A pesticide is any substance used to control pests, such as target insects, vegetation, or fungi, typically by poisoning them.
Unfortunately, pesticides can also be poisonous to humans, with some being highly toxic and capable of causing serious injury or death, while others are relatively non-toxic.
Pesticides can irritate the skin, eyes, nose, or mouth. Caution must always be exercised when working with any pesticide.
All pesticides must be toxic to be effective against targeted pests, making them potentially hazardous to humans, animals, other organisms, and the environment.
Therefore, individuals using pesticides or regularly in contact with them must understand the relative toxicity and potential health effects of these products.
The toxicity of a substance is its capacity to cause injury or illness to a living system, which may include a human body, its parts (e.g., lungs or respiratory system), or ecosystems like ponds or forests and their inhabitants.
Toxicity indicates the kind and extent of damage a chemical can cause, revealing “how poisonous” it is. The effect of a pesticide depends on several factors, with the dose-time relationship being the most critical.
Dose refers to the quantity of a substance a surface, plant, or animal is exposed to, while time indicates the frequency of exposure.
Thus, the dose-time relationship describes the amount and frequency of exposure to the substance. The toxicity of a pesticide is determined by subjecting test animals to varying dosages of the active ingredient (a.i.) and its formulated products, where the active ingredient is the chemical component controlling the pest.
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Understanding Toxicity and Its Impact on Human Health in Agriculture
The toxicity of a substance is its capacity to cause injury to a living system, such as a human body, its parts (e.g., lungs or respiratory system), or ecosystems like ponds, forests, and their inhabitants.
Toxicity indicates the kind and extent of damage a chemical can cause, revealing “how poisonous” it is. The effect of a pesticide depends on several factors, with the dose-time relationship being paramount.
Dose is the quantity of a substance a surface, plant, or animal is exposed to, and time refers to the frequency of exposure. This relationship results in two types of toxicity acute and chronic that pesticide applicators must understand.
Acute Toxicity in Pesticide Exposure
Acute toxicity refers to how poisonous a pesticide is to a human, animal, or plant after a single short-term exposure, with effects appearing promptly or within 24 hours. It typically results from accidents, suicide attempts, or careless handling.
A pesticide with high acute toxicity is deadly even in small amounts. Acute toxicity levels assess and compare pesticide toxicity and form the basis for warning statements on labels, measured as acute oral, dermal, or inhalation toxicity.
Products are categorized by their relative acute toxicity (LD50 or LC50 values).
1. Toxicity Category I: Highly Toxic Pesticides: Classified as highly toxic based on oral, dermal, or inhalation toxicity, these require the signal words DANGER and POISON in red, with a skull and crossbones symbol, and “PELIGRO” (Spanish for DANGER) on the label.
The acute oral LD50 ranges from a trace to 50 mg/kg, where a few drops could be fatal to a 150-pound person. Some products have DANGER without the skull symbol due to severe skin or eye effects.
2. Toxicity Category II: Moderately Toxic Pesticides: These require WARNING and “AVISO” (Spanish) on labels, with an acute oral LD50 of 50–500 mg/kg, where a teaspoon to an ounce could be fatal to a 150-pound person.
3. Toxicity Categories III and IV: Slightly or Non-Toxic Pesticides: These require CAUTION on labels, with acute oral LD50 valuesHive values greater than 500 mg/kg, where an ounce or more could be fatal to a 150-pound person.
The EPA’s Worker Protection Standard mandates minimum reentry intervals: 12 hours for Category III (CAUTION), 24 hours for Category II (WARNING), and 48 hours for Category I (DANGER) pesticides.
All pesticides, even slightly toxic ones, can be hazardous if label instructions are ignored, and applicators are legally responsible for misuse.
Chronic Toxicity in Agricultural Pesticide Use
Chronic toxicity refers to harmful effects from long-term, low-level exposure to pesticides, affecting the general public and those mixing, loading, applying, or working in treated fields, or through food, water, or air.
It is measured after three months of continuous or occasional exposure in experimental conditions, potentially causing immediate or long-term effects.
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Types of Pesticide Exposure in Agriculture
Pesticide exposure occurs through contact with a pesticide, categorized as acute or chronic.
1. Acute Exposure to Pesticides: A one-time contact, defined as 24 hours or less in studies, acute exposure causes readily detectable effects, often from rapidly absorbed pesticides.
2. Chronic Exposure to Pesticides: Repeated contact over more than three months may produce acute effects per exposure and chronic effects, such as reproductive issues, teratogenic effects (birth defects), carcinogenic or oncogenic effects (cancer or tumors), neurotoxicity, or immunosuppression.
3. Local and Systemic Effects: Local effects occur at the contact site (e.g., skin inflammation, lung irritation), while systemic effects occur elsewhere (e.g., cholinesterase inhibition in the nervous system).
4. Immediate and Delayed Effects: Immediate effects occur upon or shortly after exposure (e.g., sneezing from inhalation), while delayed effects, like tumors, may appear years later.
5. Reversible and Irreversible Effects: Reversible effects (e.g., skin rash, nausea) can be remedied, while irreversible effects (e.g., nervous system damage, cancer) are permanent.
Routes of Pesticide Entry in Agricultural Settings
Pesticides may enter the body through three routes, all potentially causing poisoning, sometimes unnoticed, particularly via skin or lungs.
Dermal Exposure to Pesticides in Agricultural Settings
Wet, dry, or gaseous forms of pesticides can be absorbed through the skin. This may occur if pesticides contact the skin during mixing or application, or if pesticide-contaminated clothing is not promptly removed and properly cleaned before reuse.
Oil- or paste-based pesticides allow greater absorption through the skin than water-based ones. Some pesticides do not penetrate the skin readily, while others are quickly absorbed and can be as dangerous as if swallowed.
Skin varies in its ability to act as a barrier to pesticide absorption, with the eyes, eardrums, scalp, and groin absorbing pesticides more rapidly than other areas.
Damaged or open skin permits much faster pesticide penetration than healthy, intact skin. Once absorbed through the skin, pesticides enter the bloodstream and are distributed throughout the body.
Inhalation Exposure to Pesticides in Agriculture
Pesticides, whether as dusts, spray mist, or fumes, can be inhaled into the lungs during breathing.
Inhalation exposure may occur while mixing wettable powders, dusts, or granules, or during fumigation or spraying without a self-contained breathing apparatus or proper respirator in enclosed or poorly ventilated areas such as greenhouses, apartments, or grain bins.
Larger inhaled particles tend to remain on the throat and nasal passages, while smaller particles can reach the lungs directly.
The number of particles required to cause poisoning depends on the chemical concentration within them. Even dilute pesticides can result in poisoning via inhalation.
Once absorbed through the lung surfaces, chemicals enter the bloodstream and are distributed throughout the body.
Oral Exposure to Pesticides in Agricultural Practices
Pesticides can enter the body through the mouth (ingestion), often when hands are not washed before eating or smoking, or if pesticides are mistakenly stored in food containers.
Ingested materials are primarily absorbed in the small intestine, though absorption can occur anywhere along the gastrointestinal tract. Once absorbed, pesticides enter the bloodstream by various means and circulate throughout the body.
Significance of Pesticide Exposure Routes in Agriculture
Pesticides can cause poisoning regardless of the entry route, with some being toxic via all three routes (e.g., parathion).
Dermal and inhalation routes are likely the most significant for pesticide applicators. Ingestion is less likely to occur intentionally, but inhalation of pesticide particles or dermal contact from splashes or fallout is common.
Healthy skin can slow pesticide absorption, though liquid or oil-based pesticides penetrate more quickly than dry ones. Damaged skin (chapped, cut, or abraded) loses its protective ability, allowing faster pesticide entry.
Measuring Acute Pesticide Toxicity in Agricultural Applications
Acute toxicity is determined by examining dermal, inhalation, and oral toxicity, as well as eye and skin irritation, in test animals.
Acute toxicity measures the amount or concentration of a toxicant (active ingredient) required to kill 50% of a test population, expressed as LD50 (lethal dose 50) or LC50 (lethal concentration 50).
To measure acute toxicity, a specific pesticide dose is administered (orally or dermally) to laboratory animals of the same species, age, and size, caged under identical conditions, and observed for 14 days for changes.
1. Lethal Dose Fifty (LD50) in Pesticide Evaluation: LD50 represents the dose killing 50% of test animals, measured for oral and dermal routes. A lower LD50 indicates a more poisonous pesticide (e.g., a dermal LD50 of 25 mg/kg is more toxic than 2000 mg/kg). LD50 is expressed as milligrams of pesticide per kilogram of body weight (mg/kg), equivalent to parts per million (ppm).
2. Lethal Concentration Fifty (LC50) for Inhalation: LC50 measures the pesticide concentration in air that kills 50% of test animals, expressed in milligrams per liter (mg/l), ppm, or milligrams per cubic meter (mg/m3). A lower LC50 indicates higher toxicity.
Chronic Toxicity Assessment in Agricultural Pesticide Use
No standard measure like LD50 exists for chronic toxicity. Chronic toxicity is assessed by exposing test animals to low, repeated doses of the active ingredient over days, months, or years.
Chronic effects include birth defects, tumors, blood disorders, and neurotoxic effects. Organophosphates and carbamates can slowly poison by inhibiting cholinesterase, measurable through blood cholinesterase levels.
Regular monitoring of cholinesterase levels in hospitals is advisable for those working with these pesticides.
Toxicity categories, based on LD50, LC50, and irritation effects, assign signal words (DANGER, WARNING, CAUTION) to alert users to a pesticide’s acute toxicity.
Categories of Acute Pesticide Toxicity for Agricultural Safety
The following table outlines the four categories of pesticide toxicity:
CategorySignal Word Required on LabelOral LD50 (mg/kg)Dermal LD50 (mg/kg)Inhalation LC50 (mg/l)Approximate Oral Dose That Can Kill an Average PersonHighly Toxic Class IDANGER-POISON! [Skull & Crossbones]*0–500–2000–0.2A few drops to 1 teaspoonful [or a few drops on the skin]Moderately Toxic Class IIWARNING!50–500200–20000.2–2Over 1 teaspoonful to 1 ounceSlightly Toxic Class IIICAUTION!!500–50002000–20,0002.0–20Over 1 ounce to 1 pint or 1 poundRelatively Non-Toxic Class IVCAUTION!!>5000>20,000>20Over 1 pint or 1 pound
Not used for skin and eye irritation effects.
Insecticides are classified by toxicity based on LD50 values:
1. Highly Toxic: AO LD50 = 0–50 mg/kg, AD LD50 = 0–200 mg/kg, IT LC50 = 0–2000 µg/l; DANGER, skull and crossbones, POISON on label.
2. Moderately Toxic: AO LD50 = 51–500 mg/kg, AD LD50 = 201–2000 mg/kg, IT LC50 = 2001–20,000 µg/l; WARNING on label.
3. Slightly Toxic: AO LD50 = 501–5000 mg/kg, AD LD50 = 2000–20,000 mg/kg, IT LC50 > 20,000 µg/l.
4. Relatively Non-Toxic: AO LD50 > 5000 mg/kg, AD LD50 > 20,000 mg/kg.
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