Okada Ecotech - The Environmental Friendly People

The Dose Makes the Poison

"What is it that is not poison? All things are poison and nothing is without poison.
It is the dose only that makes a thing not a poison."
Paracelsus (1493-1541)

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Understanding Pesticide Toxicity

All synthetic pesticides are designed to control pests, but they can also be toxic (poisonous) to desirable plants and animals, including humans. Some pesticides are so highly toxic that very small quantities can kill a person, while exposure to a sufficient amount of most pesticides can make a person ill. Since even fairly safe pesticides can irritate the skin, eyes, nose, or mouth, it is a good idea to understand how pesticides can be toxic so you can follow practices designed to reduce or eliminate your exposure and the exposure of others to them.

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How Pesticides Enter the Body

Before a pesticide can harm you it must be taken into the body. Pesticides can enter the body orally (through the mouth and digestive system), dermally (through the skin) or by inhalation (through the nose and respiratory system).

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Oral Exposure

Oral exposure may occur because of an accident, but is more likely to occur as the result of carelessness, such as blowing out a plugged nozzle with your mouth, smoking or eating without washing your hands after using a pesticide, splashing concentrate while mixing, or eating fruit that has been recently sprayed with a pesticide containing residues above the tolerance set for the commodity by the relevant authorities. The seriousness of the exposure depends upon the oral toxicity of the material and the amount swallowed.

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Dermal Exposure

Dermal (skin) exposure accounts for about 90% of the exposure pesticide users receive from non-fumigant pesticides. It may occur any time a pesticide is mixed, applied, or handled, and it often goes undetected.

The seriousness of dermal exposure depends upon:

• the dermal toxicity of the pesticide;
• rate of absorption through the skin;
• the size of the skin area contaminated;
• the length of time the material is in contact with the skin; and
• the amount of pesticide on the skin.

Rates of absorption through the skin are different for different parts of the body (Table 1). Absorption continues to take place on all of the affected skin area as long as the pesticide is in contact with the skin. The seriousness of the exposure is increased if the contaminated area is large or if the material remains on the skin for a period of time.

Table 1. Absorption rates through the skin on various bodily regions

Body region	Percent relative absorption
Forearm	8.6
Palm of hands	11.8
Ball of foot	13.5
Abdomen	18.4
Scalp	32.1
Forehead	36.3
Ear canal	46.5
Genitalia	100

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Inhalation Exposure

Inhalation exposure results from breathing pesticide vapors, dust, or spray particles. Like oral and dermal exposure, inhalation exposure is more serious with some pesticides than with others, particularly fumigant pesticides, which form gases.

Inhalation exposure can occur by breathing smoke from burning containers; breathing fumes from pesticides while applying them without protective equipment; and inhaling fumes while mixing and pouring pesticides. Some pesticides will have statements on their labels requiring the use of a specified respirator. Another means of inhalation exposure is smoking tobacco products containing pesticide residues.

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Toxicity

Toxicity refers to the ability of a chemical substance to damage body organs or disrupt the metabolism of living tissues. These adverse effects may range from slight symptoms such as headaches to severe symptoms like coma, convulsions, or death. Poisons work by altering normal body functions. Most toxic effects are naturally reversible and do not cause permanent damage if prompt medical treatment is sought. Some poisons, however, cause irreversible (permanent) damage.

Toxicity is usually divided into two types, acute or chronic, based on the number of exposures to a poison and the time it takes for toxic symptoms to develop. Acute toxicity is due to short-term exposure and happens within a relatively short period of time, whereas chronic exposure is due to repeated or long-term exposure and happens over a longer period (Table 2).

Table 2. Types of toxicity

Type	Number of Exposures	Time for symptoms to develop
Acute	Usually 1	Immediate (minutes to hours)
Chronic	More than a few	One week to years

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Acute Toxicity

The acute toxicity of a chemical refers to its ability to do systemic damage as a result of a one-time exposure to relatively large amounts of the chemical. A pesticide with a high acute toxicity may be deadly if even a very small amount is absorbed. The signal words on the label (Table 3) are based on the acute toxicity of the pesticide. Acute toxicity may be measured as acute oral (through the mouth), acute dermal (through the skin) and acute inhalation (through the lungs or respiratory system).

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Acute Toxicity Measurements

The commonly used term to describe acute toxicity is LD₅₀. LD means lethal dose (deadly amount) and the subscript 50 means that the dose was acutely lethal to 50% of the animals to which the chemical was administered under controlled laboratory conditions. The test animals are given specific amounts of the chemical in either one oral dose or by a single injection, and are then observed for a specified time.

The lower the LD₅₀ value, the more acutely toxic is the pesticide. Therefore, a pesticide with an oral LD₅₀ of 500 mg/kg would be much less toxic than a pesticide with an LD₅₀ of 5 mg/kg. LD₅₀ values are expressed as milligrams per kilogram (mg/kg), which means milligrams of chemical per kilogram of body weight of the animal.

LD₅₀ values are generally expressed on the basis of active ingredient. If a commercial product is formulated to contain 50 percent active ingredient, it would take two parts of the material to make one part of the active ingredient. In some cases, other chemicals mixed with the active ingredient for formulating the pesticide product may cause the toxicity to differ from that of the active ingredient alone.

Acute inhalation toxicity is measured by LC₅₀. LC means lethal concentration. Concentration is used instead of dose because the amount of pesticide inhaled in the air is being measured. LC50 values are measured in milligrams per liter. The lower the LC₅₀ value, the more toxic is the pesticide.

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Label Identification of Acute Toxicity

It is important to read the label to look for signal words identifying the product's acute toxicity. To alert pesticide users to the acute toxicity of a pesticide, a signal word may appear on the label. Four different categories are used (Table 3). Signal words are used to tell the user whether the chemical is highly toxic, moderately toxic, slightly toxic, or relatively non-toxic. These label warnings are based, for the most part, on the chemical's acute toxicity. For example, the acute oral and acute dermal toxicity of a pesticide may be in the slightly toxic category. But if the acute inhalation toxicity is in the highly toxic category, the pesticide label will have the signal words for a highly toxic pesticide. The degree of eye or skin irritation caused by the pesticide also influences the signal word.

Table 3: Acute Toxicity Classifications: Hodge and Sterner Scale

			Routes of Administration
			Oral LD50	Inhalation LC50	Dermal LD50
Class	Classification	Signal Words	(single dose to rats) mg/kg	(exposure of rats for 4 hours), ppm	(single application to skin of rabbits), mg/kg	Probable Lethal Dose for Man
1	Extremely Toxic	DANGER, POISON (skull and bones)	1 or less	10 or less	5 or less	1 grain (a taste, a drop)
2	Highly Toxic	WARNING	1-50	10-100	5-43	4ml (1 tsp)
3	Moderately Toxic	CAUTION	50-500	100-1000	44-340	30ml (1 fl. oz.)
4	Slightly Toxic	CAUTION	500-5000	1000-10,000	350-2810	600ml (1 pint)
5	Practically Non-Toxic	CAUTION (or no signal words)	5000-15,000	10,000-100,000	2820-22,590	1L (or 1 quart)
6	Relatively Harmless	No signal words	15,000 or more	100,000	22,600 or more	1L (or 1 quart)