Pesticide Formulations, Toxicity, and Environmental Fate
Objective. Students will classify common pesticide formulations by physical form and carrier, explain the signal-word toxicity scale, and describe how pesticides move through and break down in the environment.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.
VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.
What Is a Pesticide Formulation?
A pesticide is rarely sold as a pure active ingredient. Manufacturers combine the active ingredient with carriers, solvents, emulsifiers, and other additives to create a formulation — a product that is safer to handle, easier to measure, and effective when applied. Understanding formulations helps you select the right product, mix it correctly, and protect yourself and the environment.
Common Formulation Types
Emulsifiable Concentrates (EC or E) are liquids in which the active ingredient is dissolved in an organic solvent with an emulsifier added. When you add water, the mixture turns milky. ECs are easy to measure and mix but can be absorbed through the skin quickly; wear chemical-resistant gloves and eye protection.
Wettable Powders (WP or W) look like dry powder but are designed to be mixed with water and kept in suspension. They require constant agitation in the tank because they do not truly dissolve. WP products can create dusty conditions during mixing — respiratory protection is often needed.
Granules (G) are dry, sand-like particles applied without mixing. The active ingredient is absorbed onto a carrier such as clay. Granules reduce drift and dermal exposure during application but still require gloves; never handle granules barehanded.
Dusts (D) are very fine dry powders applied dry, sometimes directly to plants or soil. Because the particles are so light, inhalation risk is high. A fitted respirator and eye protection are required.
Ready-to-Use (RTU) products are pre-diluted and require no mixing. They are common for small jobs and home use. Although lower-concentration, they still carry label requirements for PPE.
Other formulations you will encounter include soluble concentrates (SC or SL), flowables (F or SC), and microencapsulated (ME or CS) products. Always read the label — it tells you exactly how the product is formulated and how to handle it.
Signal Words and the Toxicity Scale
Every pesticide label registered with the EPA carries a signal word that communicates acute toxicity to humans.
- DANGER / POISON — Highly toxic; a tiny amount can kill or cause serious injury. A skull-and-crossbones symbol also appears.
- WARNING — Moderately toxic; a larger but still hazardous dose causes serious harm.
- CAUTION — Slightly toxic or relatively non-toxic; the largest category by number of products.
The signal word is determined by testing multiple exposure routes — oral, dermal (skin), inhalation, and eye/skin irritation — and reflecting the most severe result. You must read the signal word before every use.
LD50: What It Means and What It Does Not Tell You
LD50 (lethal dose 50) is the dose of a substance that kills 50 percent of a test population, expressed in milligrams of pesticide per kilogram of body weight (mg/kg). A lower LD50 means higher toxicity — less material is needed to cause death.
However, LD50 alone does not determine real-world applicator risk. You must also consider:
- Formulation concentration — a highly diluted product with a toxic active ingredient may pose less risk than a concentrated one.
- Exposure route — a chemical with a low oral LD50 may have a high dermal LD50, making skin contact less dangerous than ingestion.
- Duration and frequency of exposure — chronic low-level exposure can cause harm even when a single dose is far below the LD50.
- PPE effectiveness — proper gloves and respirators reduce actual dose received.
Always treat LD50 as one data point, not the whole picture.
Environmental Fate: Where Does a Pesticide Go?
After application, pesticides do not simply disappear. Several processes determine their fate.
Volatilization — Some pesticides evaporate from soil or plant surfaces and move into the air, potentially drifting off-site or being inhaled.
Runoff — Rainfall or irrigation can carry pesticides dissolved or adsorbed to soil particles into surface water such as streams and ponds. Steep slopes, heavy rain, and compacted soils increase runoff risk.
Leaching — Water moving downward through the soil carries water-soluble pesticides toward groundwater. Sandy soils with low organic matter allow faster leaching. This is a primary pathway to well contamination.
Adsorption — Pesticide molecules bind to soil particles, especially clay and organic matter. High adsorption slows movement but does not eliminate risk; adsorbed pesticides can desorb later.
Degradation — Pesticides break down through microbial activity (the most important pathway in most soils), photodegradation (sunlight), and chemical hydrolysis (reaction with water). Persistence is measured by half-life — the time for half the active ingredient to break down. A long half-life increases the risk of accumulation and off-site movement.
Site Characteristics That Increase Groundwater Risk
- Sandy or gravelly soils (low adsorption, fast drainage)
- Shallow water table
- High annual rainfall
- Proximity to wells or surface water
- Low soil organic matter content
Understanding environmental fate is not just good science — it is required knowledge for the State Pesticide Applicator license and guides legal, responsible application decisions.
Safety Summary
Before handling any pesticide formulation: read the entire label, put on all required PPE, ensure eyewash is accessible, and never eat, drink, or smoke in the application area. Store products in original containers in a locked, ventilated space away from food, feed, and water sources.
Key terms10
Worked examples3
- Recall that a lower LD50 means a smaller dose kills 50% of test animals, so lower = more toxic.
- Pesticide A has the lower LD50 (50 mg/kg), so it is more acutely toxic by ingestion.
- Divide the larger LD50 by the smaller to find the relative toxicity: 1,200 ÷ 50 = 24.
- Interpret: Pesticide A is 24 times more acutely toxic by ingestion than Pesticide B.
- Identify the LD50: 25 mg/kg.
- Multiply by body weight: 25 mg/kg × 80 kg = 2,000 mg.
- Convert to grams for context: 2,000 mg ÷ 1,000 = 2 grams.
- Note the important caveat: LD50 values come from animal studies and cannot be directly applied to humans; this calculation is an educational estimate only and does not predict actual human lethality.
- Identify the number of half-lives in 90 days: 90 days ÷ 30 days per half-life = 3 half-lives.
- After 1 half-life (30 days): 400 g ÷ 2 = 200 g remaining.
- After 2 half-lives (60 days): 200 g ÷ 2 = 100 g remaining.
- After 3 half-lives (90 days): 100 g ÷ 2 = 50 g remaining.
The math in this lesson1
Materials: Sealed labeled pesticide containers (for display only), LD50 data table handout, watershed diagram worksheet, EPA pesticide environmental fate factsheets.
Check: Written quiz (15 questions) on formulation types, signal words, LD50 interpretation, and environmental fate terms; passing score 75%.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
