Extension Toxicology Network

Toxicology Information Briefs

A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Oregon State University, the University of Idaho, and the University of California at Davis and the Institute for Environmental Toxicology, Michigan State University. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.

EXTOXNET primary files maintained and archived at Oregon State University

Revised 9/93.



Cholinesterase (ko-li-nes-ter-ace) is one of many important enzymes needed for the proper functioning of the nervous systems of humans, other vertebrates, and insects. Certain chemical classes of pesticides, such as organophosphates (OPs) and carbamates (CMs) work against undesirable bugs by interfering with, or 'inhibiting' cholinesterase. While the effects of cholinesterase inhibiting products are intended for insect pests, these chemicals can also be poisonous, or toxic, to humans in some situations.

Human exposure to cholinesterase inhibiting chemicals can result from inhalation, ingestion, or eye or skin contact during the manufacture, mixing, or applications of these pesticides.


Electrical switching centers, called 'synapses' are found throughout the nervous systems of humans, other vertebrates, and insects. Muscles, glands, and nerve fibers called 'neurons' are stimulated or inhibited by the constant firing of signals across these synapses. Stimulating signals are usually carried by a chemical called 'acetylcholine' (a-see-till-ko-leen). Stimulating signals are discontinued by a specific type of cholinesterase enzyme, acetylcholinesterase, which breaks down the acetylcholine. These important chemical reactions are usually going on all the time at a very fast rate, with acetylcholine causing stimulation and acetylcholinesterase ending the signal. If cholinesterase-affecting insecticides are presentin the synapses, however, this situation is thrown out of balance. The presence of cholinesterase inhibiting chemicals prevents the breakdown of acetylcholine. Acetylcholine can then build up, causing a "jam" in the nervous system. Thus, when a person receives to great an exposure to cholinesterase inhibiting compounds, the body is unable to break down the acetylcholine.

Let us look at a typical synapse in the body's nervous system, in which a muscle is being directed by a nerve to move. An electrical signal, or nerve impulse, is conducted by acetylcholine across the junction between the nerve and the muscle (the synapse) stimulating the muscle to move. Normally, after the appropriate response is accomplished, cholinesterase is released which breaks down the acetylcholine terminating the stimulation of the muscle. The enzyme acetylcholine accomplishes this by chemically breaking the compound into other compounds and removing them from the nerve junction. If acetylcholinesterase is unable to breakdown or remove acetylcholine, the muscle can continue to move uncontrollably.

Electrical impulses can fire away continuously unless the number of messages being sent through the synapse is limited by the action of cholinesterase. Repeated and unchecked firing of electrical signals can cause uncontrolled, rapid twitching of some muscles, paralyzed breathing, convulsions, and in extreme cases, death. This is summarized below.

Exposure to:

May result in:


Any pesticide that can bind, or inhibit, cholinesterase, making it unable to breakdown acetylcholine, is called a "cholinesterase inhibitor," or "anticholinesterase agent." The two main classes of cholinesterase inhibiting pesticides are the organophosphates (OPs) and the carbamates (CMs). Some newer chemicals, such as the chlorinated derivatives of nicotine can also affect the cholinesterase enzyme.

Organophosphate insecticides include some of the most toxic pesticides. They can enter the human body through skin absorption, inhalation and ingestion. They can affect cholinesterase activity in both red blood cells and in blood plasma, and can act directly, or in combination with other enzymes, on cholinesterase in the body. The following list includes some of the most commonly used OPs:

  • acephate (Orthene)
  • Aspon
  • azinphos-methyl (Guthion)
  • carbofuran (Furadan, F formulation)
  • carbophenothion (Trithion)
  • chlorfenvinphos (Birlane)
  • chlorpyrifos (Dursban, Lorsban)
  • coumaphos (Co-Ral)
  • crotoxyphos (Ciodrin, Ciovap)
  • crufomate (Ruelene)
  • demeton (Systox)
  • diazinon (Spectracide)
  • dichlorvos (DDVP, Vapona)
  • dicrotophos (Bidrin)
  • dimethoate (Cygon, De-Fend)
  • dioxathion (Delnav)
  • disulfoton (Di-Syston)
  • EPN
  • ethion
  • ethoprop (Mocap)
  • famphur
  • fenamiphos (Nemacur)
  • fenitrothion (Sumithion)fensulfothion (Dasanit)fenthion (Baytex, Tiguvon)
  • fonofos (Dyfonate)
  • isofenfos (Oftanol, Amaze)
  • malathion (Cythion)
  • methamidophos (Monitor)
  • methidathion (Supracide)
  • methyl parathio
  • mevinphos (Phosdrin)
  • monocrotophos
  • naled (Dibrom)
  • oxydemeton-methyl(Meta systox-R)
  • parathion (Niran, Phoskil)
  • phorate (Thimet)
  • phosalone (Zolonc)
  • phosmet (Irnidan, Prolate)
  • phosphamidon (Dimecron)
  • temephos (Abate)
  • TEPP
  • terbufos (Counter)
  • tetrachlorvinphos (Rabon, Ravap)
  • trichlorfon (Dylox, Neguvon)

Carbamates, like organophosphates, vary widely in toxicity and work by inhibiting plasma cholinesterase. Some examples of carbamates are listed below:


Overexposure to organophosphate and carbamate insecticides can result in cholinesterase inhibition. These pesticides combine with acetylcholinesterase at nerve endings in the brain and nervous system, and with other types of cholinesterase found in the blood. This allows acetylcholine to build up, while protective levels of the cholinesterase enzyme decrease. The more cholinesterase levels decrease, the more likely symptoms of poisoning from cholinesterase inhibiting pesticides are to show.

Signs and symptoms of cholinesterase inhibition from exposure to CMs or OPs include the following:

  1. In mild cases (within 4 - 24 hours of contact): tiredness, weakness, dizziness, nausea and blurred vision;
  2. In moderate cases (within 4 - 24 hours of contact): headache, sweating, tearing, drooling, vomiting, tunnel vision, and twitching;
  3. In severe cases (after continued daily absorption): abdominal cramps, urinating, diarrhea, muscular tremors, staggering gait, pinpoint pupils, hypotension (abnormally low blood pressure), slow heartbeat, breathing difficulty, and possibly death, if not promptly treated by a physician.

Unfortunately, some of the above symptoms can be confused with influenza (flu), heat prostration, alcohol intoxication, exhaustion, hypoglycemia (low blood sugar), asthma, gastroenteritis, pneumonia, and brain hemorrhage. This can cause problems if the symptoms of lowered cholinesterase levels are either ignored or misdiagnosed as something more or less harmful than they really are.

The types and severity of cholinesterase inhibition symptoms depend on:

(a) the toxicity of the pesticide.
(b) the amount of pesticide involved in the exposure.
(c) the route of exposure.
(d) the duration of exposure.

Although the signs of cholinesterase inhibition are similar for both carbamate and organophosphate poisoning, blood cholinesterase returns to safe levels much more quickly after exposure to CMs than after OP exposure. Depending on the degree of exposure, cholinesterase levels may return to pre-exposure levels after a period ranging from several hours to several days for carbamate exposure, and from a few days to several weeks for organophosphates.

When symptoms of decreased cholinesterase levels first appear, it is impossible to tell whether a poisoning will be mild or severe. In many instances, when the skin is contaminated, symptoms can quickly go from mild to severe even though the area is washed. Certain chemicals can continue to be absorbed through the skin in spite of cleaning efforts.

If someone experiences any of these symptoms, especially a combination of four or more of these symptoms during pesticide handling or through other sources of exposure, they should immediately remove themselves from possible further exposure. Work should not be started again until first aid or medical attention is given and the work area has been decontaminated. Work practices, possible sources of exposure, and protective precautions should also be carefully examined.

The victim of poisoning should be transported to the nearest hospital or poison center at the first sign(s) of poisoning. Atropine and pralidoxime (2-PAM, Protopam) chloride may be given by the physician for organophosphate poisoning; atropine is the only antidote needed to treat cholinesterase inhibition resulting from carbamate exposure (9).


Anyone exposed to cholinesterase-affected pesticides can develop lowered cholinesterase levels. The purpose of regular checking of cholinesterase levels is to alert the exposed person to any change in the level of this essential enzyme before it can cause serious illness. Ideally, a pre-exposure baseline cholinesterase value should be established for any individual before they come in regular contact with organophosphates and carbamates. Fortunately, the breakdown of cholinesterase can be reversed and cholinesterase levels will return to normal if pesticide exposure is stopped.


Humans have three types of cholinesterase: red blood cell (RBC) cholinesterase, called "true cholinesterase;" plasma cholinesterase, called "pseudocholinesterase;" and brain cholinesterase. Red blood cell cholinesterase is the same enzyme that is found in the nervous system, while plasma cholinesterase is made in the liver.

When a cholinesterase blood test is taken, two types of cholinesterase can be detected. Physicians find plasma cholinesterase readings helpful for detecting the early, acute effects of organophosphate poisoning, while red blood cell readings are useful in evaluating long-term, or chronic, exposure (8).

The cholinesterase test is a blood test used to measure the effect of exposure to certain or cholinesterase-affected insecticides. Both plasma (or serum) and red blood cell (RBC) cholinesterase should be tested. These two tests have different meanings and the combined report is needed by the physician for a complete understanding of the individual's particular cholinesterase situation. Laboratory methods for cholinesterase testing differ greatly, and results obtained by one method cannot be easily compared with results obtained by another. Sometimes there is also considerable variation in test results between laboratories using the same testing method. Whenever possible, cholinesterase monitoring for an individual should be performed in the same laboratory, using a consistent testing method.

The approved methods are: Michel, microMichel, pH stat, Ellman, micro-Ellman, and certain variations of these. Micro methods have the advantage of not necessitating venipuncture, the drawing of blood from a vein by puncturing the vein with a needle attached to a collecting tube. The Ellman technique is considered better for detecting cholinesterase inhibition caused by carbamates. Many of the various "kit" methods in use are not satisfactory, particularly those which can be used only for plasma (or serum) determinations.


The following people should be concerned with having their cholinesterase levels checked on a regular basis: (a) anyone that mixes, loads, applies, or expects to handle or come in contact with highly or moderately toxic organophosphate and/or carbamate pesticides (this includes anyone servicing equipment used in the process); (b) anyone that is in contact with these chemicals for more than 30 hours at a time in one 30-day period.


Every person has his/her own individual 'normal' range of baseline cholinesterase values; cholinesterase levels vary greatly within an individual, between individuals, between test laboratories, and between test methods. The extent of potential pesticide poisoning can be better understood if cholinesterase tests taken after exposure to the cholinesterase inhibiting pesticides can be compared to the individual's baseline, pre-exposure measurement. Workers that receive routine exposure to organophosphate or carbamate pesticides should be offered an initial pre-employment check of their blood cholinesterase levels to establish "baseline values" prior to any exposure to these agrochemicals. If no pre-exposure value was obtained, however, the earliest cholinesterase value recorded can be used for later comparison. Excessive exposure to OPs and CMs depresses the cholinesterase so markedly that a diagnosis can also be made without previous baseline testing. If an individual's cholinesterase levels drop 30 percent below the original baseline level, immediate retesting should be done.

While there is no set formula for deciding the frequency of cholinesterase testing, in general, the initial baseline test should be followed by subsequent cholinesterase testing on a regular (usually monthly) basis. This testing should be done weekly during the active season, however, when workers are employed full-time and regularly using OPs and CMs labelled "DANGER." The test should be repeated any time a worker becomes sick while working with OPs, or within 12 hours of his/her last exposure.

Several factors should be considered in deciding how often someone should have his/her cholinesterase levels tested:

a) The extent and seriousness of the possible exposure. This will vary with the toxicity of the pesticides being used and how
often they are handled.
b) The type of work being done and the equipment being used may involve different risks of exposure.
c) Work practices have an important effect on worker safety. Some good practices include: the proper use of protective clothing and equipment; showering after each job; avoidance of drinking, eating and smoking in pesticide contaminated areas; prompt and effective decontamination in the event of spills.
d) The past safety record of a company and the work history and experience of an individual.
e) The physician's experience and familiarity with a specific work force may be an additional factor.


Since individual states vary in their cholinesterase monitoring programs, people that want to get their cholinesterase levels checked should consult with either their family or company physician for the specific requirements and procedures for cholinesterase testing in their particular state. After the blood is sampled and tested, test results are sent to the individual and his/her physician for interpretation.

Baseline blood samples should be taken at a time when the worker has not been exposed to organophosphate and carbamate pesticides for at least 30 days. Establishing a stable baseline requires a minimum of two pre-exposure tests taken at least 3 days but not more than 14 days apart. If these two tests differ by as much as 20 percent, a third sample should be taken and the two closest values averaged and considered the true baseline.


While cholinesterase testing is extremely valuable, it does have its limits, for the following reasons:

(a) not all hospitals are set up to complete the test within one facility, causing delays in diagnosis;
(b) the wide statistical error of the test makes it difficult to accurately detect very slight poisoning from cholinesterase inhibiting pesticides;
(c) the blood test is more effective in detecting cholinesterase depression from OP exposure than it is in detecting cholinesterase inhibition from carbamate exposure.

While carbamates (CMs) cause a depression in cholinesterase levels, the enzyme levels may return to baseline levels within hours of exposure, perhaps before test results are returned. When the effects of over-exposure to CMs are being checked, blood must be drawn during actual exposure or not more than 4 hours thereafter. If the drawing of blood and the actual completion of the laboratory test is delayed for more than 4 hours, reactivation of the enzyme will have taken place in the blood. This situation makes it hard for the physician to know the extent to which cholinesterase was inhibited, and to fully assess the seriousness of any safety problems which might exist in the work environment.


The interpretation of cholinesterase test results should be done by a physician. A 15 to 25 percent depression in cholinesterase means that slight poisoning has taken place. A 25 to 35 percent drop signals moderate poisoning, and a 35 to 50 percent decline in the cholinesterase readings indicates severe poisoning (8).

A reported change in an individual's cholinesterase level may result from something other than a pesticide exposure, or it may be the result of laboratory error, but this should never be assumed to be the case. If the report shows a worker's cholinesterase level has dropped 20 percent below his/her baseline in either plasma or RBC, he/she should be retested immediately. If the second test repeats the same low values, faulty work practices should be carefully looked for and steps should be taken to correct them.

A 30 percent drop below the individual's baseline of RBC cholinesterase or plasma cholinesterase means that the individual should be removed from all exposure to organophosphates and carbamates, with the individual not being allowed to return until both levels return to the pre-exposure baseline range. Removal from exposure means avoidance of areas where the materials are handled or mixed and avoidance of any contact with open containers or with equipment that is used for mixing, dusting or spraying organophosphates or carbamates. A worker removed from exposure to cholinesterase inhibitors may be employed at other types of work.


Because of the lack of approval of standardized test methods and laboratories in the U.S., a list of approved laboratories is not available. However, consult with your physician or local community hospital (testing laboratory) and the State Department of Health for guidance and recommendation of a good laboratory. Keep in mind that a single test method at one test laboratory should be used in your monitoring program.

The 1986 estimates on the cost of individual cholinesterase tests range from $7.00 to $60.00, with the average test costing approximately $35.00. The quality of tests will improve and prices will be lowered if and when testing methods are standardized and automated.


Current EPA worker protection standards (put into place in 1974) are incomplete, and more comprehensive rules are being proposed which would be put into effect in the Spring of 1988. The standards address reentry intervals, notification, decontamination facilities, training of workers, and emergency medical care for workers. Additional provisions are also specified on protective equipment, change facilities, medical monitoring, annual physical examinations, and maintaining contact during pesticide handling. These regulations are likely to require commercial pesticide applicators to have cholinesterase blood tests to establish individual baseline readings. Applicators would then be required to have another test for every 3 or more consecutive days of exposure to organophosphates which fall in toxicity category I ("highly toxic") or category II ("moderately toxic") or when exposed six or more days in a 21-day period. Four states currently have some type of cholinesterase testing requirement in place: California, Ohio, Arizona, and Colorado.


(1) U.S. Environmental Protection Agency. Telephone: 1-800-858-7378.

(2) Cooperative Extension Service in your area.

(3) Pesticide Unit, Epidemiological Studies Laboratory, California Department of Health, 2151 Berkeley Way, Berkeley, CA. 94704. Telephone: (415)-540-3063.

(4) Worker Health and Safety Branch, Department of Food and Agriculture, 1220 N Street, Sacramento, CA. 95814. Telephone: (916)-445-8474.

(5) Davies, J.E. and V.H. Freed (eds.). 1981. An agromedical approach to pesticide management Some health and environmental considerations. Consortium for International Crop Protection. Berkeley, CA.

(6) Goh, Kean, W.G. Smith, R.F. Pendleton. 1985. Pesticide safety for IPM field scouts. Chemicals Pesticides Program. Cornell University, Ithaca, NY.

(7) Golz, H.H. and C.B. Shaffer. 1960. Toxicological information on cyanamid Insecticides. American Cyanamid Co., Princeton, NJ.

(8) Paul, Jane. 1987. Commercial pesticide applicators may get mandatory blood tests. Agrichemical Age. March.

(9) Smith, William G. 1983. Cholinesterase. Chemicals Pesticide Program. Cornell Cooperative Extension Information. New York State College of Agriculture and Life Sciences, Cornell University, Ithaca, NY.

(10) Van Driesche, R G. 1985. Cholinesterase testing information. Pesticide Facts. Cooperative Extension Service, University of Massachusetts, Amherst, MA. June 7, 1985.

DISCLAIMER: The information in this brief does not in any way replace or supersede the information on the pesticide product label/ing or other regulatory requirements. Please refer to the pesticide product label/ing.