1 Pesticide Pollution

Dr. Anju

TABLE OF CONTENTS

1. Learning Outcomes

2. Introduction

3. Classification of various pesticides

          3.1 Organochlorines

      3.2 Carbamates

      3.3 Organophosphates

      3.4 Pyrethroids

4. Biochemical aspects of pesticides

5. Effects of pesticides on the environment 5.1 Surface water

       5.2 Ground water

       5.3 Soil quality

       5.4 Air quality

       5.5 Pesticides as environmental cancer

  6.Summary

  7.References

1.Learning Outcomes

After reading and studying this module, you shall be able to:

• To know about pesticides and their different groups.
• To gain knowledge about action mechanisms of different pesticides.
• To learn about the biochemical aspects of pesticides.
• To know about harmful impacts of pesticide use.
• To gain knowledge about environmental cancer caused by the excessive use of pesticides.
• To analyze prevention and control mechanisms of pesticide pollution.

2.Introduction

Pesticides are the chemical substances which are projected to destroy, prevent, deter and weaken the capacity of pests and weeds to compete with desired agricultural crops. The term pesticide comprises insecticides, herbicide, molluscicide, nematicide, rodenticide, avicide, insect repellent, bactericide, fungicide, antimicrobial, sanitizer and disinfectant. Pesticides are broadly used to avoid unwanted pests from attacking desired agricultural crops and livestock which led to their access into different components of the environment. Remarkable benefits have been consequent from the use of pesticides in agriculture, public health, forestry, and the domestic applications. Agriculture is a sector upon which the Indian economy is principally dependent. The applications of pesticides have provided an important aid to agricultural production, increasing yield and crop protection. On the other hand, the detection of pesticide residues in different components of the environment has raised severe concerns regarding the use of chemical pesticides. If the credits of pesticides include enhancement in the economic potential in terms of increase in food, vegetables, fruits and grain production and amelioration of vector-borne killing diseases, then their debits have outcome in severe health problems to man and his environment. The risk groups exposed to chemical pesticides include occupational health hazards, like production workers, formulators, mixers, sprayers, loaders and farmers working in the agriculture sector. Throughout manufacturing and formulation there is high possibility of hazards. In industry workers are at very high risk, since they handle different types of toxic chemicals used in the pesticides making. In recent decades, increasing population has demanded for higher production and improved quality, which has led to increase in the use of chemical pesticides worldwide. Roughly 2 million tons of pesticides are used in the world every year, of which around 24% is used in the United States of America, 45% in the European countries and rest of the world use remaining 25%. However, the quantity of pesticides used in developing countries is much less as compared to developed countries, it is growing progressively and considerably. Extensive use of pesticides for agricultural crops has caused rigorous health hazards and environmental pollution, including cases of acute and chronic human as well as animal poisoning. The introduction of new, advanced and more toxic pesticides into the environment has demanded precise identification of their potential hazards to human health as well as other components of the environment. Pesticides may contaminate and pollute water, soil, air and vegetation. In addition to killing and eliminating insects and weeds, they can be toxic and hazardous the host organism as well as other organisms including fish, birds, beneficial insects, non-target organisms and plants. Insecticides are commonly the most toxic class of pesticides; however herbicides can as well create risks to non-target organisms.

These toxic chemicals used in the pesticides have become an integral part of the ecosystem, while many of them are remarkably toxic for the physical, chemical and biological segments of the ecosystem. Some pesticides are lipid and fat soluble compounds and they are accumulated in the fatty parts of humans and other animals, such as breast milk, fatty tissues and blood in the food chain. Consequently, human beings are exposed to these micro-pollutants by consuming food-stuff which has been in contact with contaminated water and soil. These pesticides are also extremely toxic to most of aquatic life forms and soil micro-fauna as well as micro-flora.

Pesticide Class	Target Organism
Insecticides	Insects or pests
Herbicides	Weed plants
Bactericides	Bacteria
Fungicides	Fungi
Nematicides	Nematodes
Virucides	Viruses
Miticides	Mites
Mollusccides	Snails

                                             Table 1 Pesticide class and target organism

3. Pesticide Classification

Pesticides are generally classified on the basis of origin, target organism and chemical structure. Pesticides are frequently referred according to their pest control mechanism. Pesticides can also be considered as biodegradable pesticides, which can be broken down by the action of microbes and these are finally converted into harmless compounds and others are persistent chemical pesticides, which may take years before they are finally broken down and there are chances of their entry into food chain. On the basis of their chemical structure pesticides are divided into four major groups, namely organochlorines, carbamates, organophosphates and pyrethroids.

3.1 Organochlorines

                                                   Fig. 1  Stucture of DDT (Organochlorines group)

                                           Fig. 2 Structure of Lindane (Organochlorines group)

They function by misbalancing the sodium and potassium ratio of the nerve fiber, directing nerves to transmit endlessly. At the sodium channel, organochlorines stop opening end subsequent to activation and membrane depolarization. Sodium ions leak throughout the nerves and produce a subvert negative after potential with hyper excitability of the nerve membrane. This leakage originates frequent discharges in the neuron impulsively. Their toxicities differ widely, however now a day’s most of them have been phased out because of their persistence nature and bioaccumulation in the food chain.Furthermore, organochlorines are found to be carcinogenic and these types of pesticides are highly resistant to degradation.

3.2 Carbamates

Carbamates are organic compound derivative of carbamic acid. Carbamates are available as carbamate ester and carbamic acids and have functional groups that are interconnected structurally and frequently inter-transformed. Esters of carbamates are often called as urethanes. Carbamates are extensively used for the control of pests and worms in the vegetables as well as other crops. Some important carbamate pesticides are propionaldehyde-O-methylcarbamoyl oxinle, aldicarb, carbofuran, baygon, dimetilan, and 2, 3-dihydro-2, 2-dimethy1-7-benzo-furanyl methylcarbamate.

The manner of action of the carbamates is the inhibition of cholinesterase. Most widely used carbamate is aldicarb. Carbamates act by inhibiting a key enzyme in central nervous system of insects. The inhibition of acetylcholinesterase will direct the buildup of acetylcholine, which is a neurotransmitter. Thus normal functioning of the central nervous system of the organism get disturbed.

Even though carbamates are accounted as toxic compounds, their toxicity depends on the chemical structure of the pesticide. However toxicity of the carbamates has been reported lower as compared to organochlorine compounds. Carbamate pesticides vary in their spectrum of activity, toxicity in mammals and their persistence nature. Carbamates have high selectivity for insect acetylcholinesterase enzymes than the mammals, the most effective insecticides such as carbofuran and aldicarb are able to inhibit mammalian acetylcholinesterase enzymes in mammals even at very low concentrations and these carbamate pesticides create a considerable risk of poisoning to human beings.

Fig. 3 Structure of Carbofuran (Carbamates group)

Pesticide group	Absorption and mode of action
Organochlorines	Absorption:   Organophosphate  pesticides may be absorbed by respiratory tract, skin and gastrointestinal tract in the body and fat soluble. Mode  of  action:  Hormonal  imbalance,  suppresses enzymatic activities, direct action on CNS, disturb Na+ channel.
Carbamates	Absorption: Carbamates are absorbed in the body by absorbed by respiratory tract, skin and gastrointestinal Mode of action: Direct action on CNS (Central nervous system),  acetylcholinesterase  inhibitor,  acetylcholine effects are reversible.
Organophosphates	Absorption:   Organophosphate   pesticides   may   be absorbed by respiratory tract, skin and gastrointestinal tract in the body. Mode of action: Direct action on CNS (Central nervous system),  acetylcholinesterase  inhibitor  and  receptor agonists.
Pyrethroids	Absorption: Pyrethroids are absorbed by the skin and gastrointestinal tract in the body. Mode of action: Suppress enzymatic activities, disturb Na+ and Cl– channel.

            Table 2 Absorption and mode of action of selected groups of pesticides

3.3 Organophosphates

An organophosphate is the common name for esters of phosphoric acid. Organophosphate pesticides are frequently plasticized in agriculture sector globally. The first organophosphate hexaethyl-tetra-phosphate was discovered in 1942, which was applied as an insecticide in agriculture sector. Commonly used organophosphate pesticides are parathion, malathion, diazinon, methyl parathion, chlorpyrifos, , fenitrothion, dichlorvos, phosmet, , tetrachlorvinphos, sarin, tabun and azamethiphos. Organophosphates class of insecticides is extremely toxic. Organophosphorus insecticides are the highly toxic amongst the other insecticides. These are poisonous to insects as well as mammals. Organophosphates when used in liquid form these may be absorbed by all routes from the plant surface. When organophosphate pesticides are applied on the plant crops, these are absorbed by the plant sap and they stay active for a longer period of time. These types of pesticides are poisonous for the pests feeding on the plant crops but are harmless for the plant predators. Organophosphate compounds were among the most extensively used insecticides existing until the 21st century. Thirty six organophosphate pesticides are presently registered for use in United States, and most of them can cause acute toxicity. Organophosphates pesticides may kill insects as well as other animals, comprising amphibians and mammals. Malathion is least toxic from all other organophosphate compounds and it is rarely absorbed through the skin. Organophosphate pesticides cause phosphorylation of the enzymes at nerve endings inside the body of the target pest organism. Acetylcholinesterase enzyme is critical to usual control of nerve signal transmission from nerves to secretory cells and skeletal muscle cells in the central nervous system.

                                        Fig. 4 Structure of Malathion (Organophosphates group)

                                              Fig. 5 Structure of Parathion (Organophosphates group)

                                          Fig. 6 Structure of Sarin (Organophosphates group)

Since, a significant proportion of the tissue enzyme mass is disturbed by the process of phosphorylation as result symptoms of poisoning become harsh. Fenthion an organophosphate pesticide is used against biting and sucking insects and pests; it inhibits the proper functioning of cholinesterase and results in the death of the target pest.

3.4 Pyrethroids

These are the pesticides of biological origin, so these are also known as botanical pesticides. Pyrethroids group of pesticides are extracted from naturally occurring materials or substances and these naturally existing materials are derived from flora. Nature has provided ability to some plants by

providing them some kind of biochemicals. A number of these types of substances are present which can be used as pesticides or fungicides. Development of pyrethroids or botanical pesticides is one of the fastest emerging areas in the modern pesticide manufacturing industry. Combination of pyrethroids can with piperonyl butoxide, a well known inhibitor for cytochrome P-450 from metabolizing the pyrethroids, which raises its effectiveness. The toxic effect of pyrethroids is arbitrated by avoiding the closure of sodium channels in the axonal membranes of the cells. The sodium channel is a membrane protein having a hydrophilic nature of the interior part. Pyrethroids do not allow the nerves to be re-polarized which results in permanent depolarized axonal membrane which causes paralysis and finally death of the target pest. Commonly used synthetic pyrethroids pesticide is derivative of naturally occurring chrysanthemum esters which that generate biochemicals called as pyrethrins. Pyrethroids have attained desirability and these are applied to kill crop pest as well as household pests. They demonstrate selective insecticidal activity, less toxicity to mammals and completely non-persistence in nature.

                                           Fig. 7 Structure of Permethrin (Pyrethroids group)

Name of pesticide	Persistence in the environment (Approx.)
Aldrin	10 Years
Dieldrin	10 Years
Heptachlor	11 Years
DDT	12 Years
Chlordane	11 Years
Monuron	04 Years
BHC	13 Years
Atrazine	04 Years
Slimazine	02 Years

                              Table 3 Persistence of some selected pesticides in the environment

4. Biochemical aspects of pesticides

Chemical pesticides can interact with the cell’s biochemical mechanisms, which may lead to toxicity as well as mortality. Excessive use of the chemical pesticides is detrimental to all life forms including human beings. They can enter into the food chain and harm the successive trophic levels. Pesticide are used to kill the harmful organisms to protect our crops and increase the crop yield and health, unfortunately most of them are highly persistent in nature and they find their way to other non target life forms including birds, fish, mammals as well as human beings and harm them in one way or another. Pesticides affect the metabolism of proteins, carbohydrates, lipids and it has been observed in the living system exposed to organochlorines. The use of DDT, cyclodienes, TCDD, HCH, has resulted in fatty liver, hyperlipidemia and fat mobilization in mammals.

Exposure to chemical pesticides is responsible for a variety of biochemical changes in human beings as well as other life forms. DDT has been reported to weaken the egg shell of the birds which results in death of the young ones. Millions of birds die every year by consuming the pesticide laden grains. Farmers are at an increased risk of chronic toxicity, since they are frequently exposed to chemical pesticides while using them in their fields and it becomes difficult to identify a particular pesticide which has caused the damage. It has been reported that women with blood level of DDE at 19 ng/mL have four times more risk of breast cancer than the women with DDE level at 2 ng/mL. DDE may induce cytochrome P450 enzymes and alters the mechanism of toxicants metabolism or it may act as an estrogen mimic and disrupt the endocrine system. Biological parameters linked to organ functions may be altered in the human beings as well as other animals when they are exposed to chemical pesticides directly or indirectly, hepatic or renal cytotoxicity is commonly observed in such cases. Pesticides are among 9 of the 12 most unsafe, dangerous and persistent chemical pollutants according to The Stockholm Convention on Persistent Organic Pollutants. Strong confirmation too reveals negative outcomes from pesticide exposure which includes birth defects, neurological problems, fetal death and neuro-developmental disorder. Majority of pesticides have been associated with impaired or reduced fertility in males, low sperm count in males and damage to germinal epithelium. Body cell’s enzymatic and hormonal activities are disturbed or altered in the pesticides exposed group, as a result body organs fail to perform in a usual way.

5. Effects of pesticides on the environment

5.1 Surface water: Pesticides may reach the water bodies like rivers, streams, lakes, ponds, canals with the agricultural runoff and they may deteriorate the quality of the water, consequently affecting the aquatic life. There are several factors that affect a pesticide’s capacity to pollute or contaminate the water. These comprise the solubility of the pesticide in water, distance of water body from the agricultural field, type of soil, weather, slope and method used in the pesticide application. Aquatic animals, plants as well as aquatic birds are exposed to these chemical pesticides; therefore it results in a negative outcome. Pesticides may find their way into the food chain and finally succeeding trophic levels are affected.

5.2 Ground water: Pesticides are used in the agricultural fields, when these are sprayed on the plants, a part of the liquid pesticide may drop on the ground soil. There is possibility that it may leach down to the groundwater table and contaminate the groundwater, although chances are very less still there is possibility. Once the ground water has been polluted, there are very less chances that these chemicals can be removed from the groundwater and pesticide residue may remain in the groundwater for decades.

5.3 Soil quality: Soil is a precious resource. Soil formation by the process of weathering takes millions of years. Most of the pesticides are persistent soil contaminants, and their effect may continue for months, years or decades, which adversely affect soil quality. Pesticide pollution may alter physical, chemical and biological properties of the soil. Soil’s cation exchange capacity, porosity; water holding capacity and texture may get altered. Pesticides are extremely damaging for the soil flora and fauna, as a result soil quality is deteriorated, leading to decrease in crop yield. Pesticide residue present in the soil may enter into the food chain affecting the entire ecosystem.

5.4 Air quality: While pesticides are applied in the fields, especially when sprayed the tiny liquid droplets get mixed with the air and air quality is deteriorated and they travel a far distance. Air becomes poisonous which affects the health of the human beings as well as other animals and a foul smell is also produced. It affects our lungs directly and causes chronic lung diseases, heart problems, damage to eyes, skin problems and sometimes but rarely it may result into death of the living beings when they are exposed for a longer period.

5.5 Pesticides as environmental cancer

There are strong evidences, which prove that long term and high dose exposure to pesticides causes various types of cancer in the human beings as well as other animals. Today we are facing a cancer epidemic and confirmation is growing that exposure to high dose of pesticides is a key contributor to this alarming trend. There are cases of childhood brain cancer and some researchers have related it to the exposure of their parents who were exposed to high levels of pesticides before the birth of the young one. DDT is also included in the list of cancer causing substances in the environment. According to a report, women with high dose exposure to DDT are more prone of developing breast cancer. Agricultural workers in many countries of the world have overall high rates of deaths due to different types of cancer than the general population. According to a recent scientific study farmers who are concerned with the use of pesticides in their fields certain are twofold more prone to contract melanoma, a lethal variety of skin cancer. Agent Orange an herbicide was used by the United States of America during Vietnam War to kill the vegetation exposing millions of human beings to this deadly carcinogenic compound. Years later researchers learned that “Agent Orange” was responsible for cause various types of cancers in the exposed group. Pesticides are responsible for triggering cancer in many ways, which include DNA damage, hormone disruption, inflaming tissues as well as causing gene mutations. A study has reported that there is risk of bladder cancer in the pesticide exposed population. It has been revealed that the children are at risk of cancer due to pesticides. Exposure to pesticide during childhood boosts up the risk of cancer amongst children. Organochlorine pesticides are found to be concerned with numerous types of cancer such as breast cancer, prostate cancer and bladder cancer.

6. Summary

By learning this module, you may have been acquainted with:

•  Purpose of using different types of pesticides.
•  Types of pesticides and their target organisms.
•  Different groups of pesticides like: organochlorines, carbamates, organophosphates and   pyrethroids.
•  Absorption mechanism and mode of action of different groups of pesticides.
•  Biochemical aspects of different pesticides like how they affect different life forms.
•  Environmental and health effects of different pesticides. How pesticides affect life and   different components of the environment.
•  Pesticides are responsible for causing different types of cancers in human beings as well   as other animals.

    7.  References

1. Srivastava A K, Kesavachandran C., Health effects of pesticides, Book, (New age international publishers)
2. De A K., Environmental Chemistry, Book, Eighth Edition (TERI).
3. Grung ., Lin Y, Zhang H, Steen AO, Huang J, Zhang G, Larssen T., (2015) Pesticide levels and environmental risk in aquatic environments in China — A review, Environment International, 81, 87–97.
4. Yadav IC, Devi N., Syed JH, Cheng Z, Li J, Zhang G, Jones KC., (2015) Current status of persistent organic pesticides residues in air, water, and soil, and their possible effect on neighboring countries: A comprehensive review of India. Science of the Total Environment, 511, 123–137.
5. Elibariki R, Maguta MM., (2017) Status of pesticides pollution in Tanzania – a review. Chemosphere, 178, 154–164.
6. Ali U, Syed JH, Malik RN, Katsoyiannis A, Li J, Zhang G, Jones KC., (2014) Organochlorine pesticides (OCPs) in South Asian region: A review, Science of the Total Environment, 476– 477, 705–717.