Understanding Pesticides

When selecting or trying to decipher pesticide/ insecticide labels here are some points of reference:

Active Ingredient (AI)

Active Ingredient is the term used to describe the active chemical compound within a herbicide, pesticide, or insecticide. Legally this needs to be displayed on the container and is sometimes expressed as a percentage or in milliliters per liter or grams per kilogram.

An example of this would be Confidor (insecticide) where the AI is 200 grams per liter of product – the balance consists of dispersants and suspensions.

LD50 - Lethal Dose 50

Poisons are classified according to their toxicity under standard laboratory conditions, a standard set of tests are applied to provide a baseline for comparison. The LD50 is the unit of measurement for this (Lethal Dose 50%); where 50% of a population of white laboratory-bred rats would be expected to die when administered/ exposed to a fixed amount of the Active Ingredient.

This amount is expressed in milligrams per kilogram body mass or Parts per Million; this can then be extrapolated to provide a guideline in the case of human poisonings by estimating the amount of poison consumed in relation to the body weight of the patient.

Poisons with a lower LD50 are therefore more toxic, as they require less active ingredients to provide the probability of a 50% fatal result.

• In fact, most substances (not just poisons) have a calculated LD50, here are a few common examples:

  • Nicotine 0.8mg/kg

  • Capsaicin 47mg/kg (found in pepper)

  • Caffeine 192mg/ kg

  • Vitamin C 11,900mg/kg

  • Glucose 25,800 mg/kg

  • Water 90,000mg/ kg (or 90ml per kilogram)

Oral and dermal LD50

Poisoning can take place through the skin (dermal) or by ingestion (oral). As the skin is slightly porous, chemicals can be absorbed through it, in most instances, the dermal LD50 is significantly higher than the oral LD50 for the same Active Ingredient. Dermal absorption can take place simply through working with the chemical, it is therefore prudent to wear plastic/ rubber gloves when working with particularly hazardous chemicals.

Cumulative poisoning

Certain poisonous compounds once absorbed into the body are not broken down or metabolized, in particular, Organophosphates; these are stored in the fatty tissue of the body and accumulate over time until they interfere with the body's functions. Most Organophosphates are Class 1 poisons and have a low LD50. The use of Organophosphates pesticides is banned in most developed countries. Examples of Organophosphates include Parathion, Chlorpyrifos, and Diazinon - all now banned.

Fumigants

There is one specific product that I would like to mention here, this is Phostoxin (Active Ingredient aluminum phosphide). The reason this product warrants a mention is its toxicity, this is not a product to take lightly, and legally it may not even be sold to non-registered pesticide applicators, although from time to time there are instances where it has been sold to members of the public with dire and lethal consequences. The product is registered for use against storage pests (for example in grain silos). It is not under any circumstances to be used in any residential application. The LD50 of the product is 9mg/Kg if inhaled, a single tablet will kill everyone within a reasonable-sized room; sadly this has happened on occasion where pesticide applicators have used this product to fumigate rats in buildings, only to poison the residents (in some cases fatally) in adjacent apartments.

Having used this product in a past life I can attest that even the faintest whiff of the fumes is enough to make you light-headed and will leave you with a splitting headache; besides this, the tablets will combust if they are wet or come into contact with water or high humidity.

Poisoning in humans

This is not a simple matter and cannot be correlated 100% to laboratory results; factors such as age, metabolism, body fat, body mass, illness, allergies, type of Active Ingredient, resistance, etc. all can contribute to poisoning.

Scale of toxicity

All poisons are classified according to their potential toxicity, (in accordance with the laboratory LD50) and assigned a Class according to the Hodge-Sterner Scale.

Class of Poison	LD50 Value
Class 1	Extremely poisonous – LD50 of 1 mg/Kg or less
Class 2	Highly Toxic – LD50 of between 1 and 50 mg/Kg
Class 3	​Moderately Toxic – LD50 of between 50 and 500 mg/Kg
Class 4	​Slightly Toxic – LD50 of between 500 and 5,000 mg/Kg
Class 5	​Practically non-toxic – LD50 of between 5,000 and 15,000 mg/Kg
Class 6	​Relatively harmless - LD50 over 15,000 mg/Kg