The Toxicity of DDT

Although the toxicity of DDT to the amphipod Hyalella azteca decreased with increasing carbon content of the sediment, this was not the case for endrin (Nebeker et al. 1989) so that specific mechanisms of interaction even between neutral xenobiotics and the organic carbon in the sediment phase may be of determinative significance. The results with DDT are, in fact, consistent with evidence from equilibrium dialysis experiments of its association with dissolved humic material (Carter and Suffet 1982). 

Kouyoumjian HH, Villeneuve JP (1979) Further studies on the toxicity of DDT to planaria. Bull Environ Contam Toxicol 22 109-112... 

The two examples just given are of localized effects associated with the acute toxicity of DDT and DDD to organisms in higher trophic levels. A more wide-ranging toxic effect associated with population decline was eggshell thinning caused by the relatively high levels of p,p -DDE in some predatory birds (see Table 5.7). 

The fundamental chemistry, especially of the newer economic poisons, is of primary importance. The mechanism of action of the various types of economic poisons and the relation of structure to toxicity of insects are of fundamental interest. Chemical versus biological methods of evaluation should be presented. Performance methods of evaluation of these chemicals have been given careful consideration by several workers. Emphasis was placed by several workers on the need for much additional information on various aspects of the problem regarding the use of DDT, 2,4-D, and other pesticides. There is direct importance in studies on the metabolism of DDT. 

Accessibility of the deposit to the insects by contact is the chief feature of the small vial method, but fumigant action cannot be eliminated entirely. In the case of DDT this has been found to be unimportant, for flies kept in the vials out of contact with the surface are not affected. But with 7-hexachlorocyclohexane or parathion there is a noticeable toxic effect. If the vials are stood upright, laid on the side, or hung upside down, there is a decrease in the mortality produced in the order given. The position on the side has been adopted because it avoids extremes and because practical use of an insecticide often involves limited but not accentuated ventilation. 

The chemistry, insecticidal, activity and toxicity of the major organofluorine insecticides are reviewed. In the ten years since the discovery of DDT opened up a new field of endeavor for the chemist, biologist, and toxicologist, activity in the field of fluorine-containing insecticides has been great. 

The complete replacement of chlorine in the DDT molecule by fluorine gives (p-FCeH CHCFs, a compound with greatly decreased toxicity toward fruit flies and thrips and probably other species. This reduced activity is matched by the low activity of (p-ClCeH CHCFs which, when contrasted to the potency of DDT and DFDT, shows that the attenuation is caused by the replacement of the three chlorine atoms on the alkane bridge. The corresponding substitution of only one chlorine atom by fluorine gave a compound having as much activity as DDT against the chafer beetle. 

With the advent of DDT it became possible to think in terms of the eradication of insect pests instead of their control only. Although very lethal organic toxicants such as the pyrethrins and rotenone had been previously employed, their instability under normal conditions of use limited their utility. 

The situation with respect to dieldrin is altogether different. No insect toxicant hitherto available, with the exception of DDT, has been characterized by the possession of insect toxicity which continued for long periods after its application. In this respect, dieldrin is unique in that, in addition to its high order of insect toxicity, it possesses a span of residual activity comparable to that of DDT. 

For some important insect pests there are still no satisfactory chemical controls. Such problems should be given due consideration in the development program. Many of these problems appeared to be solved with the discovery of DDT, benzene hexachlo-ride (hexachlorocyclohexane), and some of the more recent insecticides. Further studies of the toxicity of some of these products to warm-blooded animals have raised the important question of the advisability of continuing their use where food and feed products are concerned. Considerable attention is being centered on finding safer analogs, such as TDE and methoxychlor, and new and better insecticides. 

The conversion in the animal body of at least some of the water-insoluble chlordan to a water-soluble degradation product must facilitate the elimination of the poison through its excretion into the urine by the kidneys. Moreover, the degradation of chlordan as shown in the present experiments may be a mechanism for its detoxification, as in the case of DDT (1). Only the isolation of the degradation product, its identification, and a study of its toxicity can determine this point. 

The detection of DDT in surface waters as (DDE + DDD)/DDT < 1 reflects minor transformation of the initial insecticide in the soil and hence the toxicants loss or leaching from recently formed RPA or so-called local pedogeochemical anomalies, LPA (former action zone of plants for DDT preparations production places of accidental spillage or output of the preparations areas of storage or burial—tombs, etc. that are characterized by extremely high contamination level (Lunev, 1997 Silowiecki et al., 1998). 

At the 1000 ppm rate, fish become lethargic within one day, started to die by day 3 and were all dead by day 15. Additional fish, added on day 16, continued to die, but at a much slower rate in the W sediment tanks (Table II). A few fish also died in the 100 ppm tanks, necessitating the addition of more fish on day 50. TLC analysis of the fish extracts show that DDT decreased slowly with time, possibly accounting for the toxic response (Figure 3). On day 3 about 90% of the total extracted from fish was DDT which decreased slowly to 60 to 70% by day 57. DDD and polar metabolite increased slowly with time. The concentration of DDT in fish from the W sediment tanks remained 10 to 20% higher than fish from the W/0 sediment tanks. 

Diffusion Experiment Results of the diffusion experiment are shown in Table VI. One or more cm of untreated soil covering 20 g of soil treated with 100 ppm c DDT was very effective in preventing toxic concentrations of DDT from diffusing into water for one year. If any DDT did diffuse through the soil into water, the concentration was not sufficiently high to affect the survival or reproduction of daphnids. A 60% reproductive impairment has been reported when daphnids were exposed to 100 ng/L DDT W. Therefore, on the basis of the daphnid bioassay, the concentration of DDT in water over the 1 cm of soil was at or below 100 ng/L. On the other hand, where untreated soil did not cover the DDT layer, daphnids never survived more than 7 days. This result is very similar to those from the microecosystem experiment. The 1-ml water samples indicated a total DDT concentration of 10 to 20 ppb. In addition, TLC analysis of treated soil extracts after one year showed the expected conversion of DDT to DDD, but only when covered by 1 or more cm of soil. For the uncovered soil, 87% of the radioactivity was DDT. Apparently, 1 cm of soil was sufficient to produce the anaerobic conditions known to be necessary for conversion of DDT to DDD (j 2). 

Toxic organic chemicals can harm organisms in a variety of ways. Many animals simply become ill after feeding on poisoned plants. Others survive but are then eaten by larger animals that prey on them. The toxins accumulate in predators bodies over time as they consume affected prey animals. Thus, the concentration of a toxic chemical tends to increase as it moves up the food chain in a process known as biomagnification. In one study, for example, the concentration of DDT was found to be 0.25 ppm in the phytoplankton in a body of water, 0.123 ppm in zooplankton, 1.04 ppm in small fish, 4.83 ppm in larger fish, and 124 ppm in birds that fed on the fish. 

It was the selective toxicity of the insecticide DDT that was destined to have a most profound effect on attitudes to chemical safety. DDT was a chemical that had first been synthesised decades before the Swiss chemist Muller discovered its potent insecticidal action in the late r930s. What was so remarkable about DDT was its selectivity. Even in extremely small doses, it was lethal to many species of insect yet it was remarkably non-toxic to humans even at quite high doses (Figure 6.3). The manufacture of DDT is... 

Figure 6.3. The LDj is the concentration of a substance that kills 50% of a test population of organisms. It is a useful measure to compare the toxicity of different substances but it is a very controversial measurement when applied to mammals because of the suffering caused to the animals used to gain the data. Note that if the LDj dose is increased by a factor of 10, nearly 100% of the population will die and if the concentration of the substance drops to one-tenth of the LDg then only a very small percentage will die. The notorious insecticide DDT has an LDj of 150-300 mg kg for rats, 1 g kg for goats, 2 g kg for ducks but <0.1 mg kg for some aquatic invertebrates.

To determine the influence of wind, velocity, and particle size on the toxicity of oil aerosols containing DDT, Latta and coworkers (31) conducted experiments in a wind tunnel at wind speeds of 2, 4, 8, and 16 miles per hour. The median lethal dose for a female mosquito was 1 particle 83 microns in diameter from a 10% DDT solution. 

The acute and/or chronic nature of the toxicity of a chemical should be part of any decision-making process about its use or subsequent release. The focus cannot be solely on reduction of acute hazards, which tends to be easily achievable. The majority of cases in which chemicals have been released into the environment, only to cause serious ecological impacts over large spatial scales, were usually identified after many years, and at chronic low-dose exposures, with low acute toxicity to nontarget organisms. The classic examples of DDT and other chlorinated pesticides such as dieldrin and toxaphene, along with PCBs, exemplify the flaws in an approach that focuses on acute hazards, with more recent examples being the perfluorinated... 

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