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- 6 - for the good of mankind, it is conceivable that society may wish to put restraints on the use of certain pesticides regardless of economic considerations and crop quality. PRODUCTION AND USE OF PESTICIDES The worldwide increase in the use of persistent pesti- cidal chemicals is cause for concern, and, if trends continue, problems of environmental contamination by such chemicals may become more serious. During the first half of the 10-year period ending June 30, 1967, the use of the prin- cipal organochlorine insecticides (DDT, aldrin-dieldrin, toxaphene, etc.) in the United States continued at about the same level. During the last half of the period, a slight decrease occurred. During the whole 10-year period, the use of DDT decreased substantially, but the use of aldrin and dieldrin increased. Undoubtedly, the decrease in the use of the principal organochlorine insecticides from 1962 to 1967, inclusive, resulted from (1) changes in government regulations and agency recommendations con- cerning use, (2) public pressure to reduce the release of persistent pesticides into the environment, (3) a marked increase in the development of strains of insects resistant to persistent pesticides, and (4) the availability of use- ful new pesticides of other types. In the United States, total production of the principal organochlorine insecticides for domestic use and export increased by about one-third in the 10-year period ending June 30, 1966, but the 225 million pounds produced in the last year for Which information is available (1966-67) represents a decline from the values of the previous 5 years. During the same 10-year period, the production of DDT increased to a peak value of about 185 million pounds in 1962-63 and decreased to about 60% of this value in 1966-67. The use of herbicides and fungicides expanded steadily in the last decade of record: the use of herbicides showed a substantial growth. Continued substantial increases are anticipated for herbicides, but little increase is seen
- 7 - for fungicides. These two groups do not contribute impor- tantly to the problems commonly associated with persistent pesticides. The total worldwide production of persistent pesti- cides is not known. This information is necessary if the residues of pesticidal chemicals entering global transport systems and accumulating in the biosphere are to be evalu- ated and if a realistic appraisal of the effectiveness of residue control measures is to be made. SIGNIFICANCE OF RESIDUES Analytical Problems Residue measurement, as performed in monitoring pro- grams, consists in detecting and measuring minute quantities of pesticidal chemicals. The task requires not only the skillful use of precise, highly sensitive analytical methods but also a proper interpretation of resultant data. For convenience and speed, pesticide residue chemists generally adopt gas-liquid chromatography (GLC) as their primary analytical technique. Where only GLC is employed, one or more of the following conditions could contribute to erroneous results: (1) failure to utilize adequate confirmatory identification of the pesticide residue, (2) lack of adequate extraction and cleanup procedures, (3) lack of standardized analytical methods, (4) lack of sufficient comparisons to permit the measurement and evalu- ation of variability between cooperating laboratories and between analyses performed in the same laboratory, (5) presence of artifacts that are interpreted as pesticide residues, and (6) lack of biological materials for controls that are free of pesticide residues.
- 8 - Health Understanding of the toxicology of persistent pesti- cide residues is increasing steadily insofar as warmblooded animals are concerned. However, extrapolation of toxico- logical data from test animals to man remains uncertain. Several recent studies that have been conducted with man as the subject do not indicate that the present levels of pesticide residues in man's food and environment produce an adverse effect on his health. These studies have been principally epidemiological. --Workers involved in the manufacture of DDT, chlor- dane, dieldrin, heptachlor, and endrin have been the sub- jects of long-term studies designed to determine whether their exposure had caused any significant alterations in disease frequency or pathology. The evaluations included comprehensive health examinations and monitoring of exposure rates and pesticide residue levels in the workers' tissues. The health of those workers was not found to differ signif- icantly from that of the general population. Yet the pesti- cide storage levels in the tissues of some of the pesticide workers were 10 to 20 times higher than those found in the general population. --A nationwide epidemiological study of persistent pesticide residues in several thousand persons has not produced evidence that pesticide residues in the body have an adverse effect on health. A substantial number of the persons involved in this study were exposed to relatively high concentrations of pesticides as a result of occupation or residence: the others were exposed in no unusual way. Data are available from recent studies concerned with the relation between dosage rates and storage levels. Studies on the storage dynamics of DDT and dieldrin in human volunteers confirmed observations, made in studies of other mammals, that a dosage-storage equilibrium develops. Many persistent pesticides have been subjected to basic toxicological reevaluations in various test animals.
- 9 - These studies were conducted to measure chronic effects, including, in most cases, effects on reproduction and carcinogenicity potentials. Studies of aldrin, dieldrin, endrin, heptachlor, heptachlor epoxide, and chlordane in animals have not shown that these chemicals produce significant toxicological effects at dosage levels under 1.0 ppm in the diet, although they produce reversible "adaptive responses" of the liver at a level of 1.0 ppm or higher. These data include results from reproduction studies of rats (in Which all six of the above chemicals were used) and dogs (endrin) and from long-term exposure studies of mice, rats, and dogs (endrin and heptachlor): dogs (chlordane): and monkeys (dieldrin). Increases in liver size induced by these chemicals are accompanied by increased activity of the hepatic microsomal enzymes, and this response increases the toler- ance of an animal to the chemical. This adaptive liver response in animals occurs with the administration of any one of more than 200 compounds, including some chemicals naturally present in the environment, many drugs, and various synthetic compounds. Hence, the response in animals is common, and it would be expected to occur in humans also if a sufficient quantity of pesticide were administered. Steroid hormones are also degraded by the microsomal enzymes of the liver. A widely held view is that increased steroid degradation occurs when persistent organochlorine compounds accumulate in man and other animals. Experi- mental evidence supports this view, but, as with other toxicological effects, the minimum dosage is apparently above 1.0 ppm of these compounds in the diet. The ultimate effects, if any, of long-term low-level exposure to persistent pesticides are not understood. Although the existing studies do not indicate the low-level exposure to persistent pesticides is a health hazard for the general public or that higher-level, but subacute expo- sure is a hazard to persons in certain occupations, addi- tional research on the effects of long-term, low-level exposure is essential.
- 10 - From this statement of the need for information about possible effects of long-term exposure, it does not neces- sarily follow that such exposure would be deleterious to health. The point is that definite information is lacking. Both the epidemiological and the mammalian experimentation should be continued, and this experimentation should include studies of the biochemical and physiological effects of prolonged low-level exposure to persistent pesticides. Food For as long as persistent organic pesticides have been used, there has been concern about their presence in human food. The primary objective in regulating pesticide use has been to keep residues in food supplies at minimal and safe levels, because food is the principal route by which pesticides normally reach man. Increasing effort has been devoted to inspecting food supplies, and supplies in which residue levels were found to exceed legal tolerances have been condemned. As a result, residues in the food supplies of the United States have been maintained at remarkably low levels during a time of great increase in pesticide use. The Committee believes that, at present, pesticidal chemical residues in food are being maintained at safe levels. The interaction of inspection and enforcement with research on agricultural practices has resulted in the discontinuance of some uses that were once approved. For example: --DDT was once used to control flies and other insects on dairy cattle, but this practice was found to result in unacceptable residues in milk. --Forage grown on fields the year after application of certain organochlorine insecticides sometimes transmits residues to the milk of dairy cattle grazing on the forage. --Many of the residue tolerances for aldrin, dieldrin, endrin, heptachlor, and DDT were recently reduced or with- drawn. These actions led to discontinuance of some pre- viously recommended uses.
- 11 - Some pesticides, notably DDT, are so widely distributed that few, if any, food-producing areas are free of their residues. Thus, most foods contain measurable traces of such residues. There is no evidence that these traces are of significance to human health. Wildlife Damage to wildlife was recorded soon after persistent pesticides came into general use. Direct mortality of some birds, mammals, and fish followed the application of organo- chlorine insecticides at heavy rates over large areas. Alarm expressed by various groups, each concerned about the fate of a particular group of animals, has generated strong public reaction to these effects of the use of pesticides. The period of large-scale, very heavy applications of persistent pesticides appears to be coming to a close in the United States~ methods are being refined, applica- tion levels are being reduced, and other materials are being substituted. Because of this change in the use of chemicals, some persons expect that serious damage to the biota will be substantially reduced or eliminated. The Committee concludes that there is substantial evidence of continuing damage in some areas,particularly to fish and birds, by pesticide residues at present envi- ronmental levels. There are examples of concentrations in food chains at levels that are lethal to predators. Exposure to pesticides at sublethal levels probably pro- duces more subtle effects, causing changes in the physi- ology, biochemistry, or behavior of animals that may be harmful to the population as a whole. Certain game fish accumulate pesticidal chemicals by storage in the body and in the fat-rich yolk of egges~ there may be no injury to adult fish, but lethal or harmful amounts are acquired by the newly hatched offspring when they absorb the egg yolk. Studies on two continents show that the reproductive success of certain birds of prey is impaired by DDT and its metabolites, which apparently act to reduce eggshell
- 12 - thickness and thus to increase premature breakage of the eggs. Certain organochlorine pesticides are hazardous to many species because they are readily absorbed, are stored in body lipids, and are slowly metabolized and excreted. The body lipids are metabolized continually to support the life processes and are continually being replaced. If a predator loses the pesticidal chemical it receives in food as fast as it takes it in, accumulation of the chemical is avoided, whether the loss is by metabolism, excretion, or chemical degradation. If it loses the pesticide at a lesser rate, it accumulates the chemical in its fat. The concentration attained is tolerated without any ill effect, or it causes sublethal injury, or it is lethal. Assuming that a given organism in a food chain is not killed by the pesticide it stores, the next higher predator generally increases the concentration by accumulation. This process repeated several times may result in tissue residue levels several thousand times greater than the level existing in the environment. The same process occurs in the food supply of man, who is at the top of many food chains. However, it has been possible to stabilize the accumulation of residues in man's food below tolerance levels by controlling the pesticide input for the major food chains. Where the ecology of pesticide residues is concerned, there are important parts of the biota about which almost nothing is known. Research on the effects of persistent pesticides in the ecosystem is concerned almost entirely with the relatively few forms of life of direct interest to man. These include economic plants, economic insects (both harmful and beneficial), economic fish, shellfish, sport fish, game animals, and birds. Reports of new research may suddenly increase interest in other compo- nents of the biota. For example, interest in soil micro- organisms increased because of their role in the biochemical degradation of pesticides.
- 13 - Research on natural populations is a difficult, undeveloped field that is poorly supported at the national level. Methods of field study are imprecise, yet they must be used for populations that fluctuate continually. It is not difficult, for example, to demonstrate the massive lethal effect of a control agent, but it is almost impossible to measure a low-order change in the balance between reproduction and mortality by ordinary field studies. Information on subtle effects can come only from carefully coordinated field and laboratory investi- gations. Large-scale field experiments will be required to establish whether effects found in the laboratory occur in the field and to allow more general extrapolation from laboratory results to what may be expected in the field. There are no research programs of this magnitude. No public agency is conducting or sponsoring research on the broad ecological effects of pesticides. The Committee believes that such research programs on natural popula- tions should be initiated. The Environment Residues of DDT and, occasionally, of other organo- chlorine insecticides have been recorded in biota in all parts of the world. After being released into an eco- system, these pesticides are transported about the bio- sphere by a variety of little-understood mechanisms and are concentrated in the biota. The pesticides about which the greatest concern is felt are those having, to a marked degree, all of the following characteristics: toxicity, persistence, mobility in natural environments, and affinity for the biota. Wide distribution of DDT and some of the other organo- chlorine insecticides is inferred from scattered observa- tions. The amounts in environmental stores cannot be compared with the amounts dispersed, because there is too little information on the amounts in the environment. The general presence of DDT and its metabolites in man and other animals is taken by some to imply that large quanti- ties are stored in the biosphere: possibly the observations
- 14 - reflect only a relatively small circulating store concen- trated in the biota. Scattered measurements of DDT in rain and the atmosphere emphasize the need to know what is in the atmosphere. The ready sorption of pesticidal chemicals by silt suggests that the chemicals may be stored in silt deposits in reservoirs, rivers, and estu- aries and on the continental shelf, but a few observations support or refute this supposition. Order-of-magnitude estimates of even a few of the important environmental stores of residues would be valuable. Accumulations are investigated at sites of applica- tion in areas Where the organochlorine insecticides have been applied heavily. There is increasing evidence that when they are applied to farm fields year after year, the measurable residues in the soil reach a steady state, the amount disappearing each year being about the same as the amount that is applied. In a number of fields in the South, the steady-state level approximates the amount applied in 1 year. Other studies show much greater persistence at the site of application. Nevertheless, continued use of even the long-lived organochlorine insecticides does not result in a constantly increasing accumulation at the site of application. Therefore, the problem of high-level local contamination by pesticidal chemicals is not the same as that presented by arsenical insecticides in orchards. Disappearance of a pesticide from the site of appli- cation may or may not mean that the chemical is being destroyed. Two processes act to reduce the local concen- tration of pesticide. The first transforms the chemical by biological, chemical, and photochemical degradation. The second transports the chemical to another place. Organic pesticides are degraded in a number of ways. As a result of work in the last few years, much new infor- mation exists on the degradation, metabolism, and fate of persistent pesticides in soil, water, animals, and plants. Some persistent pesticides are subject to photo- chemical decomposition or are destroyed by nonbiological components of the environment. Some are degraded biolog- ically. Biological degradation occasionally takes place
- 15 - through enzymatic processes that may result in a complete conversion of organic chemicals to simple, well-known products. In other instances, biological degradation yields organic compounds that are normal constituents of living organisms. Although the return from this research investment has been impressive, vast gaps in knowledge remain. There is relatively little information about the ultimate fate of persistent pesticides in soil or in other parts of any ecosystem, or about the sequence in which the degradation processes take place. For some chemicals of interest, the initial products formed are known and are measurable in monitoring programs: but for these chemicals, the products formed next and the sequence in which they are formed are not known or are known for only a few types of natural habitats. Until these products are identified and their potential biological activities are ascertained, it is impossible to assess meaningfully their toxicity to man or to the biota or their residence times in nature. Rates of degradation differ with the chemical, the type of degradation, and the place in the environment. In some instances, the parent material or a product generated from it is highly resistant to degradation in all major environments into which it enters. In other instances, the pesticide is long-lived in one habitat but short-lived in another, or it is short-lived in almost any environment. With some chemicals, the degradation is partial: in others, it is extensive. Whether the overall ecological storage of a chemical increases or decreases in a given period depends, of course, on the balance between the rate of input and the rate of degradation during the period. Like many other chemicals, organochlorine insecticides move about the biosphere. Knowledge concerning this trans- port is growing, but it is still incomplete with respect to the relative importance of the various mechanisms. Most of the persistent pesticidal chemicals have an extremely low water solubility and a high affinity for colloidal sur- faces. These two characteristics explain why the materials are resistant to leaching and move no more than a few inches in the soil profile during a period of several years.
- 16 - Evidence suggests that general environmental contamination by the chemicals is more likely to occur from wind and water erosion of the soil than through leaching into ground water. One means of transport is the sorption of pesticides to soil particles, especially the fine silt and organic fractions, both at the site of application and in the aquatic environment. Normal processes of erosion transport the soil into streams, rivers, estuaries, and the sea. There are records of pesticides being carried on the ocean winds in association with particles of the talc diluent used in application, and pesticides are prob- ably carried on particles of soil in the same way. Al- though many chemicals used as pesticides have low vapor pressures, substantial amounts can be transported in the vapor state. Pesticides in the atmosphere are returned to the earth in rain. In addition, a direct air-soil exchange may take place. Pesticides are carried in the biota, but the relative importance of biological transport is unknown. On the one hand, living things carry relatively high concentrations of residues because of the process of biological concen- tration; on the other hand, the mass of the biota is very small compared with the total environment. Fish and shell- fish may remove certain organochlorine insecticides from their water environment, despite the very low concentrations present, and, if mobile, excrete the chemicals or their metabolites back into the water at a new site. Consumption of one species by a mobile predator species, with consequent transfer of a portion of their body burden of residues, provides an additional avenue of distribution. Where little or no information is available concern- ing the transport, accumulation characteristics, and degradation products of a long-lived (persistent) synthetic chemical, it would appear prudent for man to refrain from needlessly releasing the chemical into the biosphere. The crucial questions are: What are the degradation products? How toxic are they? What is their behavior in nature? If a long-lived chemical is released and is later found to be toxic, nothing can be done about the store already dispersed; it must remain in the biosphere until dissi- pated by the slow processes of degradation and removal.
- 17 - Release of short-lived material is a different matter. If such a material is released and is later found to be hazardous, the store in the biosphere will be rapidly reduced once the use of the material is discontinued. Biosphere Stores The threat of continued accumulation of residues in the environment is responsible for much of the appre- hension about the future use of the organochlorine insec- ticides. Therefore, their rate of degradation to nontoxic compounds of known biological behavior is a crucial factor. Evidence that degradation destroys residues about as fast as the pesticides are released would be reason to believe that global environmental levels can be controlled by raising or lowering the rate of release. Evidence of continued accretion in the biosphere should force a prompt reappraisal of continued use. Three kinds of evidence can be examined in deciding whether chemical residues are increasing in the biosphere: --Evidence gained in studying degradation processes to determine how rapidly chemicals are being destroyed. --Evidence from monitoring to detect changes in con- centration in selected compartments of the biosphere. --Evidence from measuring residue storage in the total biosphere. The first two procedures are producing considerable knowledge: however, the third is receiving too little attention. Chemical degradation of pesticidal chemicals has been investigated extensively in the laboratory. Some studies have been made under selected field conditions, but it is difficult to evaluate the significance of transport away from the study area. Available infor- mation indicates that rates of degradation vary with
- 18 - environmental conditions. Evaluation of the overall rate of disappearance from the biosphere requires quantitative knowledge concerning (1) partitioning of the total residue store and (2) rates of degradation in the principal envi- ronmental components. The data now available do not permit estimation of the rate of disappearance of any of the persistent pesticides from the biosphere. The main objective of the present monitoring programs is to detect changes of pesticide concentration in selected parts of the biosphere. To obtain information on changes, it is necessary to operate a program for several years. The best-organized parts of the present programs relate to pesticide residues in man and his food supply. They cannot furnish reliable indications of changes in the total envi- ronment. (See "Monitoring Residues in the Environment," page 19.) Estimates of the total amounts of residues stored throughout the biosphere would make it possible to establish whether present stores are about the same as the amounts of pesticides that have been manufactured or whether degrada- tion processes have already disposed of most of the pesti- cides that have been manufactured. It seems essential at this time to estimate the store of persistent chemicals in the environment and determine whether this store is increasing or decreasing. To be meaningful, the estimate must be on a global scale. Modern sampling design would probably make such an under- taking feasible if international cooperation could be obtained. Presumably, residues are stored primarily in the soil, the water, and the atmosphere~ the biota is probably too small to store a significant mass of pesticides. Metallic Compounds Several metallic elements are used as pesticides in both the organic and the inorganic form. No matter what form is used, the element persists. Traces of arsenic, lead, and copper occur naturally and are widely
