|
||||||
|
||||||
|
|
|
|||
|
Water Quality Ambient Water Quality Guidelines for Chlorophenols 5. BIOLOGICAL EFFECTS OF CHLOROPHENOLS The effects of chlorophenols on organisms are reported under the following headings: cytotoxicity, immunotoxicity, embryotoxicity, fetotoxicity, teratogenicity, mutagenicity, carcinogenicity, and enzymatic/metabolic effects. Most cellular, organ and whole-body effects have been determined on laboratory animals, usually rats and mice, and most genetic effects have been determined on bacteria. Since the effects of chlorophenols are the same in all eukaryotic aerobic organisms, data from one species are generally applicable to others. The main reason for different guidelines in different habitats or for different organisms is the relative efficiency of uptake and transport of chlorophenols to the active site in the mitochondria. Table 5.0 summarizes some cytotoxicity, teratogenicity, fetotoxicity, and embryotoxicity data of the chlorophenols.
The only cytotoxicity effects reported consist of chromosomal aberrations and effects on mitosis. These are summarized in Table 5.0. 4-MCP Root cells of the bean, Vicia faba, were exposed to 250 mg/L. This caused genetic malfunctions including a decrease in the mitotic index, anaphase bridges, lagging chromosomes, cytomixis and disturbances in telophase (448, 502, 503).
Buds of the bean, Vicia faba, were sprayed with 16.3 g/L 2,4-DCP and root cells were exposed to 62.5 mg/L. The buds and root cells had genetic malfunctions including meiotic alterations of chromosome stickiness, lagging chromosomes and anaphase bridges; in addition, root cells had cytomixis, disturbed prophase and metaphase, and chromosome disintegration (448, 502, 503).
The roots of the bean, Vicia faba, were exposed to PCP at 174, 87 and 43.5 ng/L. This caused genetic malfunctions including a decrease in the mitotic index, anaphase bridges, lagging chromosomes, cytomixis and disturbed meta- and ana-telophase (448, 502, 503). Chromosomal aberrations, breaks and gaps, were examined in workers in a wood treatment plant. Air, serum and urine PCP levels were monitored. No differences were found between workers exposed year-round and controls, but the sizes of the study groups were only six people and four people, respectively (573).
PCP Immunotoxic effects of PCP are reported in mice, rats, cattle and chickens (199); the ability of organisms to resist bacterial infection is compromised (306). PCP causes thymic hypoplasia in cows; suppresses total leucocyte counts, gamma globulins and IgG in young pigs; reduces humoral immunity and impairs T-cell cytolytic activity in vitro in mice; and increases cell-mediated immunity in rats (736).
PCP Neurotoxic effects are reported for PCP, but many are likely due to dioxin contaminants instead of the PCP (199, 307). PCP does apparently increase cell detachments in mouse neuroblastoma cells and cause a transient alteration in brain tissue enzyme activity in rats (736).
Some chlorophenols are reported as fetotoxic but not teratogenic (288, 294, 314, 320, 321, 322, 737, 738).
Pregnant mice received 9.0 or 0.9 mg/kg orally during days six to 15 of gestation. Embryo mortality increased at 9.0 mg/kg as did the rate of resorptions. No fetotoxic effects were reported at the 0.9 mg/kg dose rate(480).
The tetrachlorophenols reduce the number of offspring, neonatal survival and the growth rate of the weanlings; but higher dose rates are required than for PCP (38, 288, 294, 322).
Female 250 g Sprague-Dawley rats were given oral doses of 2,3,4,6-TTCP in oil and sacrificed on day 21 of gestation. The dose rates were 10 and 30 mg/kg/d. There was delayed ossification of the skull in 17% of the 30 mg/kg/d group; the 10 mg/kg/d dose was a no effect level. There was no difference between purified and commercial grade 2,3,4,6-TTCP (322). In a similar experiment doses of 99% pure 2,3,4,6-TTCP were given at 25, 100 and 200 mg/kg/d . Maternal toxicity occurred at 200 mg/kg/d and reduced weight gain at 100 mg/kg/d, but no effects were seen at 25 mg/kg/d. Pre-implantation losses of 3 to 4% occurred at 100 mg/kg/d and 200 mg/kg/d. Malformed female fetuses were found, but only at non-statistically significant rates (531).
High dose rates of pure PCP, 26 to 30 mg/kg, reduced the number of offspring, neonatal survival and weanling growth rates of rats (38, 288, 244, 322). PCP is fetotoxic, delays development, reduces litter size, reduces neonatal body weight, survival and growth (199). PCP is embryotoxic but not teratogenic (225, 227). No teratogenicity was observed but some fetotoxicity and maternal toxicity occurred in rats, hamsters and mice whose mothers were given doses of PCP in the 0.34 to 60 mg/kg range during gestation (558, 564, 565, 566, 567). PCP was usually found to be fetotoxic to rabbits and rats but there were some experiments which did not show such effects at the dosages used. Dose related effects were sometimes absent and NOEL values were sometimes found to be in the tens of mg/kg/day range (737, 738, 739, 740, 741).
Some chlorophenols are fetotoxic but not teratogenic (288, 294, 314, 320, 321, 322, 737, 738). 2,4,5,-TCP Mice were given 0.9 or 9.0 mg/kg orally at days six to 15 of gestation. No teratogenic effects were noted at either dose level (480).
In rats, 2,3,4,6-TTCP is not teratogenic (322).
Though it is embryotoxic, PCP is not teratogenic (227, 288) at doses up to 30 mg/kg/day for 2 months prior to mating and continuing through lactation. PCP is not teratogenic (737, 738).
Rasanen et al., 1977, considered that, due to negative Ames tests, there was little chance of carcinogenic or mutagenic activity in any of 2,3-, 2,4-, 2,5-, 2,6-, 3,4- or 3,5-DCP; 2,3,5- 2,3,6-, 2,4,5- or 2,4,6-TCP, or 2,3,4,6-TTCP (231). There is, however, some question in the literature about the ability of the Ames test to identify organochlorine mutagens consistently. Table 5.6 summarizes the results of the Ames assays with Salmonella typhimurium, which are discussed below.
Ames assays, at 200 µg/plate, were carried out on Salmonella typhimurium -TA1535, TA1537, TA1538, TA100 and TA98. Only TA1537 showed a slight increase over background rates and 4-MCP may be considered a borderline mutagen (446).
The Ames assays with Salmonella typhimurium -TA98, TA100, TA1535, and TA1537 were carried out at 0.5, 5, 50 and 500 µg/plate, using male Wistar rat livers. The 500 µg plates were toxic but negative for mutagenicity; no tests showed any increase in revertant colonies (231).
The Ames assays at 0.1, 1, 100 and 1000 µg/plate, using Salmonella typhimurium -TA100, were negative for mutagenic effects (430).
The Ames assays using Salmonella typhimurium -TA1538 were negative for mutagenicity (429). This reference also used the Salmonella strains and concentrations listed under reference (231).
The mutation rate in Saccharomyces
cerevisiae was increased
by 2,4,6-TCP (200), but this congener was not active in the Ames
test (231). The MP-1 strain of this yeast was treated with 400
mg/L 2,4,6-TCP and no difference was noted in inter- or intragenic
recombinations. There was a weak forward mutation response, but
no increase in mitotic crossovers or gene conversions (465). Treating V79 chinese hamster cells with 12.5, 25, 50 or 100 µg/mL (mg/L) of 2,3,4,6-TTCP did not produce any mutants (528). In a similar experiment, a weak mutagenic response was noted (529).
PCP is not mutagenic in the Ames test (437), host-mediated assay (285) or sex-mediated lethal test on Drosophila (286). PCP was negative for mutagenicity in Salmonella typhimurium, Escherichia coli, Serratia marcescens and Drosophila melanogaster (406). It was negative in the intergenic recombination test in Saccharomyces cerevisiae, but positive for forward mutation and intragenic recombinations in the yeast (200, 410). PCP is reported as positive in the Bacillus subtilis assay (408), the mouse spot test (200) and in cultured human white blood cells or lymphocytes (410). These latter positive results are all reported as slight or weakly positive. PCP is not considered mutagenic (199). In Saccharomyces cerevisiae MP-1, a yeast, forward mutations were induced by 400 mg/L PCP. Cell survival was only 59%, but of the survivors there were about 3x as many mutations under the very heavy PCP dose (563). Mitotic crossovers were not affected by this dose, but mitotic gene conversions doubled. Using the mammalian spot test, and 50 or 100 mg/kg PCP injected into the mothers peritoneal cavity, the authors concluded that PCP is a weak mutagen (563). PCP is negative in the in vivo micronucleated polychromated erythrocyte assay on mice up to 120 mg/kg (742). Salmonella typhimurium strains TA 100, TA 98, TA 1535, TA 1537, and TA 1538, and Escherichia coli strain WP2hcr, gave negative results for mutagenicity, both with and without metabolic activation, (743, 744, 745, 746). PCP did not cause DNA damage in cultured Chinese hamster ovary cells at up to 10µg/mL (747) and a genotoxicity review indicated that PCP is, at most, a very weak inducer of DNA damage producing neither DNA strand breaks nor differential toxicity in bacterial recombinant assays in the absence of metabolic activation, it did not cause an increase in SCE induction nor induce gene mutations (748).
2-MCP In the mouse, Mus sp., a skin test of 2-MCP in benzene gave either papillomas, or papillomas and carcinomas (413).
Skin tests of 20% 3-MCP in benzene on the mouse, Mus sp., did not result in any carcinomas but did produce papillomas. The test was repeated twice a week for 15 weeks using 25 µL of the test solution (413).
Female Sutter mice were given dermal applications of 2,4-DCP at 41 mg/kg body weight, twice a week for 15 or 24 weeks. In both tests the mice had more papillomas than the controls, 1.07/0.07 and 1.62/0.15 respectively. No controls had epithelial carcinomas while the rate was 11/27 and 6/16 respectively for the test animals. EPA reviewed these data and concluded: firstly the high concentration of 2,4-DCP was enough to cause damage by irritation which may have been responsible for the papillomas, secondly no malignant neoplasia was observed unless an initiator was used, thirdly pathology was done only on a gross level, and fourthly the mice were in creosote-treated wood cages which could cause a carcinogenic response by itself (413).
Available data (in 1979) do not permit one to make an evaluation of the carcinogenicity of 2,4,5-TCP (464). Mice given 600 mg/kg in the diet for 6 months did not form hepatomas (482). Dermal application of 1 drop of 21% 2,4,5-TCP to the backs of mice twice a week for 16 weeks resulted in half the mice with observed skin papillomas, as opposed to none in the controls (483). Mice given 600 mg/kg in the diet had increased liver weights and tumor formation (484), and mice given one subcutaneous injection at a one g/kg rate did not have increased tumor numbers after 78 weeks. These were both male and female mice that were 4 weeks of age when treated (464). Mice given skin treatments of 2,4,5-TCP in benzene produced many papillomas (413).
High dose rates of 2,4,6-TCP are carcinogenic to mammalian laboratory animals (228, 38, 319, 232). The National Cancer Institute index finds 2,4,6-TCP carcinogenic, inducing lymphomas or leukemias in male but not female rats and in both sexes of mice where it induces hepatocellular carcinomas or adenomas (232). The IARC indicated that available data did not permit an evaluation of carcinogenicity (464). Commercial grade 2,4,6-TCP was given to young mice by stomach tube, at the rate of 100 mg/kg, daily for days seven to 28. For the next 74 weeks their diets contained 260 mg/kg of 2,4,6-TCP. Hepatoma and sarcoma rates were not significantly higher than in controls. The same strains of mice were given a single subcutaneous injection of 464 mg/kg of 2,4,6-TCP and observed after 78 weeks. Tumor incidences did not increase over the control group (464). Mice given skin treatments of 2,4,6-TCP in benzene did not produce any papillomas or carcinomas (413). F344 rats were put on a diet of 0, 5 or 10 g/kg of 2,4,6-TCP beginning at 6 weeks of age and continuing until they were 107 weeks old. Mean weights of treated rats were lower than controls, but there was no dose-related trend in deaths. Lymphomas or leukemias occurred in male rats, and were dose related, but did not occur in females at rates higher than the control group. Leukocytosis and monocytosis of peripheral blood, and hyperplasia of the bone marrow, occurred at a higher rate than in controls (232). Mice received 2,4,6-TCP in their diets. Male rats received 5 g of 2,4,6-TCP/kg of rat, or 10 g/kg, for 105 weeks. One group of female rats received 10 g/kg for the first 38 weeks followed by 2.5 g/kg for the next 67 weeks. A second group received 20 g/kg for the first 38 weeks followed by 50 g/kg for the next 67 weeks. Body weights of treated rats were lower than in the controls but there was no dose-related trend in mortality. Hepatocellular carcinomas or adenomas occurred and were dose related (232). To put these high dose-rate experiments into perspective, the drinking water guideline for TCPs is 2 µg/L, or a daily dose of 3 µg for a 75 kg man; which is a rate of 40 ng/kg/day. In contrast the female mice receiving 50 g/kg in their food were receiving about 250 mg/25 g animal or a rate of 10 g/kg/day. This is about 2.5 x 108 times the guideline level for man.
The dioxin contaminants in some PCP products are carcinogenic (227) but PCP itself is not (227, 38, 319, 232), even when rats receive chronic doses in their diet (38). PCP may be responsible for some Hodgkin's disease and leukemia in woodworkers, but there is no adequate epidemiology. Toxic impurities such as dioxins may be responsible rather than the PCP (233, 234, 235). PCP was not found to be carcinogenic in mice and rats after oral doses that caused obvious toxicity (411), nor in sub-cutaneous doses to rats (412). It does not promote DMBA-induced skin carcinogenesis in Sutter mice (413). PCP is not carcinogenic in rats (199). Mice were given 46 mg/kg PCP orally, daily, for 3 weeks, and then fed PCP at 130 mg/kg in their food for another 1.5 years. No increase in tumors was noted. Both sexes of two strains of mice were used (319,411). Female and male Sprague-Dawley rats were given PCP orally for 24 months (females) or 22 months (males) at 0,1,3,10 and 30 mg/kg. There was no increase in tumors at any dose (539). Mice given skin treatments of PCP in benzene did not produce papillomas or carcinomas (413).
5.8.1 Mode of Action The chlorophenols, pentachlorophenol in particular, have a number of effects on organisms which are the basis for their uses as pesticides. They are weak acids and their solubilities increase in alkaline solutions. Although absolute solubilities are low (mg/L), the solubilities are high relative to solute concentrations which cause toxic effects in aquatic life (µg/L). In the mitochondria, stored energy in foods is converted to a flow of electrons which converts ADP to ATP on its way down an enzyme pathway to the oxygen of the air. Although PCP uncouples oxidative phosphorylation in all eukaryotic cell mitochondria, it does not inhibit electron transport. The net effect is to stop the formation or release of organic phosphate from ATP, thus greatly reducing the energy supply to the cell, while increasing oxygen demand and respiration rates. Oxidative phosphorylation is uncoupled at low concentrations (39, 60, 61, 289) and inhibited at higher levels (39). This affects all aerobic organisms and is the primary mode of action of the chlorophenols. Food reserves quickly become depleted but no useful work can be accomplished since the energy is not trapped as ATP but is wasted in respiration. Enzymatic activities are affected (59, 62, 63, 64, 139) and limb regeneration in crustaceans is inhibited (74). Although food intake increases, growth still decreases due to decreased food conversion efficiency resulting from the uncoupling of oxidative phosphorylation (78, 94, 140, 151, 156, 308, 309). Since oxidative respiration increases with rising temperature, doubling for approximately every 10C rise, chlorophenols have greater effects at higher temperatures, especially in poikilotherms (5, 86). The chlorophenols, and PCP in particular, are lipophilic and the increasing ionization which occurs with rising pH reduces uptake. Thus acute toxicity and bioconcentration are reduced about six-fold over the range of pH 4 to pH 9 (6). The differences in LC50 values may vary 14-fold from season to season depending upon life-stage and metabolic activity. In any one season there can be a 40-fold difference in LC50s from species to species (113). There is a trend towards greater toxicity as the degree of chlorine saturation increases. This is likely due to greater uptake since the Ko/w increases concomitantly (78). In homeotherms, the basal metabolic rate is about 13 µg 02/min/g - 0.8; in poikilotherms it is about 2 µg O2. Basal ATP production is thus about 6.5 times less in fish than mammals. If the sizes of the enzyme pools were different in fish and mammals, but reaction rates the same, the lethal PCP dose would be 6.5 times higher in mammals than fish. However, if the enzyme pools were the same size but the rate differed, the LD50 of PCP would be about the same in fish and mammals (587, 588, 589). In mammals, from mice to cows, the LD50 is quite uniform at about 150 µg/g (315, 287) and the EC50 for mitochondrial activity is about 280 µg/g (63). In fish the majority of species tested fall at about the 200 µg/g range, 60 to 2000 is the spread between salmonids and guppies. Considering the amount of variability in the data due to pH and temperature effects, this is good agreement and seems to indicate that enzyme pools are much the same size and the rate of reaction varies with organism size. Thus a similar amount of PCP ties up the same proportion of oxidative enzymes in all eukaryotic life, and most variation in dose effects for whole organisms are a function of how readily the PCP can reach the active enzyme sites in the mitochondria. Thus, the LC50 concentration in the same animal may vary about 10-fold depending upon how the dose is given: whole body exposure, dermal patches, inhalation, oral, intraperitoneal, sub-cutaneous or intravenous. The effect of PCP is additive; it is the integrated total dose over a period of days which counts, and several small doses equal one large one. This results in the product of the LC100 and the time course over which it was determined being a constant for relatively short periods of time (69), as shown in Table 5.8.1. One sublethal effect of PCP is to cause increased oxygen consumption. In the eel, Anguilla rostratus, 0.1 mg/L doubles the basal respiration rate. Thus it can be estimated that one molecule of PCP blocks the functioning of two ATP sites (590). During PCP toxicity in fish there is a large loss in lipids, but net growth still occurs, and thus protein or carbohydrate increases must occur. Since aerobic energy production is blocked, anaerobic catabolism (breakdown) of fats provides energy more effectively than burning carbohydrates. The rate of use of the fats must be higher than the equivalent use of carbohydrates under aerobic metabolism since the ATP production rate is much lower. The electron transport process thus increases and oxygen demand rises. Effects include slowed glycolysis (carbohydrate metabolism), increased lipid catabolism, increased O2 consumption, and increased heat loss due to failure to capture the energy in the electron transport process. There is an apparent anomaly of purified PCP being more toxic than the dioxin-contaminated commercial grade, i.e. it causes an LC50 response at a lower dose rate. The commercial product has high levels of many different toxic compounds which induce the mixed-function oxidase enzyme system which serves to detoxify many toxins, including PCP. This same effect can be achieved by pre-treating the test organisms with some similar toxic material and generating a high enzyme level before treatment with purified PCP. If this pretreatment is not carried out the slower induction of these enzymes to useful levels by pure PCP alone, leads to the organism being subjected to higher levels of PCP for longer periods of time. Thus, even though the applied dose is the same, the length of time it is active at that dose level in the animal differs significantly. To achieve the same effect on animals who have had their enzymes pre-induced, requires almost three times the dose of pure PCP (288, 586).
A great many similar experiments have been performed on rats and mice and enzymatic extracts in in vitro systems. Only a few examples are given here since there is no doubt about the mode of action of chlorophenols and their effects on enzymatic systems by shutting down energy flow. Mitochondrial oxidative phosphorylation is uncoupled by chlorophenols. This affects the microsomal enzyme system by disturbing electron transport between flavine and cytochrome P-450, resulting in inhibition of hydroxylation (433, 477).
Oxidative phosphorylation in rat liver mitochondria was 50% inhibited by 2-MCP at 67 mg/L, as measured by polarographic oxygen consumption techniques (425). At 5 mg/L 2-MCP caused reversible inhibition of etiolated-pea-tissue-culture-cell-catalase activity and of crystalline beef-liver catalase (315).
Oxidative phosphorylation was inhibited 50% at 20 mg/L in rat liver mitochondria, as measured by oxygen consumption techniques (425).
Oxidative phosphorylation was inhibited by 50% at 20 mg/L of 4-MCP in rat liver mitochondria, as measured by polarographic oxygen consumption techniques (425). In an in vitro system 4-MCP inhibited the activity of lactate dehydrogenase and hexokinase by 50% at concentrations of 3.6 and 7 g/L respectively (449).
Mitochondrial oxidative phosphorylation is uncoupled by 2,4-DCP
(504, 505); inhibition is 50% at 6.8 mg/L as measured by polarographic
oxygen consumption (425). Lactate dehydrogenase activity was
inhibited by 50% at 1.3 g/L and hexokinase activity inhibited
50% by 711 mg/L 2,4-DCP (449). Liver microsomal detoxification
processes are disturbed by inhibiting P-450, the terminal oxygenation
enzyme (505). This chlorophenol inhibits phosphorylation and the electron transport system (433).
A tissue culture cell line, BF-2, derived from bluegill sunfish, Lepomis macrochirus, was exposed to 2,3,5-TCP and assayed by the uptake of neutral red dye. The EC50 was 65 mg/L (423).
In 200 g female rats fed 0.05% 2,4,5-TCP in their diet, effects were seen in liver microsomal and nuclear membrane cytochrome P-450 in 14 days (477). In rats, 400 mg/kg of 2,4,5-TCP given orally decreased microsomal NADPH-cytochrome C reductase activity and cytochrome P-450 content (479). At 200 mg/kg daily for 14 days, taken orally by male rats, 2,4,5-TCP was not toxic to the liver as indicated by hepatic glucose-6-phosphatase and serum sorbitol dehydrogenase (479). The demethylation of P-nitroanisole was inhibited, in vitro, by 50 mg/L of 2,4,5-TCP, and 20 mg/L inhibited liver microsomes from conjugating naphthol (479). The chlorophenol 2,4,5-TCP is an effective -SH group inhibitor in enzymes of wood rotting fungi (487). Mitochondria are completely uncoupled or inhibited at 10 mg/L, but 5 mg/L albumin in the mitochondria counters this inhibition (488). In rat liver mitochondria, oxidative phosphorylation was inhibited by 50% in 0.6 mg/L of 2,4,5-TCP, as measured by polarographic oxygen consumption techniques (425).
In E. coli, 2,4,6-TCP inhibits the lactose permease system (B-D-galactoside transport) in proportion to its concentration, beginning at 10 mg/L for 10 minutes (470). Chloride transport in mammalian red blood cells is inhibited 50% by 260 µg/L of 2,4,6-TCP (471). Oxidative phosphorylation in rat liver mitochondria is inhibited 50% by 3.6 mg/L 2,4,6-TCP, as measured by polarographic oxygen consumption techniques (425). In the marine mollusc, Navonox inermis, 2,4,6-TCP uncouples oxidative phosphorylation. Applied to neurons in an isolated ganglion, it causes dose-dependent reversible increases in membrane potential and conductance. The effect is on the permeability of potassium relative to other alkali cations and conduction of potassium relative to chloride (469).
The rat liver microsome detoxification system is affected, likely by inhibition of electron transport between flavine and cytochrome P-450, by as little as 7 mg/L of 2,3,4,6-TTCP in vitro (505).
A tissue culture cell line, BF-2, derived from Bluegill sunfish, Lepomis macrochirus, was exposed to 2,3,5,6-TTCP and assayed by the uptake of neutral red dye. The EC50 was 62.6 mg/L (433).
The livers of rats have enzymes which dechlorinate PCP to tetrachloro- or trichloro-p-hydroquinones (40). The microsomes of liver are active in detoxification processes. This important activity is disrupted by PCP and makes the organism susceptible to other toxins which it might otherwise be able to neutralize. PCP uncouples oxidative phosphorylation and selectively inhibits the terminal oxygenation enzyme P-450. PCP is toxic below 1 mM or 266 mg/L (505, 574). In vitro, lipase is 50% inhibited by 26.6 mg/L PCP and shows threshold inhibition at 2.128 mg/L (575).
Chlorophenols affect the mitochondria of all eukaryotic aerobic organisms in the same way by uncoupling oxidative phosphorylation. This affects the microsomal enzyme system by disturbing electron transport between flavine and cytochrome P-450, resulting in inhibition of hydroxylation. In the mitochondria, stored energy in foods is converted to a flow of electrons, which convert ADP to ATP. ATP is essential for all biological activities which require energy. Uncoupling allows respiration to continue, but stops the conversion of ADP to ATP. Thus respiration rates rise, but no energy is stored and food reserves become depleted. The major reason for different LC50 values in different organisms is the relative efficiency of uptake, transport, and elimination of the chlorophenols by different organisms. Chlorophenols are immunotoxic, fetotoxic, and embryotoxic but not neurotoxic or teratogenic. Chlorophenols are probably not mutagenic or carcinogenic; even at the very high doses used in some tests, the results are ambiguous. In almost all mammals the LD50 for PCP is quite uniform at about 150 µg/g. There is no biological justification for extrapolation of toxicity testing to concentration ranges many orders of magnitude over the normally encountered levels (466). One would expect that under abnormally high toxicity loads the normal metabolic operations of the organism would be disturbed, overloaded or bypassed and that the normal bodily processes that protect against carcinogens could be impaired or unable to cope. Organisms evolved amidst mutagens, carcinogens and other challenges to their survival, and developed the defenses to meet these challenges, but only at the concentrations which would normally be expected in the natural environment. Testing mutagens, and other toxins, at levels far above the levels at which they are, or will be, encountered, does not necessarily prove anything about whether or not they are mutagenic or toxic at normally encountered concentrations.
|
||||
|
|
