Routes of Exposure
Terrestrial wildlife (creatures living on land) may be exposed to chlorpyrifos by ingesting plants or dead and dying insect pests that have been treated with the chemical. Greenhouse and field studies indicate that plants do not readily take in chlorpyrifos through roots, although they do retain and absorb some of the product in their leaves. Chlorpyrifos is quickly broken down and detoxified on leaf surfaces and within the plant. Dissipation of the product from leaf surfaces occurs in as brief a time as a single day and as extended a period as 7 days. Although chlorpyrifos application to aquatic environments (lakes, streams, etc.) is not permitted, any traces of residues which may inadvertently enter waterways through spills, spray drift or surface runoff could be absorbed by fish and other aquatic organisms.
Chlorpyrifos Absorption and Breakdown in Animals
Animals, birds, and insects vary in the routes by which they’re exposed and the speed with which they absorb and metabolize chlorpyrifos. Many insect pests absorb chlorpyrifos through their body surfaces, and at a rate of up to 90% of what they’re exposed to. Some animals, such as rats, can absorb up to 60% of the exposure dose through their skin, whereas humans tend to absorb only 3% of the chlorpyrifos to which their skin is exposed. While some species of bird and small mammal are particularly susceptible to chlorpyrifos, in general low levels of the product are metabolized quickly — in a matter of hours — and excreted without harmful effect. Fish and other aquatic life can absorb chlorpyrifos from the surrounding water. Although the compound is absorbed by the organism’s tissue, it is quickly excreted — so it isn’t stored and accumulated in the fish and doesn’t increase in concentration as it travels up the food chain. Most of the chlorpyrifos in fish, shellfish, and other aquatic life is excreted within half a day to just over two days.
Wildlife, birds, fish and other organisms vary greatly in their sensitivity to chlorpyrifos. Many insect species, including important pests, are extremely sensitive to chlorpyrifos and this provides a broad-spectrum of control. Honeybees which are directly exposed to chlorpyrifos may also be highly sensitive and managing applications so as to avoid direct exposures (e.g., not spraying during crop blooming) is an important use practice. Under laboratory test conditions, sensitivity of birds, mammals and fish to chlorpyrifos ranges from moderate to high, whereas sensitivity of water fleas, shrimp, and other aquatic life ranges from high to very high for all pesticides. The significance of these levels of sensitivity for wildlife populations depends on the nature, magnitude, and duration of exposures, which in turn are influenced by use practices and characteristics of the pesticide as well as environmental variables.
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Barron M.G. et al. “Absorption, tissue distribution and metabolism of chlorpyrifos in channel catfish following waterborne exposure.” Environmental Toxicology and Chemistry, 1993, Volume 12: 1469–1476.
Barron, M.G. and Woodburn, K.B. “Ecotoxicology of Chlorpyrifos.” Reviews of Environmental Contamination and Toxicology, 1995, Volume 144:1-93.