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.



The human body is a vastly complex biochemical organism, finely tuned and adaptable. It contains many different regulatory systems to make sure that things work properly in response to external conditions. When it becomes too warm inside the body, the water cooling system is turned up, and more sweat is secreted by the skin. The sweat evaporates, cools the blood underneath the skin, which in turn cools the body core. The sensors in the brain detect that things are back within normal limits, and turn off the sweat glands. This type of regulation (known as homeostasis) occurs for all bodily processes, and usually without any awareness or thought on our part.

When external circumstances (like extreme heat or cold) or internal conditions (disease or poisoning) cannot be adjusted by normal mechanisms, the signs of discomfort and disease appear. The types of physical effects seen or felt (signs and symptoms) depend on the type of stress to which the body has been exposed. Because there are so many complex interrelationships between the systems within the body, a single change in any one system may result in numerous effects in other systems. In addition, the types of response to disease are limited, thus signs and symptoms of disease are often quite similar for different diseases. For example, headache, fever, nausea, vomiting and diarrhea are very common non-specific symptoms of disease, produced by many, many conditions. Because of the generality of most physiological responses to disease, many other methods have been developed to help diagnose the actual causes of disease. These methods include physical, biochemical and immunological techniques upon which modern clinical medicine is based.

A body's homeostasis can be upset by physical, chemical and/or biological agents which put stress on the body. The body's reaction to prolonged stress depends on the nature of the agent, the degree of stress, and the duration of stress. When the stress is too strong or too long, and homeostasis cannot be maintained or restored, disease occurs. Poisoning by chemical agents is nothing more than chemically induced disease, and the symptoms of chemical poisoning often are the same as symptoms caused by biological agents such as bacteria or viruses. To better understand how disease is caused by exposure to toxic chemicals, we must first understand how poisons work within the body.


Poisons work by changing the speed of different body functions, increasing them (for example, increasing the heart rate or sweating), or decreasing them (sometimes to the point of stopping them entirely, like breathing). For example, people poisoned by parathion (an insecticide) may experience increased sweating. Increased sweating due to parathion poisoning results as follows. The first step is the biochemical inactivation of an enzyme. This (1) biochemical change leads to a (2) cellular change (in this case an increase in nerve activity). The cellular change is then responsible for (3) physiological changes, which are the symptoms of poisoning that are seen or felt in particular organ systems (in this case the sweat glands). The basic progression of effects from biochemical to cellular to physiological occurs in most all cases of poisoning. Depending on the specific biochemical mechanism of action, a poison may have very widespread effects throughout the body, or may cause a very limited change in physiological functioning in a particular region or organ. Parathion causes a very simple inactivation of an enzyme which is involved in communication between nerves. The enzyme which parathion inactivates however, is very widespread in the body, and thus many varied effects on many body systems are seen besides sweating.


Toxicity is a general term used to indicate adverse effects produced by poisons. These adverse effects can range from slight symptoms like headaches or nausea, to severe symptoms like coma and convulsions and death.

Toxicity is normally divided into four types, based on the number of exposures to a poison and the time it takes for toxic symptoms to develop. The two types most often referred to are acute and chronic. Acute toxicity is due to short-term exposure and happens within a relatively short period of time, whereas chronic toxicity is due to long-term exposure and happens over a longer period.

Most toxic effects are reversible and do not cause permanent damage, but complete recovery may take a long time. However, some poisons cause irreversible (permanent) damage. Poisons can affect just one particular organ system or they may produce generalized toxicity by affecting a number of systems. Usually the type of toxicity is subdivided into categories based on the major organ systems affected. Some of these are listed in table 1. Individual Toxicological Information Briefs (TIBs) are available which more fully explain skin and reproductive toxicities. Another is available which covers the formation of tumors and cancer.

Because the body only has a certain number of responses to chemical and biological stressors, it is a complicated business sorting out the signs and symptoms and determining the actual cause of human disease or illness. In many cases, it is impossible to determine whether an illness was caused by chemical exposure or by a biological agent (like a flu virus). A history of exposure to a chemical is one important clue in helping to establish the cause of illness, but such a history does not constitute conclusive evidence that the chemical was the cause. To establish this cause/effect relationship, it is important that the chemical be detected in the body (such as in the blood stream), at levels known to cause illness. If the chemical produces a specific and easily detected biochemical effect (like the inhibition of the enzyme acetylcholinesterase), the resulting biochemical change in the body may be used as conclusive evidence.

People who handle chemicals frequently in the course of their jobs and become ill and need medical attention should tell their physicians about their previous exposure to chemicals.

Table 1. General toxicity categories

Category System affected Common symptoms
respiratory nose, trachea, lungs irritation, coughing, choking, tight chest
gastrointestinal stomach, intestines nausea, vomiting, diarrhea
renal kidney back pain, urinating more or less than usual
neurological brain, spinal cord headache, dizziness, behavior confusion, depression, coma, convulsions
hematological blood anemia (tiredness, weakness)
dermatological skin, eyes rashes, itching, redness, swelling
reproductive ovaries, testes fetus infertility, miscarriage

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.