Environmental Health
Integrated Pest Management (IPM)
  IPM - Pesticide Review Process  


Thurston County Active Ingredient Rating System

Thurston County reviews the active ingredients in pesticides and rates them as one of the following:

Passed: Indicates low hazard; the County’s first choice for product selection.

Conditional: Contains a known hazard; the County’s second choice for product selection. Hazard(s) may be minimized by application method or location.

Failed: Indicates high hazard; the County’s last choice for product selection if no lesser hazard product is known to be effective to control pest problem.

Pesticide active ingredients with one of the following elements receive a “Failed” review rating:

  • High hazard for persistence along with high hazard for bioaccumulation potential.
  • Produces reproductive toxicity in laboratory testing that is not a result of maternal toxicity.
  • Produces developmental toxicity in laboratory testing that is not a result of maternal toxicity.
  • Contains ingredient with a cancer designation of:
    • EPA Group A: Human carcinogen
    • EPA Group B: Probable human carcinogen
    • EPA Group C: Possible human carcinogen
    • EPA “Likely to be carcinogenic to humans” or “Known / Likely”
    • International Agency for Research on Cancer (IARC) Group 1, Group 2A, or Group 2B
    • National Institute of Environmental Health Sciences (NTP) designation - “Known to be human carcinogens” or “Reasonably anticipated to be human carcinogens”
  • Contains ingredient with positive evidence of mutagenicity or genotoxicity in a test evaluated and accepted by the EPA, World Health Organization, Health Canada, European Union, National Institute of Health, National Library of Medicine, or referenced peer-reviewed studies evaluated by an expert scientific panel (like the information within the Hazardous Substances Data Bank).

Elements for an active ingredient to receive a “Conditional” review rating:

  • Active ingredient or toxic degradation chemical rates high in hazard for persistence and mobility but low in hazard for human and wildlife toxicity.
  • Risk assessment calculates potential human exposure to applied pesticide to be equal or greater than 10%, but less than 50%, of the EPA’s level of concern for toxicity.

Elements for an active ingredient to receive a “Passed” review rating:

  • Contains none of the criteria listed in the conditional or failed rating.
  • Risk assessment calculates potential human exposure to applied pesticide to be less than 10% of the EPA’s level of concern for toxicity.
  • Risk assessment for active ingredient has at least ten-fold safety factor above the EPA’s level of concern for human acute or chronic toxicity.
  • Risk assessment for active ingredient does not exceed the EPA’s level of concern for non-target organisms.

Thurston County's Pesticide Review Process

Thurston County’s Integrated Pest and Vegetation Management Policy (IPM Policy) requires a review of all pesticide products recommended for use by County departments. The review process ensures that only pesticides with the lowest possible hazard are used.

The following outlines the review process used by the Environmental Health Division to identify potential hazards associated with specific pesticide product active ingredients. A pesticide evaluation is complete after the product’s material safety data sheet has been reviewed to determine if any other known ingredients have a hazard that is unacceptable according to the County’s IPM policy. Product-specific information is not available on the County’s website.

Human Toxicity

Thurston County’s IPM policy states that pesticide chemicals have an unacceptable degree of hazard if they have the characteristics of carcinogenicity, mutagenicity, reproductive toxicity, or developmental toxicity. To ensure that pesticide chemicals do not pose an unacceptable degree of hazard, each active ingredient (and any degradation chemical of toxicological concern) is reviewed for known toxic effects. These toxicity hazard reviews document a chemical’s potential to cause mutagenicity, reproductive or developmental toxicity (not caused by maternal toxicity), and carcinogenicity.

Not all chemicals produce a concern for carcinogenicity, mutagenicity, developmental or reproductive toxicity, although exposure to them may cause other toxicities. To rate the hazard of these other potential toxic effects, an evaluation of human health risk assessments is performed. A risk assessment evaluates the toxic effects observed in laboratory testing from ingestion, skin absorption, and inhalation exposures and compares the toxic dose concentration to potential exposures from expected environmental concentrations (EEC) following pesticide use. These risk assessments determine if expected pesticide uses could result in exposures that exceed a level of concern. A level of concern is calculated to include safety factors so that the level of concern is conservative enough to provide a margin of safety from potential exposures to a pesticide chemical and the dose that could cause toxicity. Nearly all pesticide chemical registrations by the EPA include a risk assessment evaluation (or a waiver from the requirement for specific chemicals and uses).

Thurston County uses risk assessments to identify which pesticides create an expected environmental concentration that is at, or near, concentrations that create an adverse effect in laboratory testing. Risk assessments are performed for various lengths of potential exposures; acute (one-time exposure), short-term (exposures up to a week in duration), intermediate-term exposures (longer than a week, but less than a year), and chronic or long-term exposures that can represent occupational or lifetime exposures. Risk assessments also calculate an exposure concentration for each potential route of exposure (ingestion, inhalation, and skin absorption). These potential exposures are calculated for maximum application rates for proposed use areas (residential, indoor, turf, crops, etc.), type of application equipment, and expected types of people exposed (applicator, adults, and children). These calculated exposures are then compared to a dose of concern. Thurston County establishes a dose of concern by using the no observable adverse effect level concentration (NOAEL), which is the highest dose that DOES NOT produce an adverse effect in laboratory testing or a benchmark dose limit (BMDL), divided by the safety factor, which is established by the risk assessor. Typically the safety factor for human risk assessments is 100, 300, or 1,000, depending on the type and severity of the adverse effect, the type of animal tested, etc. The difference between the dose of concern and the calculated exposure is the margin of safety.

  In laboratory testing of chemical X on rats, no adverse effects were observed at dose concentrations of 1 milligram of chemical X per kilogram of animal weight (mg/kg). But, the animals tested had abnormal skeletal formation at dose concentrations of 10 mg/kg. Therefore, the no observable adverse effect concentration (NOAEL) was determined to be 1mg/kg. The risk assessor then placed an uncertainty factor of 10 for using animal test data instead of human test data (interspecies extrapolation) and another uncertainty factor of 10 for the potential differences between the same type of animal (intraspecies variability), for a total uncertainty value of 100. Therefore, the dose of concern is 1mg/kg divided by 100 for a value of 0.01mg/kg. So, if chemical X is applied to a lawn and a child playing on the lawn could get a calculated exposure (dose) of chemical X equal to 0.005mg/kg, then that dose would be 50% of the dose of concern and the margin of safety would be 2.  

Pesticide active ingredients with a small margin of safety (less than 2) between the calculated exposure and the dose of concern are rated high in hazard. Active ingredients with a margin of safety at least 10 times greater than the calculated dose of concern are rated low in hazard. Pesticides that have the potential to create exposures that are calculated to have margins of safety between two and ten times the dose of concern are rated moderate in hazard.

When evaluating risk assessments, the EPA begins with a highly conservative “Tier 1” risk assessment. Tier I assessments use many general assumptions (default values for dermal absorption, residue transfer coefficients, etc.) and are used as a first round screen for risk. If a Tier I risk assessment calculates an exposure that exceeds the level of concern, then a more refined “Tier II” risk assessment may be performed using test data specific to the chemical proposed for use. Tier II risk assessments are preferred for Thurston County reviews when they are available.

Mobility Hazard

Pesticide mobility rates the potential of an active ingredient (or other chemical of concern) to move away from the site of application with water. These reviews look at an active ingredient’s potential to move when introduced to the environment and do not take formulation types (oil-based liquid, water-based liquid, pellet, etc.) into account.

Mobility rating assessments include the following: a) water solubility, b) adsorption to inorganic soil - Kd value (sand and gravel) and c) adsorption to organic soil - Koc value (clay, sediment, etc.). In general, pesticides with the greatest mobility potential have high solubility values and low adsorption values. However, a chemical that is very soluble in water that also has a very high soil adsorption value, are rated more on adsorption potential than solubility.

Water Solubility

Solubility is a measure of a chemical’s ability to dissolve in a liquid. To evaluate chemical mobility potential, Thurston County looks at the water solubility of an active ingredient. Water soluble chemicals are more likely to move with rain or irrigation water off the site of application. Typically, solubility is measured in milligrams per liter (mg/L), but are sometimes expressed in parts per million (ppm).

Water Solubility (mg/L or ppm) County Rating
< 10 Low
10 - 1,000 Moderate
> 1000 High

Chemical Adsorption to Organic Soil

In a chemical adsorption test, a pesticide solution is added to soil and the soil organic partition coefficient (Koc) is determined by the amount of pesticide adsorbed to the soil compared to amount of pesticide remaining in the liquid. The unit of measure for Koc is expressed in milliliters per gram of carbon (ml/g). The more chemical that adheres to the soil the higher the Koc value), the less likely the chemical is to move in soil. Chemical adsorption varies with soil type and the amount of organic matter the soil contains. This rating reflects the potential for chemical mobility without site-specific soil data.

Koc Value County Rating
> 5,000 Low
5,000 - 500 Moderate
< 500 High

Chemical Sorption to Inorganic Soil

The soil partition coefficient (Kd) is the ratio of adsorbed chemical to inorganic soil to of the amount of pesticide remaining in the liquid solution. The Kd value reflects a chemical’s ability to attach to inorganic soil. Typically, Kd values are much lower than Koc values and usually represent the worst-case scenario for chemical mobility. Most chemicals applied to soil without organic material are likely to soak into the ground with rain or irrigation water (although there are exceptions).

Kd Value County Rating
> 100 Low
100 - 5 Moderate
< 5 High

Persistence Hazard

Persistence evaluates the time it takes for a pesticide to break down (degrade) or be removed (dissipate) from the environment. Persistence is rated on professional judgment (selection) of the most likely combination of the routes of degradation or dissipation based on expected use (e.g. herbicides used on land plants will be rated on terrestrial field dissipation, aerobic dissipation, and potentially photolysis and volatilization).

Evaluation of persistence includes the chemical that has the toxic mode of action to the intended pest and also any degradation chemical that is included in the risk assessment. For example, the active ingredient triclopyr triethylamine (TEA) will dissociate to triclopyr acid shortly after it is applied. This chemical conversion may be so rapid that the original form (triclopyr TEA) is not included in the risk assessment. Or, the degradation may be slow; resulting in both the original chemical and the degradation chemical being included in the risk assessment. The persistence hazard is rated on all of the chemicals included in either the human or wildlife risk assessment. The persistence rating of “low” to “high” will consider the following criteria:


Volatilization is scored using a chemical’s vapor pressure. The greater a chemicals vapor pressure the more likely it will evaporate into the air. Since vapor pressure is greatly influenced by temperature, the score should is based on a vapor pressure at 25° Celsius.

Vapor Pressure (mmHg) County Rating
> 0.0001 Low
<0.0001 but >0.0000001 Moderate
< 0.0000001 High

Chemical Half-life

To better understand the likely persistence of a pesticide, several degradation routes are evaluated. Degradation routes are then compared to the environment the pesticide is expected to be used in to best predict what is most likely to occur. The following chemical half-life time scale is used to rate all forms of degradation.

Half-life (Days) County Rating
1 - 7 Low
8 - 60 Moderate
< 60 High

Dissipation Rate – terrestrial and aquatic (field tests)

Field dissipation studies rate the expected half-life of a product in similar environmental conditions as the proposed application area (terrestrial or aquatic). As much as possible, the score is based on chemical breakdown, not off-site migration or dilution.


Hydrolysis is scored on the time it takes for a chemical to break down by interacting with water. Hydrolysis and photolysis are important routes of abiotic degradation in aquatic environments.


Photolysis is scored on the time it takes for a chemical to degrade in sunlight when it is applied to soil or water.

Biotic (aerobic) Degradation

Biotic degradation is scored on the amount of time it takes for a chemical to be broken down by microbial activity in a soil containing oxygen (aerobic).

Anaerobic Degradation

Anaerobic degradation half-life is the time it takes for a chemical to break down to half of its original concentration in an environment without oxygen. If a pesticide is used in an aquatic environment, anaerobic degradation reflects chemical’s breakdown in sediment. Pesticides that can leach into soils due to high mobility are likely to degrade anaerobically at depths of two feet or more below the ground’s surface.

Bioaccumulation Hazard

Bioaccumulation is an increase of a chemical within an organism from eating and absorbing the chemical from a surrounding medium (ex. water) at a rate that is faster than its ability to metabolize, depurate, or excrete it. Bioaccumulation of pesticide chemicals can result from contaminated air, water, soil, sediment, or food. Chemicals that bind strongly to oil and fats and are very slowly metabolized or excreted, tend to bioaccumulate.

Bioaccumulation factor values (BAF) are not always found, and may be substituted by bioconcentration factor values (BCF) from fish studies or a chemical’s octanol-water partition coefficient (Kow). Bioconcentration values determined from fish studies only take into account chemical accumulation absorbed from contaminated water and not contaminated food (Reference 3). The best BCF studies include a test for depuration, which evaluates how much of an accumulated chemical is be removed from the fish when it is moved to clean water. Fast rates of depuration infer that accumulation will be minimal if the chemical is not persistent in water. Animal metabolism studies are reviewed to determine if a chemical is quickly metabolized and excreted. Quick elimination of a chemical from an animal (50% elimination within one week) indicates that even though there is accumulation, it is short-lived and the bioaccumulation hazard is lessened. If there are no bioaccumulation or bioconcentration studies, then a chemical’s octanol/water partition coefficient can be used as a rough estimate for accumulation potential (preferably combined with an animal metabolism assessment).

Bioaccumulation factor or Bioconcentration factor

To be considered likely to bioaccumulate through the food chain, a substance must be characterized by either a BAF value greater than 1,000 or a BCF value greater than 5,000. BCF and Kow values are rated less conservatively because they are less accurate measures of bioaccumulation potential.

Octanol-Water Partition Coefficient (Kow)

The octanol-water partition coefficient (Kow) is used to estimate a chemical’s likelihood to bind to organic substances like fat and oil. The Kow value (which is documented in the logarithmic scale) is measured by adding a chemical to a mixture of water and octanol (an organic solvent) and determining the ratio of chemical within each medium. The more chemical found in the octanol, the larger the Kow value. A very large Kow value implies a greater attraction to organic matter and a greater potential for bioaccumulation. Because Kow values are used to estimate bioaccumulation potential without any fish or animal testing, they are the least accurate measure used for predicting bioaccumulation potential.

BAF BCF log Kow value County Rating
< 100 < 100 < 2 Low
100 - 999 >100 - 4,999 2 - 4.9 Moderate
> 999 > 4,999 . 4.9 High

Non-Target Organism Hazard (Ecotoxicity)

To help select pesticides that are expected to have minimal impact to non-target organisms, Thurston County reviews ecological risk assessments. These risk assessments compare expected chemical concentrations from registered (allowable) non-agricultural pesticide uses to concentrations that cause adverse effects in non-target mammals, birds, and fish in laboratory testing. Although the hazard rating for potential impacts to non-target organisms is only based on an evaluation of ecological risk assessments, Thurston County displays the hazard to pets and wildlife in review fact sheets in two ways. First, the acute (one-time exposure) toxicity categories to animal species (mammals, birds, fish and other aquatic organisms) are documented and second, ecological risk assessments for each species type are evaluated for pesticide applications.

The EPA uses the toxicity category of active ingredients to determine the environmental hazard statement needed for pesticide products containing them. Thurston County displays these toxicity categories to help the user understand why there is a pesticide label precaution. Thurston County uses risk assessments to rate the hazards of using a pesticide to pets, birds, mammals, fish and other aquatic organisms.

Ecological Risk Assessment

Ecological risk assessments evaluate the potential impacts a pesticide chemical may have on pets and wildlife. Ecological risk assessments are performed like human risk assessments, but the dose of concern may be based on very limited data for individual species. Often toxicity data for one type of animal is used to represent the toxicity potential to all other animals. For instance, toxicity studies using rats are often used to assess the risk to all terrestrial animals, and one species of freshwater fish may be used to evaluate the risk to all freshwater species. But, calculating a dose of concern and applying a safety factor is the same as it is for human risk assessments.

First, a dose of concern is determined for a species group (EPA species groups; small mammals, large mammals, birds, fish, and aquatic invertebrates), then the dose of concern is compared to the expected environmental concentration that the species group is likely to be exposed to. The risk is determined by the difference between the dose of concern and the potential dose the species group may be exposed to.

For each species group there are levels of concern calculated for the length of expected exposures; the first level is for short-term exposures (acute risk); the second level is for long-term exposures (chronic risk) to the species. The risk assessment evaluation includes a numerical margin of safety to ensure that the potential exposure to a species is lower than the dose of concern. Short-term exposure risk assessments include a safety factor of 2 from the dose that kills half of the individuals within a species group (LD50 or LC50). The resulting dose of concern for short-term risk assessment is half (50%) of the LD50 value for the species group being evaluated. Risk assessments for endangered species evaluation have a safety factor of 10 from the LD50 value, resulting in a dose of concern that is 10% of the LD50. Chronic risk (from long-term or life-long exposures) is typically evaluated by comparing the expected environmental concentration (EEC) to the no observable adverse effect concentration (NOAEC), which is determined by laboratory testing of animals within the species. The resulting dose of concern is equal to the NOAEC. Chronic risk is evaluated when the chemical of concern is expected to be available for a lifelong diet due to chemical persistence and/or expected re-applications to the same area.

Thurston County rates the hazard for ecological risk based on the assessment for the general species population. When pesticides are proposed for use in an area with a known threatened or endangered species, a re-evaluation of risk will be performed utilizing the appropriate safety factor.

Levels of Concern and Associated Risk - Mammals, Birds, Fish and Aquatic Invertebrates
Pesticide concentration compared to dose to wildlife Risk Determination
Expected Environmental Concentration > Dose of Concern (50% LD50 or LC50) High acute risk
Expected Environmental Concentration > Dose of Concern (10% LD50 or LC50) High risk for endangered species only
Potential Long-term Exposure to Wildlife > Chronic No Observable Adverse Effect Concentration High chronic risk

Terrestrial Animal Toxicity

The following table rates lethal dose concentrations to 50% of the tested rats (LD50) and is used to evaluate mammalian toxicity. When both oral and dermal toxicity values are found, the worst-case of the two toxicity hazard values is recorded.

Oral LD50 (mg/kg) Dermal LD50 (mg/kg) Toxicity Category County Rating
> 2,000 >20,000 Practically non-toxic Low
51 - 2,000 200 - 20,000 Slightly or moderately toxic Moderate
< 50 < 200 Highly toxic High

Bird Toxicity, Bee Toxicity and Aquatic Toxicity

Avian studies also use lethal dose concentrations (LD50) to indicate the acute toxicity of the chemical in a bird’s diet. Toxicity studies of aquatic organisms generally use lethal chemical concentration (LC50) to indicate acute toxicity (because the chemical is not put into their diet, instead it is added to the water they live in).

Avian  Fish & Aquatic Organisms Insects Worms    
LD50 (mg/kg) LC50 (mg/L) (ug/bee) (mg/kg) Toxicity Category County Rating
> 2,000 > 100 > 11 > 1,000 Practically non-toxic Low
50 - 2,000 1 -100 2 - 11 10 - 1,000 Moderately toxic Moderate
< 50 < 1  < 2  < 10 Highly toxic High

References and Supporting Documents:

  1. European Union – Global Harmonization System (the following information was copied from… to help the reviewer gain a better understanding of mutagenicity testing and evaluation.)
  2. Franke, C., et al. Federal Environmental Agency (Umweltbundesamt). The Assessment of Bioaccumulation. Chemosphere, Vol. 29, No.7. 1994.
  3. National Pesticide Information Center (NPIC) – “Signal Words” Topic Fact Sheet. Revised July 2008.
  4. Purdue University Cooperative Extension Service, Purdue Pesticide Programs. Pesticide Toxicology, Evaluating Safety and Risk. Reviewed 3/03.
  5. Stockholm Convention. Persistent Organic Pollutants Review Committee. Guidance Material on New POPS, Draft Version. October 2009.
  6. The University of Hertfordshire Agricultural Substances Database Background and Support Information. http://sitem.herts.ac.uk/aeru/iupac/
  7. USEPA. Label Review Manual. Revised July 2011. http://www.epa.gov/oppfead1/labeling/lrm/
  8. USEPA. Office of Prevention, Pesticides and Toxic Substances. Corrected version of EFED's eco portion of SRRD's April 2001 Disulfoton IRED. March 25, 2002.
  9. USEPA. Office of the Federal Register. 40 CFR 156.64: Toxicity Category13 Code of Federal Regulations. Retrieved 11/26/2009.
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This page last updated: 10/12/17