. None of these were associated with serum cholinesterase inhibition. We are also following up on the question of alcohol and Tylenol use in a separate study among healthy individuals not exposed to pesticides.

Results of the Pacific Northwest Agricultural Safety and Health study suggest that we should focus on preventing exposure during high-risk activities such as mixing and loading pesticides and cleaning spray equipment. We should also encourage handlers to use personal protective equipment that offers better protection (such as full-face respirators and chemical-resistant boots), and to use lockers for storage of protective equipment.

For questions or more information about this study, contact Jon Hofmann at or call (800) 330-0827.