6 Pesticides

Dermal absorption of pesticides - evaluation of variability and prevention

6.1 History

Even earlier than 2500 BC, humans have utilized pesticides to protect their crops. The first known pesticide was sulphur dusting used in Sumeria about 4500 years ago. By the 15th century, toxic chemicals such as arsenic, mercury and lead were being applied to crops to kill pests. In the 17th century, nicotine sulphate was extracted from tobacco leaves for use as an insecticide. The 19th century saw the introduction of two more natural pesticides, pyrethrum which is derived from chrysanthemums, and rotenone which is derived from the roots of tropical vegetables (Miller, 2002).

In 1939, Swiss chemist Paul Müller revealed DDT as a very efficient insecticide. DDT rapidly became the most used pesticide in the world. However, in the 1960s it was discovered that DDT was preventing many fish-eating birds from reproducing, which posed a significant risk to biodiversity. DDT was also found to cause birth defects in animals and humans. DDT is now banned in most industrialized countries, but is still used in some developing nations to prevent malaria and other tropical diseases by killing mosquitoes and other disease-carrying insects.

German scientists experimenting with nerve gas during World War II produced the organophosphorous insecticide parathion, marketed in 1943 and still commonly used today. Throughout the 1950s and 60s, these types of chemicals became major pest control agents.

“Silent Spring”, Rachel Carson's landmark challenge to the abuse of synthetic pesticides, was published in 1962 and initiated the movement towards agrochemical regulation that is still fiercely debated (Nationmaster, 2003).

Pesticide use in Denmark has decreased during the last decade. Data from the Danish Environmental Protection Agency show that the pesticide sale has decreased from 19,400 ton in 1995 (6,600 ton active ingredient) (Miljøstyrelsen, 1998) to 12,234 ton in 2006 (3,200 ton active ingredient) (Miljøstyrelsen, 2007).

Pesticides of today are designed to persist for shorter periods of time in the environment and are supposedly less lethal than the early days of calcium arsenate and DDT. There might even be evidence for the fact that alternatives to pesticides can be more effective then the use of chemicals. Sweden has reduced its use of pesticides by half with hardly any reduction in crops. In Indonesia, farmers have reduced pesticide use on rice fields by 65% and experienced a 15% crop increase (Miller, 2004).

Today the most frequently used pesticides are non-persistent organophosphates, including glyphosate (the active ingredient in Roundup), which is currently the world's most used herbicide (Nationmaster, 2003).

6.2 Definition

A pesticide is a substance or mixture of substances used for preventing, controlling, or lessening the damage caused by a pest. By their very nature, most pesticides create some risk of harm. Pesticides can cause harm to humans, animals, or the environment as they are designed to kill or otherwise adversely affect living organisms. At the same time, pesticides are useful to society. Pesticides can kill potential disease-causing organisms and control insects, weeds, and other pests (US Environmental Protection Agency, 2007).

The use of pesticides is a way to control organisms which are considered harmful such as mosquitoes that can spread potentially lethal diseases like malaria and insects that can cause allergic reactions. Insecticides can protect animals from illnesses caused by parasites like fleas. Pesticides can prevent sickness in humans caused by diseased products. Pesticides are used in grocery stores and food storage facilities to manage rodents and insects that infest food such as grain, but each use of a pesticide carries some associated risks. Correct pesticide use decreases these risks and is therefore of great importance.

6.3 Use

Pesticides such as rodenticides, herbicides, fungicides and insecticides are used not only to prevent harmful organisms in occupational areas, but also in private homes where pesticides are used in the garden fighting weeds, ants, bees etc., and some like malathion are used directly on humans against lice.

Rodenticides are chemicals intended to kill rodents. Rodents include mice, rats, squirrels, chipmunks etc. The poison is used in households as well as in agriculture. It is most effective if tasteless and odorless in lethal concentrations and if it has a delayed effect. Rodenticides are e.g. Kiltin Bromanol B 100 (bromadiolon) or Frunax-D (difenacoum).

Herbicides are used to kill unwanted vegetation. They can destroy specific targets while leaving the desired crop relatively unharmed. Some herbicides work by restricting the growth of the weed by interfering with the plant hormones. Other herbicides are non-selective and kill all plant material with which they come into contact. Herbicides are e.g. Roundup (glyphosat), or NF-M 750 (MCPA).

Fungicides are chemical compounds used to avoid the spread of fungi in gardens and crops, which can cause harm to the plants. Fungicides are e.g. Octave (paclobutrazol) or Tachigaren 70 WP (hymexazol)

Insecticides are used against insects.They target the insect organism in all developmental forms from eggs and larvae to insects. Insecticides are used in agriculture, medicine and the household. Practically all insecticides have the potential to change ecosystems; several are toxic to humans and mammals and can therefore alter the food chain. Agricultural needs must be considered together with environmental and health issues when using insecticides. Insecticides are e.g. Pirimor (pirimicarb), Cygon (dimethoate) or DDT (dichlordiphenyltrichlorethan).

6.4 Toxicity

A toxin is often referred to as a toxic substance which is naturally produced where as a toxicant is a toxic “human-made” substance. The distinction is not always clear.

Toxic substances are classified in many ways, depending on the classifiers – their needs and interests. Substances can be discussed in terms of their use, target organ, source and effects.

Pesticides achieve desired effects but also have a spectrum of undesired effects. These effects are refered to as the adverse, deleterious, or toxic effects of the pesticides.

Toxicity can be divided into immediate or delayed toxicity and reversible or irreversible toxic effects.

Immediate toxic effects can be defined as those that occur or develop rapidly after a single administration of a substance, whereas delayed toxic effects are those that occur after some time.

Whether an effect is reversible or irreversible often depends on the ability of the injured tissue to regenerate. Some tissues are more susceptible to injury and the regeneration is therefore different. The liver has a high ability to regenerate as opposed to the CNS. Injuries to the two different organs therefore have very different outcomes and the damage is referred to as reversible and irreversible (Klaassen CD, 1996).

The undesired effects of pesticides may also be divided into different targets:

Developmental effects - ability to affects fertility or foetal development.
Carcinogenicity - ability to produce cancer or to assist carcinogenic chemicals.
Mutagenicity - ability to cause genetic changes.
Liver damage - death of liver cells, jaundice (yellowing of the skin), fibrosis and cirrhosis.
Reproductive disorders - such as reduced sperm count, sterility, and miscarriage.
Neurotoxicity - including accumulative effects on cholinesterase depression associated with organophosphate insecticides.
Allergenic sensitization - development of allergies to pesticides or chemicals used in formulation of pesticides.

These undesired effects can either be produced immediately or delayed and be reversible or irreversible. As an example teratogenicity and carcinogenicity caused by pesticides are usually considered irreversible toxic effects.

6.5 Symptoms

Symptoms based on toxicity may be shown:

As acute toxicity. Thus, exposure to a pesticide may cause acute effects such as nausea, chest pain and vomiting as well as chronic effects resulting from kidney, liver and lung damage.
As a slowly progressive form of toxicity without any previous clinical signs of acute intoxication. An example could be breathing difficulty or skin sensitization (allergy) following repeated exposure to a pesticide.
As the occurrence of a disease or condition initiated by previous exposure. Delayed appearance of neurotoxicity and development of cancer years after a period of exposure are organophosphates (neurotoxicity).