I organic air pollutants I 1 Volatile Organic Compounds (vocs)




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I.2.7. PESTICIDES



Jenny EMNEUS


Pesticides are compounds that are used in order to eliminate pests of different kinds such as insects, weeds and fungi. The different agents are then referred to as insecticides, herbicides and fungicides. There is a large group of chemicals used as pesticides and they can be divided into several classes. Each class is more or less efficient towards the different pests.

I.2.7.1. Chlorinated Pesticides



The development of chlorinated pesticides started when it was discovered that DDT (Figure I.2.12.) could be used as an efficient insecticide.

The use of DDT escalated during the Second World War when there was a big demand of insecticides for preventing food spoilage and diseases from spreading. There were several benefits with DDT: It was relatively cheap and easy to produce, it was persistent, which prolonged the effect after treatment and had low toxicity towards mammals. However, concern against the product started to develop during the 60s when it was discovered that it caused reproduction damage for birds of prey. It disrupted the enzyme system responsible for the egg production, which resulted in eggs with very thin shell. The number of surviving off springs decreased as the eggs broke before they were ready for hatching.




Figure I.2.12. Chemical structure for dichlorodiphenyl trichloroethane, more known as DDT.


I.2.7.2. Organo-phosphoric Pesticides



Organo-phosphoric pesticides originate from the military gases that were developed during the Second World War. They are not considered bio-accumulative since they are hydrolyzed when they come in contact with water. The toxic effect comes from interference with the transmission of nerve impulses and they affect all organisms from mammals to insects.

I.2.7.3. Carbamate Pesticides



Carbamate pesticides are very similar to the organophosphoric pesticides regarding use and characteristics. They are often used when organophosphates are inadequate but are at the same time more expensive. Another difference is that carbamates do not have the long-term effect as organophosphates presents.

I.2.7.4. Pyrethrins and Pyrethroids



The Chrysanthemum plant contains the natural insecticide pyrethrum, which belongs to the group pyrethrins. These compounds act as neurotoxins and affect the ion balance. Mammals are generally not affected since they are able to rapidly degrade the pyrethrins and the compounds toxicity towards mammals is also low.

Synthetic compounds that mimic the molecular geometry of pyrethrins have been developed and they are named pyrethroids. These compounds are also effective as insecticides and their stability towards moisture and light have been improved compared to the natural pyrethrins. However, some of these pyrethroids have an increased toxicity towards mammals and are in general highly toxic towards fish.

I.2.7.5. Phenoxyacetic Acid Herbicides



Phenoxyacetic acids are a group of herbicides, which are also known as hormone weeds. They were, as many other pesticides, developed during the Second World War and the development was based on the structure of the natural hormone auxin. These herbicides show low toxicity towards aquatic organisms and also a low persistence. The problem with these compounds is that during the production some very toxic dioxins are also produced. These contaminate the product and are the reason why this type of herbicide is banned in many countries.


I.2.8. RADIONUCLIDES

Diana Cristina ANDREI



Among the 2300 nuclides that have been identified, most of them are radioactive. However, in their daily life, people are likely to encounter only few radionuclides. Among those, there are the radionuclides corresponding to the natural radiation (cosmic radiation, primordial radioactive elements in the earth’s crust and their radioactive decay products, etc.) as well as the radionuclides routinely used for medical, military and commercial purposes.

Example: Decay of uranium - 238 and thorium - 232 conduct to the existence of trace elements in rocks and soils. Table I.2.2 presents a list of few radionuclides of interest and their type of emitting radiation.


Table I.2.2. Radionuclides and their characteristics


Name

Atomic Number

Radiation Type

Alpha

Beta

Gamma

Americium-241

95








Cesium-137

55








Cobalt-60

27








Iodine-129 &-131

53








Plutonium

94







Radium

88








Radon

86









Strontium-90

38









Technetium-99

43








Tritium*

1









Thorium

90








Uranium

92








*tritium is a specific isotope, H-3.


Usually, alpha emitters occur naturally, but several can come from man-made sources. They may be found in ground water and surface water. Beta and gamma emitters are mainly man-made and are frequently associated with nuclear power plants, facilities that use radioactive material for research, manufacturing, etc.

The three exposure pathways of humans to this radiation are inhalation, ingestion and direct (external) exposure. Living tissues in the human body can be damaged by ionizing radiation (from radionuclides). If at first, this was learned through observation, today the statement is a well-known fact. Moreover, it is difficult to set a “safe” level of exposure above background; any exposure carries some risk and that risk increases as the exposure increases.

Several types of cancer may occur with death as result of extremely high dose of radiation. Thus radon can cause lung cancer, with affecting water drinking and leading to stomach cancer. High exposure of radium-226 and radium-228 has been known to cause bone, stomach, lung or other forms of cancer. Uranium may have a role in the development of bone cancer and is toxic to kidneys too.


I.3. SONIC AND ELECTROMAGNETIC POLLUTION


Diana Cristina ANDREI

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