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Pesticide Half-life

Topic Fact Sheet

What is a pesticide half-life?

A half-life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaks down in the environment. In general, a pesticide will break down to 50% of the original amount after a single half-life. After two half-lives, 25% will remain. About 12% will remain after three half-lives. This continues until the amount remaining is nearly zero. See Figure 1.

Figure 1. Approximate amount of pesticide (shaded area) remaining at the application site over time.

Each pesticide can have many half-lives depending on conditions in the environment. For example, permethrin breaks down at different speeds in soil, in water, on plants, and in homes.

Why is a pesticide's environmental half-life important?

The half-life can help estimate whether or not a pesticide tends to build up in the environment. Pesticide half-lives can be lumped into three groups in order to estimate persistence. These are low (less than 16 day half-life), moderate (16 to 59 days), and high (over 60 days). Pesticides with shorter half-lives tend to build up less because they are much less likely to persist in the environment. In contrast, pesticides with longer half-lives are more likely to build up after repeated applications. This may increase the risk of contaminating nearby surface water, ground water, plants, and animals.

However, pesticides with very short half-lives can have their drawbacks. For example, imagine that a pesticide is needed to control aphids in the garden for several weeks. One application of a pesticide with a half-life of a few hours will probably not be very effective several weeks out. This is because the product would have broken down to near-zero amounts after only a few days. This type of product would likely have to be applied multiple times over those several weeks. This could increase the risk of exposure to people, non-target animals, and plants.

What can influence a pesticide's environmental half-life?

Many things play a role in how long a pesticide remains in the environment. These include things like sunlight, temperature, the presence of oxygen, soil type (sand, clay, etc.), how acidic the soil or water is, and microbe activity. See Table 1. Pesticide half-lives are commonly reported as time ranges. This is because environmental conditions can change over time. This makes it impossible to describe a single, consistent half-life for a pesticide.

A pesticide product's formulation can also change how the active ingredient behaves in the environment. In fact, the properties of the formulation may dominate initially, until enough time has passed to allow the ingredients to separate This is because small amounts of an active ingredient are 'formulated' with larger amounts of 'other' ingredients to make a whole pesticide product.

Table 1. Environmental factors that affect pesticide persistence.4

Environmental Factors Role in Chemical Degradation
Sunlight Radiation from the sun breaks certain chemical bonds, creating break down products.
Microbes Bacteria and fungi can break down chemicals, creating biodegradation products.
Plant / Animal Metabolism Plants and animals can change chemicals into forms that dissolve better in water (metabolites). This makes removal from the body easier.
Water Water breaks chemicals apart to make pieces that dissolve better in water (hydrolysis). This is typically a very slow process.
Dissociation Chemicals can break apart into smaller pieces (dissociation products).
Sorption Chemicals that stick tightly to particles can become inaccessible and/or move away with those particles.
Bioaccumulation Some chemicals can be absorbed by plants/animals from the soil, water, food, and air. When the plant/animal is exposed again before it can remove the chemical(s), accumulation can occur.

How is a pesticide's half-life determined?

Pesticide half-lives are often determined in a laboratory. There, conditions like temperature can be controlled and closely monitored. Soil, water, or plant material is mixed with a known amount of a pesticide. The material is then sampled and tested over time to determine how long it takes for half of the chemical to break down.

Field studies are also performed for some chemicals. A known amount of the pesticide is mixed with soil, water, or plant material. It is then placed in an outdoor environment where it is exposed to various environmental conditions and tested over time. Field studies provide researchers with a more realistic idea of how the pesticide will act in the environment. However, half-life values from such studies can vary greatly depending on the exact conditions. See Figure 2.

Before a pesticide product is registered, manufacturers measure their half-lives. You can find their research results in a variety of databases, books, and peer-reviewed articles. If you need help, call the National Pesticide Information Center.

What happens to pesticides after they "go away"?

When a pesticide breaks down it doesn't disappear. Instead, it forms new chemicals that may be more or less toxic than the original chemical. Generally, they are broken into smaller and smaller pieces until only carbon dioxide, water, and minerals are left. Microbes often play a large role in this process. In addition, some chemicals may not break down initially. Instead, they might move away from their original location. It all depends on the chemical and the environmental conditions.

Inorganic pesticides like iron phosphate and copper sulfate don't break down in the same way as organic pesticides.10,12 The "half-life" concept only applies to organic pesticides, those that contain carbon components.

Figure 2. The soil half-life of five pesticides.8,9,11,13,15

Where can I get more information?

For more detailed information about pesticide half-lives please visit the list of referenced resources below or call the National Pesticide Information Center, Monday - Friday, between 8:00am - 12:00pm Pacific Time (11:00am - 3:00pm Eastern Time) at 1-800-858-7378 or visit us on the web at http://npic.orst.edu. NPIC provides objective, science-based answers to questions about pesticides.

Date Reviewed: May 2015

Please cite as: Hanson, B.; Bond, C.; Buhl, K.; Stone, D. 2015. Pesticide Half-life Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/half-life.html.

References:

  1. Hornsby, A. G.; Wauchope, R. D.; Herner, A. E. Pesticide Properties in the Environment; Springer: New York, 1996, pp 1-3, 7-15.
  2. Mackay, D.; Shiu, W-Y.; Ma, K-C. Physical-Chemical Properties. Illustrated handbook of physical-chemical properties and environmental fate for organic chemicals; CRC Press: Boca Raton, FL, 1997, pp 3-9.
  3. Newman, M. C.; Unger, M. A. Environmental Contaminants. Fundamentals of Ecotoxicology, 2nd ed.; CRC Press: Boca Raton, FL, 2002, pp 24-27, 34-35, 42-43.
  4. Ney Jr., R. E. Fate and Transport of Organic Chemicals in the Environment: A Practical Guide, 2nd ed.; Government Institutes, Inc.: Rockville, MD, 1995, Chapters 1-3.
  5. Environmental Persistence. Design for the Environment Program Alternatives Assessment Criteria for Hazard Evaluation Version 2.0; U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics, U.S. Government Printing Office: Washington, DC, 2011, Chapter 4, Table 12.
  6. Ingredient Statement. Label Review Manual; U.S. Environmental Protection Agency, Office of Prevention, Pesticides and Toxic Substances, Office of Pesticide Programs, U.S. Government Printing Office: Washington, DC, 2012, Chapter 5.
  7. Data Requirements for Pesticides, Product Chemistry. Code of Federal Regulations, Part 158, Title 40, 2015.
  8. Gervais, J. A.; Luukinen, B.; Buhl, K.; Stone, D. 2008. 2,4-D Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/2,4-DTech.html.
  9. Gervais, J. A.; Luukinen, B.; Buhl, K.; Stone, D. 2008. Capsaicin Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/Capsaicintech.html.
  10. Boone, C.; Jervais, G.; Luukinen, B.; Buhl, K.; Stone, D. 2012. Copper Sulfate Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/cuso4tech.html.
  11. Bunch, T. R; Gervais, J. A.; Buhl, K.; Stone, D. 2012. Dicamba Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/dicamba_tech.html.
  12. Buhl, K.; Bond, C.; Stone, D. 2013. Iron Phosphate General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/ironphosphategen.html.
  13. Wick, K.; Bond, C.; Buhl, K.; Stone, D. 2012. Methoprene General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/methogen.html.
  14. Toynton, K.; Luukinen, B.; Buhl, K.; Stone, D. 2009. Permethrin Technical Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/Permtech.html.
  15. Bond, C.; Buhl, K.; Stone, D. 2014. Pyrethrins General Fact Sheet; National Pesticide Information Center, Oregon State University Extension Services. http://npic.orst.edu/factsheets/pyrethrins.html.

NPIC fact sheets are designed to answer questions that are commonly asked by the general public about pesticides that are regulated by the U.S. Environmental Protection Agency (U.S. EPA). This document is intended to be educational in nature and helpful to consumers for making decisions about pesticide use.

Related Topics:

What are pests?

Learn about a pest

Identify a pest

Control a pest

Integrated Pest Management

What are pesticides?

Herbicides

Disinfectants

Fungicides

Insecticides

Natural and Biological Pesticides

Repellents

Rodenticides

Other types of pesticides

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