WHAT WE DO...AND WHOM WE SERVE
Keeping Agricultural Workers Safe
Clinical poisoning from organophosphate pesticides is relatively rare among Washington state agricultural workers. However, when it happens, the potential consequences are severe. There is a relatively small margin between a dose that produces symptoms and one that is life-threatening.
Ofelio Borges of the Washington State Department of Agriculture trains pesticide handlers in the proper use of protective equipment.
Environmental measurements are little help in identifying overexposures before they produce symptoms. One reason is that workers differ in susceptibility, partly because of their genetic makeup. Another is how they use protective equipment; significant exposures can occur even with very brief lapses of protection. The most commonly used way to find out whether a worker is being affected—even before symptoms show—is with a blood test that uses the worker’s own cholinesterase levels as a baseline.
Cholinesterase is an enzyme that acts as the nervous system’s “off switch.” Many pesticides, known as carbamate and organophosphate pesticides, act by disrupting the function of this type of enzyme in insects (and in people), so the nervous system works overtime. Workers whose cholinesterase levels have been lowered by contact with these pesticides could become sick if they continue to be exposed. Monitoring workers’ cholinesterase levels can provide an early warning of overexposure, so steps can be taken to prevent further harm.
In February 2004, Washington State Department of Labor & Industries implemented a cholinesterase monitoring rule, which requires agricultural employers to provide blood testing to workers who handle certain pesticides. Our department is helping the state departments of Health and Labor and Industries develop a monitoring system to implement the rule.
Faculty members have been active on the scientific advisory committee formed to oversee data collection and analysis. Dave Kalman, PhD, is chair of the committee, and Matt Keifer, MD, MPH, Gerald van Belle, PhD, and David Bonauto, MD, MPH, are members. Keifer also serves on a stakeholder committee and works with the community clinics that collect the blood samples.
Kalman, who for 19 years directed the Environmental Health Laboratory, called this an example of “taking science out of the lab and into the regulatory world.” The idea behind the monitoring, he said, is to identify possible poisoning before symptoms develop, providing a margin of safety (see next page). Kalman’s committee is reviewing monitoring results to assist the state to determine whether the monitoring program is, in fact, protecting a significant number of workers.
Farmworkers spray an orchard with pesticide.
Keifer described the goals of the monitoring program as preventing poisonings, identifying hazardous conditions and practices, increasing hazard awareness among workers and employers, and helping determine a safe time frame for returning to work after a poisoning event.
The rule requires employers to record all hours that a worker mixes, loads, or applies organophosphate or n-methyl-carbamate pesticides with the words “danger” or “warning” on the label. Examples are azinphos-methyl (Guthion), chlorpyrifos (Lorsban), carbaryl (Sevin), diazinon, and phosmet (Imidan).
Growers whose workers handle these pesticides for 30 or more hours in any 30-day period are required to offer these workers an opportunity to participate in the cholinesterase-monitoring program. The workers will be directed to a clinician who will explain the program and the worker may decide to have the blood tests.
A blood sample will be taken before the growing season to determine the worker’s baseline cholines-terase level. Repeat tests will be made when he or she handles these pesticides for 30 or more hours in a 30-day period.
The depression of the enzyme activity is only understandable by comparing a blood test taken during or after exposure with one taken before exposure occurs, Keifer said. Since the range of normal values for cholinesterase is very broad, workers must have a personal unexposed baseline available for comparison.
A drop in cholinesterase level prompts a review of the workplace to find out how the overexposure happened—perhaps a wash station was not available or a respirator was improperly fitted.
In the relatively few cases where the cholinesterase level is very low, the worker will be reassigned to a job without pesticide exposure until the cholinesterase returns to a safe level. This occurred in 10 cases in 2005 —fewer than 2% of the total cholinesterase tests.
After two seasons of study, the screening found that nearly 20% of monitored workers sustained at least one cholinesterase depression of 20% or greater, Keifer said. A 20% depression prompts a workplace investigation, 30% a removal from work, and 50% constitutes poisoning. None of the workers had symptoms.
In most cases, decreases in cholinesterase of up to 50% do not cause illness. However, at some point an additional exposure could make the worker ill. Symptoms can include headaches, diarrhea, dizziness, blurred vision, difficult breathing, unconsciousness, and even death.
Because the purpose of the rule is to protect workers, results can’t be interpreted in the same way as a population-based study, Keifer said. For example, individual workers aren’t followed from year to year. Still, departmental researchers have found useful data.
The Pacific Northwest Agricultural Safety and Health (PNASH) center plans follow-up studies to investigate the causes for cholinesterase depressions and to evaluate a field test kit that clinics can use.
FOR FURTHER READING
L&I website
http://www.lni.wa.gov/Safety/Topics?AtoZ/Cholinesterase/default.asp.
Washington state cholinesterase monitoring rule, WAC 296-307-148
http://www.lni.wa.gov/WISHA/Rules/agriculture/HTML/part-j-1.htm.
Pacific Northwest Agricultural Safety and Health Center website
http://depts.washington.edu/pnash/.
TIMELINE OF THE WASHINGTON STATE CHOLINESTERASE MONITORING RULE
1986-1997
Discussions between farm workers’ repre-sentatives and L&I about cholinesterase monitoring. UW involved with Technical Advisory Group
1997
Following a series of organophosphate pesticide poisonings in Washington state, two farmworkers sue L&I
2002
State Supreme Court decides that L&I must develop a rule for pesticide biomonitoring
2003
L&I rule is announced; Department of Health laboratory is selected to conduct year 1-2 monitoring; training materials developed for health care providers
2003-2004
PNASH training sessions for health care providers
2003-2004
PNASH develops informed consent audiotape for Spanish-speaking workers
Jan - Sept 2004
Year 1 monitoring
Jan - Sept 2005
Year 2 monitoring
2006
PNASH field research on etiology of pesticide exposure. Comprehensive report due in September
ANALYZING THE NUMBERS
It’s more than a simple “yes or no” question to determine whether a farmworker’s cholinesterase level has fallen enough to require action. Statistical evidence is tempered with ethical and policy considerations when setting a standard such as Washington state’s new cholinesterase monitoring rule.
Professor Emeritus Gerald van Belle, one of the nation’s leading biostatisticians and former chair of our department, helped assess the measures for action. He describes the state’s dilemma as how to set a level that adequately protects workers, but doesn’t unduly burden their employers.
Biostatisticians speak of “false negatives”—people who really have something wrong that doesn’t show up in tests—and “false positives” —people whose tests suggest a problem that isn’t borne out in further testing.
In cholinesterase testing, workers’ levels are compared to their own baseline tests. The action level could be set high, perhaps at a 40% depression in cholinesterase. This would virtually eliminate false positives, but could put some workers near the threshold of serious harm. If the action level were low, perhaps at 10%, it would identify almost all of the true cases, but would remove many workers from the field unnecessarily.
An agency such as Washington State Department of Labor & Industries (L&I) has a mandate to protect workers, and may build in a margin of safety, using what is called the “precautionary principle.” The question is how large that margin should be. “Statistics tries to quantify the issue,” van Belle said.
“We try to describe what happens at each level, but it is up to the regulators to decide where to draw the bright line.” After many discussions with the scientific advisory and stakeholder committees, L&I adopted action levels of 20%, 30%, and 40% (see main story, pages 12–14).
van Belle, who retired at the end of the biennium, considers the cholinesterase project one of his most interesting in a 31-year career at the university. His résumé includes many projects, including the Tacoma smelter arsenic study that first brought him to our department’s attention. Though not part of the Environmental Health faculty at the time, biostatistician van Belle became the department’s interim chair in 1990 and its permanent chair from 1991-1998. At the time of his appointment, he said he “knew little about the department, but I know good science when I see it.”
FOR FURTHER READING
van Belle G, Fisher L, Heagerty P, Lumley T. Biostatistics:
A Methodology for the Health Sciences. Second edition.
New York: John Wiley and Sons, 2004, 871 pages.