General classification of pesticides: fumigants

by Wan Zainal Azman Wan Abdullah
The Sun, April 19, 1997

FUMIGANTS ARE THE MOST TOXIC OF all pesticide chemicals used in agriculture. They are used to kill insects, nematodes, weed seeds and fungi in soil, silo- stored cereal grains, fruits and vegetables. Fumigants may be liquids (ethylene dibromide, dibromochloropropane, formaldehyde, etc) that readily vaporise at ambient temperature, solid (aluminium phosphide, hydrogen phosphide, etc) that can release a toxic gas on reacting with water or acid, or gases (methyl bromide, hydrogen cyanide, ethylene oxide, etc). They are highly biologically reactive compounds. Many are alkylating agents, mutagens, carcinogens, neurotoxins and hepatotoxins.

These chemicals are gases penetrating readily into the lungs where they enter the blood and are rapidly distributed throughout the body. Full respiratory protection is mandatory when working with these pesticides. Death could occur rapidly if the fumes are inhaled, even for a short period of time. However, skin absorption can be significant hazard as well. Fumigants, as a class, have caused severe human illness and death.

Methyl Bromide

Methyl bromide is a widely used soil sterilant and post-harvest fumigant in agriculture against insects, termites, rodents, weeds, nematodes and soil- borne diseases. It is used to fumigate agricultural commodities, grain elevators, mills, ships, clothes, furniture and greenhouses.

Non-pesticidal uses include de-greasing wool and extracting oils from nuts, seeds and flowers. Methyl bromide is also a methylating agent in the chemical industry. In the past, it was used as a refrigerant and as a fire-extinguishing agent in aircraft. The chemical name for this fumigant is bromomethane.

Methylbromide, labelled with the word "DANGER", is an extremely toxic vapour. It is a colourless fas with a burning taste and sweet chloroform-like odour at high concentrations.

It is non-flammable in air but does burn in oxygen. It is stable and non- corrosive. In humans, methyl bromide is readily absorbed through the lungs. Most problems occur as a result of inhalation.

Methyl bromide is a dangerous cumulative poison. First symptoms often are due to damage to the nervous system and may be delayed from 48 hours to as long as severak months after exposure. This delay, combined with methyl bromide's lack of odour, means that the victim may not realise that exposure is occurring until much time has passed.

Symptoms of poisoning vary widely. Soon after inhalation of large doses, symptoms may include headache, dizziness, nausea, chest and abdominal pain and a dry throat. Three to 12 hours after vapour inhalation, symptoms include slurred speech, blurred vision, temporary blindness, mental confusion and sweating. More severe symptoms may include lung swelling, congestion, haemorrhaging of the brain, heart and spleen, severe kidney damage and numbness. Death may occur within one to 30 hours, usually from respiratory failure.

Although skin absorption is not an important route foe methyl bromide intocication, the skin is affected by contact with this chemical. methyl bromide can cause enormous blisters that are rarely deep enough to destroy the entire skin layer. Small amounts of skin or eye contact brings on shortness of breath and itching. If absorbed through skin, nausea and vomiting may result. Clothing that cannot "breathe" may delay the evaporation of the pesticide from the skin. Continued contact with skin can cause death. Ingestion of methyl bromide may cause hand tremors and convulsions.

Chronic exposure to methyl bromide can cause dizziness, vision and hearing disturbances, depression, confusion, hallucinations, euphoria, personality changes and irritability. If exposure is severe enough, lung irritation, followed by lung swelling and bronchial pneumonia may occur.

Hydrogen Cyanide

Hydrogen cyanide is an insecticidal fumigant used mainly for stored products, especially grains and flour in mill, warehouses and ships. In the United States, it has been used extensively for combating scale insects on citrus trees while each tree is covered by a gas-proof tent.

It is colourless gas or liquid with a characteristic almond-like odour. It burns with a blue flame. Hydrogen cyanide may be absorbed in toxic amounts through the unbroken skin (the amount is increased if the skin is moist), inhalation of gas and by ingestion.

The cyanide ion has essentially the same toxicity, regardless of the route by which it is absorbed. However, accidental or intentional poisoning (excluding executions) is more likely to involve ingestion of a cyanide salt whereas occupational poisoning is more likely to involve inhalation of hydrogen cyanide, whether it is volatilised from liquid or is generated at the site of use by the actionof a mineral acid on potassium or sodium cyanate.

A large doses of cyanide, whether oral or respiratory, is followed by almost instantaneous collapse and cessation of respiration. A dosage only a little above the threshold of danger causes only headaches, weakness, confusion and sometimes nausea and vomiting. respiration and heart rate are stimulated at first, but respiration later becomes slow and gasping. The heart may continue for some time after respiration stops.

While respiration is active, the venous blood remains oxygenated and the patient's colour florid. In fact, this condition may persist if death is sudden. If respiration is extremely poor or if it stops some moments before the heart, the blood will become dark and the patient cyanotic.

The toxic action is reversible and a person who is completely unconscious from the effects of cyanide but whose heart is still beating, may still recover if suitable antidotes and remedial measures are applied on time.

Ethylene dibromide

Ethylene dibromide has been used as a fumigant for stored grain, fruits and vegetables for the treatment of soil against nematodes and for spot treatment in flour mills. The compound is sometimes mixed with carbon tetrachloride or dicholorpropene to increase the spectrum of activity. It is a heavy, colourless liquid with a chloroform-like odour. It is stable and non-flammable.

Ethylene dibromide is a severe irritant to skin, eyes and the respiratory tract. The liquid causes blistering and erosion of the skin and is corrosive to the eyes. Once absorbed, it may cause pulmonary oedema and central nervous system depression. Long-term exposure may have some damaging effect on testicular tissue. Persons poisoned by ingestion have suffered chemical gastroentetitis, liver necrosis and renal tubular damage.

Death is usually due to respiratory or circulatory failure. A powerful disagreeable odour is advantageous in warning occupationally exposed workers of the presence of this gas.

Dibromochloropropane

Dibromochloropropane is used as a soil fumigant and nematocide. It forms a brown liquid with a pungent odour.

Dibromodichloropropane is irritating to the skin, eyes and the respiratory tract. Eye damage has resulted from repeated exposure to the vapours. When absorbed, it causes headache, nausea, vomiting, ataxia and slurred speech. Liver and kidney damage are prominent features of acure poisoning.

Chronic exposure to relatively low concentrations has led to permanent sterility of workers in a mnaufacturing plant. Because it is much less odiferous then ethylene dibromide, exposure of workers to toxic concentrations of dibromochloropropane is more likely.

Ethylene oxide and propylene oxide

Ethylene oxide and propylene oxide are irritants to all tissues they come in contact with. Aqueous solutions of ethylene oxide cause blistering and erosion of the affected skin. The area of the skin may thereafter be sensitised to the fumigant.

Inhalation of high concentrations is likely to cause pulmonary oedema and cardiac arrhythmias. Headache, nausea, vomiting, weakness and a persistent cough are common early manifestations of acute poisoning. Coughing of bloody frothy sputum is characteristic of pulmonary oedema.

Phospine gas

Phosphine is a fumigant for grain and other stored products. The fas is liberaed by the interaction of aluminium phosphide with water. Phosphine is a colourless gas with an odour of carbide or decaying fish. Phosphine in the form of zinc phosphide is also used in the control of rodents and moles.

Phosphine gas is only slightly irritating to the respiratory tract but is at least as toxic systematically as hydrogen cyanide. It is slowly released into treated produce or storage spaces by hydrolysis of solid aluminium phosphide. Mechanisms of toxicity are not well understood. The principle manifestations of poisoning are fatigue, anusea, headache, dizziness, thirst, cough, shortness of breath, paraethesia and jaundice. Pulmonary oedema is a common cause of death.

The writer is a Science Officer at the National Poison Centre, Universiti Sains Malaysia, 11800 Minden, Penang.


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