Fumigants

Nature and Uses of Fumigants

Robert D. Chisholm.

Fumigants are chemicals that give off poisonous vapors. Their value for killing insects was known to the Greeks and Romans, Homer referred to the use of sulfur for the purpose. About 200 B. C. Cato mentioned that the fumes from a mixture of sulfur and asphalt would kill tree-infesting insects. Since then many compounds have been found to be valuable as fumigants for a large number of species for some, in fact, fumigation offers the only practical means of control. For others it provides an alternate means to supplement spraying or dusting.

The selection of the right fumigant depends on several factors besides its ability to kill. It must not injure the commodity or thing attacked by the insects or nearby objects. It must not leave a residue that is toxic to humans or that imparts an unpleasant odor or taste if it is used on foodstuffs. Its cost must be less than the value of the materials saved from the insects. It must have certain properties adequate vapor pressure or rate of vaporization, ability to penetrate the commodities fumigated, little sorption by the commodities, and chemical stability.

Because of the large number of insect species and the variety of their environments, the ideal fumigant has not been discovered. Fumigants are not equally effective against all insect species. A fumigant, furthermore, that can be used to kill insects in one environment perhaps cannot be used in another environment.

Fumigants usually are applied in enclosed spaces vaults, houses, warehouses, mills, ships, bins, tanks, tents, or vacuum chambers, all of sufficiently tight construction to prevent undue loss of the vapors. Air pressure is used sometimes to test for tightness. Losses from enclosed spaces that are protected from wind are much less than the losses from exposed places. Within practical limits, cracks or other openings should be sealed before a fumigant is applied. Sometimes fumigants are applied to soils.

Some fumigants are gases at ordinary room temperatures (about 70 F.). Others are liquids or solids that vaporize slowly at ordinary temperatures or require heating for effective use. The gases are usually compressed in cylinders, from which they are released as gases or liquids. (An exception is sulfur dioxide, which is generated by the burning of sulfur.) Fumigants that are liquids can be sprayed or sprinkled onto the commodity or throughout the enclosed space. Some are vaporized from pans, often with the aid of heat. Liquids used for soil fumigation are injected into the soil or applied as solutions or water emulsions. Solid fumigants can be vaporized by heating or scattered on and throughout the commodity or within the space, where they vaporize slowly.

Successful and economical fumigation depends on uniform distribution of the vapors. Some vapors are lighter than air and tend to concentrate in the upper level in an enclosed space, particularly if they are released near the top. Others, heavier than air, may stratify in the lower levels. Such undesirable features may be overcome partly if light vapors are released near the bottom and the heavy ones near the top. More uniform distribution can be had by using a circulating fan to mix the vapors with the air. After mixing, stratification at different levels is often of little importance.

Distribution in soils depends on the structure, moisture content, and temperature of the soils and the uniformity and depth of application.

The vapors of many compounds will kill insects, but relatively few are used for the purpose.

CARBON DISULFIDE (CS₂) is a liquid at ordinary room temperatures. The chemically pure form is colorless, but commercial grades are slightly yellow. It has a disagreeable odor. Other properties : Boiling point 46.3 C., melting point 111.6 C., specific gravity 1.261 22 /20 C., and a vapor pressure of 297.5 millimeters at 20 C. It evaporates rapidly at ordinary temperatures and its vapors are 2.6 times as heavy as air (calculated from molecular weights).

The vapors of carbon disulfide are explosive when mixed with 1 to 99 volume of air. At 147 C. it ignites spontaneously. The mixtures may be exploded on contact with flames, live coals, sparks from electrical fixtures, or hot steam pipes. Great caution in its use is essential.

People should avoid lengthy exposure to the vapors. Short exposures may cause only headache or nausea. If exposure is continued, the symptoms become severe as a result of pathological changes due to the solubility of lipoids in carbon disulfide. Such changes may cause death.

Carbon disulfide has been employed as a fumigant since 1854. Its use has increased steadily until recent years. Other fumigants or a mixture of 1 part of carbon disulfide with 4 parts of carbon tetrachloride have largely replaced it. It has been used extensively for the fumigation of houses, warehouses, and stored products such as grains. Because it is toxic to all forms of life, it cannot be used for the fumigation of plants in greenhouses. Grains fumigated with carbon disulfide when they are moist are apt to germinate poorly.

Carbon disulfide is used also as a soil fumigant, originally having been found useful for the treatment of soil about the roots of grapes infested with root aphids. Later it was used in emulsions for the control of larvae of the Japanese beetle and other soil-inhabiting insects.

CARBON TETRACHLORIDE, or tetrachloromethane (CCl₄), is a liquid at ordinary room temperatures. It smells like chloroform. Other properties: Boiling point 76.8 C., melting point 23.0 C., specific gravity 1.595 20 / 4 C., vapor pressure 159.6 millimeters 20 C., and vapor weight about 5.3 times that of air.

Its vapors are non-inflammable it is safe for use where a fire hazard is present.

In liquid or vapor form, carbon tetrachloride is toxic to humans. Symptoms of poisoning may be produced by absorption through the skin. Some of the characteristic symptoms are fatigue, backache, burning of the eyes, stomach disturbances, and liver injury. A constant exposure to more than 100 parts per million of air is considered dangerous. Exposure to 10,000 parts per million or less for an hour may produce symptoms that last only a short time. In the presence of an open flame, carbon tetrachloride is converted to phosgene and its toxicity is increased greatly.

Carbon tetrachloride has rather low insecticidal value. Consequently its cost is too high for many purposes. Its use is largely limited to operations where a fire hazard is present or in small-scale fumigations where cost is unimportant. Its principal use is in mixture with other fumigants, such as carbon disulfide or ethylene dichloride, to reduce fire hazard. It is used also as a diluent for more toxic fumigants, such as methyl bromide or ethylene dibromide, to assist in the distribution of the vapors of the more toxic compounds. Large quantities are used in such mixtures, particularly in grain fumigation.

CHLOROPICRIN, or trichloronitromethane (CCl₃NO₂), is a colorless liquid at room temperatures. It causes vomiting and intense irritation of the eyes and throat at relatively low concentrations. Other properties: Boiling point 112.4 C., melting point 64 C., specific gravity 1.651 20 /4 C., and a vapor pressure of 18.3 millimeters at 20 C. Its vapors are about 5.7 times as heavy as air.

It is non-inflammable and is substantially free of fire or explosion hazards. In that respect it excels certain other fumigants, such as carbon disulfide or ethylene oxide.

It is toxic to humans. It was used in mixture with other more toxic gases during the First World War and was known as vomiting gas soldiers who removed their masks were thus exposed to higher concentrations of the other gas in the mixture. It is sometimes added to hydrocyanic acid and methyl bromide as a warning agent.

To fumigate stored products, it may be poured or sprayed on the infested material. Because it has a low rate of volatility, it is often mixed with carbon tetrachloride or ethylene dichloride to promote vaporization and distribution of the vapors. It has the disadvantage of being retained by the fumigated product and can only be removed by prolonged airing. It is apt to injure living plants and seeds.

To control soil-inhabiting insects, it is usually injected into the soil in mixture with the diluents previously named or with xylene. Sometimes it is emulsified in water, and the emulsion is sprinkled on the surface or poured into holes. Such treatments will control certain species of fungi, nematodes, and weeds, but it must not be used where plants are growing.

D-D MIXTURE is essentially a mixture Of 1,3-dichloropropylene and 1,2-dichloropropane obtained as a byproduct in the manufacture of allyl alcohol from petroleum.

Its composition is somewhat variable. A typical lot contained 30 to 33 percent of low-boiling and 30 to 33 percent of high-boiling 1,3-dichloropropylene, 30 to 35 percent 1,2-dichloropropane, and about 5 percent of heavy trichlorides of propane. It is a dark-colored liquid at ordinary temperatures and has a sharp, disagreeable odor. A typical lot had a boiling point of 93 C. On distillation 95 percent was recovered at 142 C. and dryness resulted at 163 . It had a specific gravity of 1.198 20 /4 C., a vapor pressure of about 31.3 millimeters at 20 C., and a flash point (Tag. open cup) of 80 F.

D-D mixture is inflammable and is dangerous to use in enclosed spaces in the presence of sparks or open flames.

It is dangerous to humans. Prolonged breathing of its vapors may cause the symptoms associated with the inhalation of the vapors of chlorinated hydrocarbons. The seriousness of such symptoms depends on the concentration of the vapors and the length of the exposure. It is very dangerous if spilled on the skin, shoes, or clothing and is likely to cause irritation, a burning sensation, and blistering. If it is spilled on clothes, the garment should be removed immediately and the skin in contact with it washed thoroughly. The garment should be washed and aired until the odor of D-D mixture can no longer be detected.