A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.
A single chemical can enter the body through all three routes of exposure -- inhalation, ingestion and skin penetration (dermal exposure). A compound, such as chloroform, which evaporates readily and which may be found in drinking water illustrates this point. When this water is used for drinking, ingestion is the route of exposure. When it is used for showering, exposure may occur due to inhalation of the steam or mist and from direct contact through the skin. Similarly, pesticide use can involve more than one route of exposure if precautions are not taken. A pesticide which is sprayed can be inhaled during use; penetrate through the skin during mixing and application; and be ingested through food if not washed off hands or food before eating.
Once a chemical is absorbed into the bloodstream, it can have several different fates. In many cases, it is rapidly removed from the body through the urine or feces. In other situations, it may be stored in various parts of the body, such as fat or bone, and remain in the individual for many years. A compound may also lead to a toxic effect through interaction with certain organs or tissues in the individual or with other compounds in the body.
Often, a substance which is absorbed into the body interacts with particular body chemicals and is changed into one or more other chemicals. This process is called metabolism and the products are called metabolites. Metabolism may lead to products which are easier for the body to excrete and so can protect the body from possible adverse effects. In other cases, however, the metabolites may be more toxic than the original chemical which was absorbed. The variety of products resulting from metabolism may have the same possible fates as the original chemical -- storage, excretion or toxicity.
In the case of (repeated) multiple exposures to a chemical, it is not only the total amount of exposure, but also the rate or timing of exposure that is quite important. All processes in the body normally proceed at specific rates so that metabolism, excretion and storage occur during a particular period of time after a chemical is absorbed. For a one occurrence exposure, the time needed for the various processes that breakdown the compound to be completed will determine the length of time that a toxic response, if any, persists.
However, if there are repeated exposures to the same chemical, the situation is more complicated. If there is enough time between exposures so that all of the chemical from the initial exposure is excreted, and no effects persist, then each exposure is essentially independent of the previous one and can be treated as a single exposure. However, if the time between exposures is so short that some of the chemical remains from the first exposure, then a buildup of the chemical can occur. Over time this buildup can lead to levels which are toxic.
The total amount of exposure can have different results depending on whether the exposure occurred all at once or repeatedly over time (the time course of exposure). A high dose given once may have a toxic effect while the same total dose given in small amounts over time will not. For example, drinking several ounces of alcohol at once may cause inebriation while drinking one ounce every few hours may not. Also, a particular dose given a few hours apart may have an adverse effect while the same total dose given a few days apart will not.