The pesticides used for protecting plants are biologically active substances and act not only on their targets, but are also a potential hazard for nature and the health of humans. Pesticides are characterized by a different toxicity to humans and warm-blooded animals. Their action manifests itself not only directly during the period of application, but may also be chronic.
Some of them, accumulating on objects of the environment, gradually concentrate in the food and fodder, become a real hazard to beneficial animals and to humans, and may lead to serious alterations in the ecosystems, biocoenosis, and landscapes. The contamination of the environment by pesticides cannot be prevented by purification facilities and trapping devices. Enormous areas of agricultural lands and hundreds of thousands of tonnes of products are treated with pesticides; consequently, they continuously circulate in the habitat of humans. Residues of pesticides are detected in the air, water, soil, and in plant and animal organisms.
This is why the chemicals used in agriculture must be studied from a general sanitary standpoint, with a view to their behaviour in the environment and to all the sources from which they may enter a human organism. The main task in ensuring safety in pesticide application is the finding of effective measures and means for the sanitary protection of the environment, especially of the atmospheric air, food products of a vegetable and animal origin, sources of water supply, and the soil, from contamination with chemicals in amounts dangerous for the health of people.
A sanitary classification makes it possible to give a comparative characteristic of various pesticides and determine what pathological effect is of the greatest danger when using a given substance. Depending on their toxicity and degree of danger, pesticides as regards their main criteria are divided into a number of categories or groups. I. According to their toxicity when inge ste d into the stomachs of warm-blooded laboratory animals (rats) (oral toxicity). l Powerful toxicants: LD_{50} up to 50 mg/kg. l Highly toxic substances: LD_{50} equals 50 to 200 mg/kg. l Moderately toxic substances: LD_{50} equals 200 to 1000 mg/kg. l Slightly toxic substances: LD_{50} over 1000 mg/kg. Following this classification, the pesticides recommended for use in agriculture are divided into the following groups: Group 1. Powerful toxicants: antu, chloropicrin, diphaci none, methylmercury chloride, gliftor, mercurbenzene, mercurhexan, methyl bromide, warfarin, zinc phosphide. Group 2.
Highly toxic substances: diazinon, dichloroethane and its distillation residues, dichlorvos, dimethoate, dinob uton, DNO C, endosulfan, “gamma” smoke pots, heptachlor, hexachlorobutadiene, lindane technical and a 50% wettable powder, metasystox, parathion-methyl, phenthoate, phosalone, phosmet, sodium pentachlorophenolate. Group 3 . Moderately toxic substances: actril AS, actril M, barium, binapacryl, blue vitriol, butyl captax + magnesium chlorate, calcium cyanamide, cambilene, carbaryl, chlorthiamid, copper oxychloride, copper trichlorophenolate, cuprosan, cyanophos, 2,4-D butyl ester, 2,4-D crotyl ester, DAEP, dazomet, D-D, DDB -te chnic al, DEF, dicofol, dimethoate granules, diquat, FDN, fenitrothion, fenthion, fenthiuram, fenthiuram molybdate, formalin, formothion, fosfaman, gamma-HCH, HCH (technical, 12% dust, 25% powder on phosphorus flour), hexathiuram, lindane ( granulated 4% and 2% fine – grained, granulate d 2% coarse-grained, 16% emulsifiable concentrate), malathion, MCPA, MCPP, metaldehyde , metam – sodium , methallyl chloride, miltox special, nitrafen, pebulate, phytobacteriomycin, phenmedipham, polychloropinene, polychor, propachlor, pyrazophos, rodan, 2,3,6-TBA, thiram, tigam, toxaphene, trichlorfon, trichlorol-5, trichloromethane-3. Group 4. Slightly toxic substances: agelon, anilat, arenarin, atrazine, az ipro tr y ne, ba nle n, banvel D, beauverin, benomyl, Bordeaux mixture, bromophos, bromopropylate, calcium chloride, captan, carboxin, chloramben, chlorbromuron, chloroxuron, chlorpropham, chlorthal-dimethyl, copper naphthenates, cuprozin-1, cycloate, 2,4-D amine salt, 2,4-D low-volatile esters, 2,4-D octyl ester, dalapon, 2,4-DB, desmetryne, dialen, diamet-D, dicamba, dichloralurea, dichlofluanid, dilor, dino c ap, di phe nami d, diuron, do di ne, entobacterin, EP TC, fenuron, fluometuron, folpet, formulations No. 30, 30A, 30M, 30S, 30SS, green soap, green vitriol, hexachlorobenzene, lenacil, lime sulphur, linuron, lumeton, magnesium chlorate, menazon, metiram, metobromuron, metoxuron, meturin, milbex , molinate , monolinuron, monuron, nitrofor, norea, oxadiazon, oxine – copper, pentanochlor, petroleum oils, picloram, polymarzin, poly-triazine, prometryne, pro panil, propazine , pyrazon, quintozene, simazine, sodium TCA, sulphur (colloidal, wettable powder, ground, and lump), tetrachlorvinophos, tetradifon, thiophanate -methyl, triallate, trifluralin, zineb. Pesticides relating to powerful toxic ants and highly toxic substances are a great hazard because of their ability to cause acute poisoning.
To predict the danger of acute poisoning, the zone of toxic action of a pesticide is determined from the ratio of the median lethal dose (LD_{50}) to the threshold dose. The assortment of pesticides in 1979 contained 20% of powerful toxicants and toxic substances, while in the nearest future their amount will diminish to 5%. Work with pesticides of the first group is performed by a constant staff; the procedure of obtaining, transporting, storing, and keeping accounts of them is regulated by special instructions. Food products containing residues of pesticides belonging to powerful toxicants and highly toxic substances in amounts exceeding the tolerance levels may not be used as food. Exceptions are food products that are used only after heat treatment that completely decomposes these pesticides. I I .
According to their toxicity in entering through the skin (dermal toxicity). l Acute: LD_{50} below 300 mg per kg, the skin-oral coefficient is less than 1. l Pronounced: LD_{50} equals 300 to 1000 mg per kg, the skin-oral coefficient is 1 to 3. l Slightly expressed: LD_{50} over 1000 mg per kg, the skin-oral coefficient is over 3. The skin-oral coefficient is defined to be the ratio of the value of LD_{50} determined when a substance is applied to the skin to the value of LD_{50} when it is introduced orally into the stomach. For example, if LD_{50} when entering through the skin is 300 mg per kg, and when introduced orally is 400 mg per kg, the skin-oral coefficient will be 300 : 400 = 0.75.
Pesticides characterized by acute dermal toxicity such as DEF, bromophos, endosulfan, and dichlorvos are especially dangerous. Reliable means for protecting the skin are needed when working with them. III. According to the degree of volatility. l A very dangerous substance: the saturating concentration is larger or equal to the toxic one. l A dangerous substance : the saturating concentration is larger than the threshold one. l A slightly dangerous substance: the saturating concentration has no threshold action. Substances having a high volatility penetrate into an organism through the respiratory organs and are characterized by an inhalation toxicity.
Pesticides used for fumigation and mercury-containing substances for the treatment of seeds are very dangerous. When working with them, one must reliably protect the respiratory organs, using gas masks with the corresponding filter. IV. According to cumulation (chronic toxicity). l Substances having supercumulation: the cumulation coefficient is less than 1. l Pronounce d c umulation: the cumulative coefficient is 1 to 3. l Moderate cumulation: the coefficient is 3 to 5. l Slightly expressed cumulation: the coefficient is over 5. By cumulation is the accumulation of a poison in an organism as a result of incomplete detoxification and excretion from the organism or an increase in the effect of its action. Material and functional cumulation are distinguished.
Material cumulation is the name given to the accumulation of a toxic substance in an organism as a result of repeated contacts. The ability of material cumulation is a property of many pesticides from among the group of organochlorine compounds and of mercury pesticides. Functional cumulation signifies not accumulation of a poison, but summation of the effect of its action. Certain organophosphorus compounds (parathion-methyl, malathion) have such a property. When the coefficient of cumulation is lower, the cumulative action of the relevant pesticide is more pronounced. Supercumulation and pronounced cumulative action are properties of pesticides exhibiting a high persistence in biological media that are capable of circulating in food chains and of gradually accumulating in organisms. V. According to the persistence (in the soil).
l Very persistent substances: the time needed for decomposition into non-toxic components exceeds two years. l Persistent substances: the time ne e de d for decomposition into non-toxic substances is 0.5 to 2 years. l Moderately persistent substances: the time needed for decomposition is 1 to 6 months. l Slightly persistent substances: decomposing into non-toxic components within a month’s time. Very persistent substances must never be used in agriculture.
Among them are simazine, monuron, thiram, mineral oils, and DNOC. Substances having a comparatively low volatility and slowly changing their chemical properties such as HCH, malathion, trichlorfon, dimethoate, zineb, metam-sodium, and esters of 2,4-D belong to the group of moderately persistent pesticides. Pesticides subject to considerable evaporation and chemical changes under the influence of the environment relate to the group of slightly persistent substances. They include fumigants, mercury formulations, and parathion-methyl dust. They have to be stored in sealed containers. In addition to the basic criteria listed above that make it possible to give a sanitary appraisal of pesticides; other pathological effects of their action are studied such as blastomogenesis, mutagenesis, teratogenesis, and embryotropism.
(THE WRITER IS AN ASSOCIATE PROFESSOR (RETD.) & FORMER HEAD, DEPARTMENT OF BOTANY, ANANDA MOHAN COLLEGE)