Fetal methylmercury poisoning. Relationship between concentration in single strands of maternal hair and child effects

Pregnant women consumed bread that was prepared from methylmercury-treated wheat. Single strands of maternal head hair were analyzed by x-ray fluorescence spectrometry. The index of fetal exposure was the maximum hair mercury concentration during gestation. Effects were measured by the frequency of psychomotor retardation, seizures, and neurological signs in the children. A dose-response relationship was demonstrated for fetal effects of methylmercury. Analysis of single hair strands provides a better index of acute or subacute fetal exposure than analysis of bundles of hair; the duration and degree of exposure are more accurately defined. A sex difference in response is discussed.

The role of biomarkers in reproductive and developmental toxicology

A massive outbreak of methylmercury poisoning took place in the winter of 1971-1972 due to the consumption of homemade bread contaminated with a methylmercury fungicide. The longitudinal analysis of the mother's head hair, collected after delivery of the baby, provided a means of recapitulating exposure to methylmercury during pregnancy. Methylmercury is incorporated into newly formed hair at a concentration that is proportional to the simultaneous concentration in blood. Since hair grows at a rate of approximately 1 cm/month, longitudinal analysis of the hair strand, centimeter by centimeter, will give a month by month recapitulation of blood levels. Depending on the length of the hair strand, it is possible to recapitulate several years of exposure. Using longitudinal hair analysis, it was possible to compare the methylmercury levels in the mother during pregnancy with the severity and frequency of effects in her offspring. As in the previous incidents, high levels of prenatal exposure led to severe brain damage. However, it was also possible to identify milder effects of methylmercury as manifested by delayed development. It was possible to demonstrate a dose-effect and dose-response relationship between the maximum concentration of methylmercury in maternal hair during pregnancy and evidence of delayed development and mild neurological abnormalities in the offspring. These relationships provided quantitative evidence that the developing nervous system is more susceptible to damage than the mature brain.

Comparison of the protection given by selenite, selenomethionine and biological selenium against the renotoxicity of mercury

The protective effect of selenite, seleno-dl-methionine and biological selenium against the renotoxicity of mercury was tested in rats. As the source of biological selenium, the liver soluble fraction of rats given 60 mumoles/kg selenite 3 days before sacrifice was used. The aim of the experiments was to test whether protective efficiency follows the reported order of ability to form HgSe. Mercury was given subcutaneously in doses of 2.5, 5.0 and 7.5 mumoles/kg HgCl2 and selenium was given in equimolar doses at the same time as Hg2+. Liver soluble fraction, biological selenium or liver soluble fraction supplemented with selenite or seleno-dl-methionine were given orally, while in experiments without liver soluble fraction the two selenium compounds were given subcutaneously. Biological selenium was tested only at the two lower dose levels. Both biological selenium and seleno-dl-methionine decreased the urinary excretion of mercury in the first 48 h, but less so than selenite and only selenite decreased the renal content of mercury at the end of this period. Urinary alkaline phosphatase activity and plasma urea nitrogen at the 2.5 and 5.0 mumoles/kg dose levels decreased in the order of no selenium greater than biological selenium greater than seleno-dl-methionine greater than selenite. As the reported HgSe formation increases in the same order, the experiments support the role of HgSe formation in the protective effect. The degree of necrotic damage in the P2 and P3 regions of the proximal tubular cells increased in the same order as the biochemical indicators at the 5.0 and 7.5 mumoles/kg dose levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Homeostatic control of manganese excretion in the neonatal rat

Previous studies in neonatal and suckling animals showed that immature animals have a greatly diminished capacity to excrete manganese and therefore were considered to be unable to regulate tissue manganese concentrations. In contrast, the present studies indicate that suckling rats have the capacity to excrete excess manganese at rates nearly comparable to those of adults. Eight- to 10-day-old rats given a tracer dose of 54MnCl2 (essentially carrier free), either via gavage or by intraperitoneal injection showed little elimination of the 54Mn until the 18-19th day of life, when there was an abrupt increase in the rate of the metal's excretion. However, when manganese was given in doses of 1 and 10 mg/kg, the young animals excreted from 30-70% of the dose in only 4 days, at which time a new rate of excretion was achieved. This enhanced rate of excretion remained constant until the 18-19th day of life, when it was again accelerated. Biliary excretion of manganese, the primary route for the elimination of the metal, was only 30-60% lower in 14-day-old rats compared with adults at doses ranging from tracer to 10 mg 54Mn/kg. For both the 14-day-old and adult rats, an apparent biliary transport maximum was reached at a dose of 10 mg Mn/kg. These studies indicate that the excretory pathways for manganese are well developed in the neonatal rat. The avid retention of tracer quantities of manganese by the neonate may be a consequence of the scarcity of this essential trace metal in its diet.

Biological monitoring of metals with special references to the early stages of the life cycle

In summary, the biological monitoring of toxic metals in the early stages of the life cycle has not yet received the attention it deserves. Most metals have simply not been studied at all. Some successful applications with mercury, lead, and cadmium have been reviewed here. But even for these widely studied metals, there is a great need to study physiological models for metal disposition in developing animals.

The therapeutic use of 2,3-dimercaptopropane-1-sulfonate in two cases of inorganic mercury poisoning

Two patients had heavy metal intoxication resulting from industrial exposure to mercury vapors. One patient remained asymptomatic despite high urinary mercury levels. The second presented with a documented toxic reaction, including abnormal electromyograms and hematuria. Treatment was begun with 2,3-dimercaptopropane-1-sulfonate, and investigational drug used under Food and Drug Administration guidelines (IND 19,276). Use in both patients resulted in a dramatic change in excretion half-life of the mercury, from 33.1 days before treatment to 11.2 days during therapy. No significant toxic effects related to drug therapy were observed. Clearing of hematuria and reversal of electromyographic abnormalities were prompt in the symptomatic patient. Therapy with 2,3-dimercaptopropane-1-sulfonate was effective in these two patients.

Spectrum of tests available to evaluate occupationally induced renal disease

The traditional tests used to screen workers for renal disease are inadequate to detect early or moderate loss of renal function. More sensitive tests are seldom measured reliably in the occupational setting. Several noninvasive tests of kidney function have proven useful in monitoring the effects of special toxins. This paper will discuss the advantages and disadvantages of various tests of renal function that may be used to detect nephrotoxicity in workers. The discussion includes a brief overview of the tissue types that can be injured by toxic agents and the specific tests that may be used for each. Special research projects are needed to validate the clinical significance of these and other medical tests through careful study in a controlled clinical setting.

An examination of the oxidation of mercury vapor by rat brain homogenate

The oxidation of mercury vapor (Hg degrees) to divalent inorganic mercury (Hg2+) was studied in rat brain homogenates. By using a "degassing" method, it was possible to speciate the mercury present in the homogenate and, for the first time, to measure the rate of oxidation as a function of the substrate (Hg degrees) concentration. Mercury oxidation was first-order with respect to substrate concentration at all concentrations tested, and the first-order rate constant for the oxidation process was proportional to homogenate concentration. The role of catalase compound I in mercury vapor oxidation by brain homogenate was examined by observing the effects of two inhibitors of catalase (catalase compound I) on homogenate mercury-oxidizing activity and catalase activity. Sodium azide (50 mM) completely inhibited both mercury-oxidizing activity and catalase activity. Aminotriazole (3-amino-1H-1,2,4-triazole) (50 mM) completely inhibited only mercury-oxidizing activity; some residual catalase activity was found in the aminotriazole-treated homogenate. It was concluded that catalase compound I plays a major role in the oxidation of Hg degrees, but the possibility that catalase-independent pathways make a minor contribution cannot be excluded.

Impact of effects of acid precipitation on toxicity of metals

Acid precipitation may increase human exposure to several potentially toxic metals by increasing metal concentrations in major pathways to man, particularly food and water, and in some instances by enhancing the conversion of metal species to more toxic forms. Human exposures to methylmercury are almost entirely by way of consumption of fish and seafood. In some countries, intakes by this route may approach the levels that can give rise to adverse health effects for population groups with a high consumption of these food items. A possible increase in methylmercury concentrations in fish from lakes affected by acid precipitation may thus be of concern to selected population groups. Human exposures to lead reach levels that are near those associated with adverse health effects in certain sensitive segments of the general population in several countries. The possibility exists that increased exposures to lead may be caused by acid precipitation through a mobilization of lead from soils into crops. A route of exposure to lead that may possibly be influenced by acid precipitation is an increased deterioration of surface materials containing lead and a subsequent ingestion by small children. A similar situation with regard to uptake from food exists for cadmium (at least in some countries). Human metal exposures via drinking water may be increased by acid precipitation. Decreasing pH increases corrosiveness of water enhancing the mobilization of metal salts from soil; metallic compounds may be mobilized from minerals, which may eventually reach drinking water. Also, the dissolution of metals (Pb, Cd, Cu) from piping systems for drinking water by soft acidic waters of high corrosivity may increase metal concentrations in drinking water. Exposures have occasionally reached concentrations which are in the range where adverse health effects may be expected in otherwise healthy persons. Dissolution from piping systems can be prevented by neutralizing the water before distribution. Increased aluminum concentrations in water is a result mainly of the occurrence of Al in acidified natural waters and the use of Al chemicals in drinking water purification. If such water is used for dialysis in patients with chronic renal failure, it may give rise to cases of dialysis dementia and other disorders. A possible influence on health of persons with normal renal function (e.g., causing Alzheimer's disease) is uncertain and requires further investigation.(ABSTRACT TRUNCATED AT 400 WORDS)

Biliary secretion of glutathione and of glutathione-metal complexes

As bile is the main route of elimination of many metals, a large number of studies have been directed toward the characterization of the hepatobiliary transport of both endogenous and exogenous metals. Although some progress has been made, we still know little of the basic mechanisms involved in the hepatocellular uptake of metals, in their intracellular translocation and metabolism, or in their transport into bile. Our recent studies have focused on the last step in the hepatobiliary transport of mercury, namely, the secretion of the metal from liver cells into bile. The rate of secretion of methyl and inorganic mercury into bile was low in suckling rats and rapidly increased to adult rates soon after weaning. These changes closely followed similar developmental changes in the biliary secretion of reduced glutathione (GSH). When GSH secretion into bile was completely inhibited, without changing hepatic levels of GSH or mercury, mercury secretion was also completely blocked. Mercury secretion paralleled individual and sex-related differences in GSH secretion. At the same time, the secretion of mercury was independent of bile flow, of the thiol and mercury concentration gradients between bile and liver cells, and of those between bile and plasma. Our results, therefore, indicate a close coupling between the secretion of mercury and that of GSH. These in vivo findings, along with in vitro studies by others in vesicles isolated from the canalicular membrane of the liver cell, indicate a carrier-mediated transport system for GSH, but the nature of the linkage of this transport system with mercury secretion is not yet fully established. Our data and those in the literature are consistent with the involvement of at least two steps in the movement of mercury from liver cells to bile--the formation of a mercury-glutathione complex in the liver cell, followed by the secretion of this complex through a process closely linked to GSH secretion. The identification of GSH as an endogenous complexing agent in the transport of metals between tissues and body fluids now permits the design of therapeutic strategies aimed at exploiting this transport vehicle to effect the removal of metals via physiological routes of excretion. The present discussion considers the role of GSH in the hepatobiliary transport of metals. In doing so, a brief review is given of current understanding of hepatic GSH metabolism and transport.

Prediction of kidney mercury content by isotope techniques

A 61-year-old female patient accidentally aspirated liquid mercury during a medically ordered diagnostic procedure. To develop animal-based guidelines, liquid mercury was introduced into the lungs of four dogs. Based on the study of these animals, a method of predicting the kidney inorganic mercury burden was developed using radioactive isotope dilution techniques. It was further demonstrated in dogs that oral administration of dimercaptopropane sulfonate (DMPS) increased mercury excretion and reduced the kidney burden. A rat experiment was performed permitting a statistical evaluation of the assumptions basic to the use of the method. The method was applied to the patient with the result that the kidney inorganic mercury burden was predicted to be 28.1 mg, 8 months after the accident. Treatment with DMPS increased urinary excretion and the post-treatment kidney burden was estimated at 19.6 mg Hg. Inasmuch as the radioactive dose to the subject may be kept at a negligible level and because sensitive methods exist for measurement of radioactive and stable mercury concentrations, the technique may be applicable in special cases to the estimation of kidney inorganic mercury burdens incurred by industrial exposure.

Reproductive and developmental toxicity of metals

This paper discusses metal exposure in the male, the nonpregnant female, and the maternal-offspring unit. In the first two situations, the primary targets are the gonads. In the mother-offspring unit, consideration must be given to effects on the fertilized ovum, the growth of the embryo, and, finally, to the fetal and perinatal stages. The central nervous system may be especially vulnerable during development. The placenta also undergoes development, and either the placenta or the fetus may be the primary target. In humans, certain metals may cause abortion or other effects on the conceptus. Effects may also be produced by metal exposure both in utero and in the suckling infant. For example, methylmercury gives rise to a range of effects on the central nervous system at doses lower than those producing damage to the mature nervous system. Effects of lead and arsenic are associated mainly with postnatal exposures during infancy and early childhood, but there is reason to believe from animal experiments that some effects may occur from prenatal exposures to certain metal compounds.

Potential human health effects of acid rain: report of a workshop

This report summarizes the potential impact of the acid precipitation phenomenon on human health. There are two major components to this phenomenon: the predepositional phase, during which there is direct human exposure to acidic substances from ambient air, and the post-depositional phase, in which the deposition of acid materials on water and soil results in the mobilization, transport, and even chemical transformation of toxic metals. Acidification increases bioconversion of mercury to methylmercury, which accumulates in fish, increasing the risk to toxicity in people who eat fish. Increase in water and soil content of lead and cadmium increases human exposure to these metals which become additive to other sources presently under regulatory control. The potential adverse health effects of increased human exposure to aluminum is not known at the present time.

Early biochemical effects of an organic mercury fungicide on infants: "dose makes the poison"

Phenylmercury absorbed through the skin from contaminated diapers affected urinary excretion in infants in Buenos Aires. The effects were reversible and quantitatively related to the concentration of urinary mercury. Excretion of gamma-glutamyl transpeptidase, an enzyme in the brush borders of renal tubular cells, increased in a dose-dependent manner when mercury excretion exceeded a "threshold" value. Urine volume also increased but at a higher threshold with respect to mercury. The results support the threshold concept of the systemic toxicity of metals. gamma-Glutamyl transpeptidase is a useful and sensitive marker for preclinical effects of toxic metals.

Sulfobromophthalein inhibition of glutathione and methylmercury secretion into bile

The mechanism through which sulfobromophthalein (BSP) inhibits the biliary secretion of glutathione (GSH) and methylmercury was examined in male rats anesthetized with pentobarbital sodium. The biliary secretion rates of GSH and methylmercury were measured following the bolus intravenous administration of various doses of BSP, the GSH conjugate of BSP (BSP-SG), and phenol-3,6-dibromphthalein disulfonate (DBSP, a nonmetabolizable analogue of BSP). The effects of BSP on GSH secretion were dose dependent; at a dose of 120 mumol/kg the rate of GSH secretion fell close to zero. DBSP also inhibited GSH secretion, although the inhibition was not as complete as observed after BSP administration; at a dose of 180 mumol/kg GSH secretion fell to 18% of control. BSP-SG, in contrast, had no effect on GSH secretion into bile when given at a dose of 120 mumol/kg. At doses of 240 and 360 mumol BSP-SG/kg, there were only minor changes in the rate of GSH secretion. The changes in GSH secretion induced by these dyes were accompanied by proportional changes in glutathione disulfide (GSSG) secretion into bile, so that the molar ratio of GSSG to GSH in bile remained within the range of 0.07-0.18. In all experiments the changes in methylmercury secretion were parallel to the changes in GSH secretion. The results suggest that the BSP-induced inhibition of GSH, GSSG, and methylmercury secretion into bile is due to the direct inhibition of the biliary GSH transport process.

Treatment of acute methylmercury ingestion by hemodialysis with N-acetylcysteine (Mucomyst) infusion and 2,3-dimercaptopropane sulfonate

A case of acute methylmercury ingestion was treated sequentially with oral D-penicillamine, hemodialysis during N-acetylcysteine (NAC) infusion, and 2,3-dimercaptopropane sulfonate (DMPS) an experimental oral agent. Urinary organic mercury elimination rate increased almost 40-fold during and 84-fold after hemodialysis with NAC infusion, compared with elimination during initial D-penicillamine therapy. Mean clearance during hemodialysis was only 13 ml/min with an extraction rate of 3.7 mcg/min. Although whole blood mercury concentrations decreased from 568 to 265 ng/ml during dialysis, a rebound to 525 ng/ml occurred. A total of 1.6 mg mercury was renally eliminated during hemodialysis and in the following 24 hours. A total of 3.3 mg of predominantly organic mercury was renally eliminated during 18 days of combined therapies. Since renal elimination of inorganic mercury is seen with chronic methylmercury poisoning, the high ratio of organic to inorganic mercury in urine supports the acute nature of this exposure. DMPS was begun on day 4 and during the two weeks of administration whole blood concentrations fell by 15% to 355 ng/ml. An expected decrease in elimination half-life to 10 days was not observed during DMPS therapy, possibly due to concurrent administration of vitamins containing zinc and copper. The amount of methylmercury ingested was estimated as 45 mg, based on a post-distribution blood concentration of approximately 450 ng/ml. The patient developed no symptoms of methylmercury poisoning during the one year after the episode. We conclude that NAC may be useful to enhance renal elimination of methylmercury and merits further investigation as a potential binding agent to reduce the body burden of methylmercury.

Dependence of biliary secretion of inorganic mercury on the biliary transport of glutathione

The interrelation between the biliary transport of glutathione (GSH) and of inorganic mercury was investigated in suckling and adult male and female rats. The 14-day-old rat secreted inorganic mercury into bile at one-seventh the rate of the 28-day-old rat. Development of the ability to secrete mercury paralleled development of the ability to secrete GSH. The inability of the 14-day-old rat to secrete mercury and GSH occurred despite hepatic tissue concentrations of both of these compounds which were similar to those of adult rats. In adult rats, inhibition of GSH secretion by sulfobromophthalein (BSP) administration resulted in a parallel inhibition of mercury secretion. Conversely, the increase in the rate of GSH secretion into bile after cysteine or GSH administration was accompanied by an increase in the rate of mercury secretion into bile. The changes in the biliary secretion of mercury and of GSH after treatment with cysteine or GSH were not closely parallel, probably because of the tissue redistribution of mercury effected by these sulfhydryl-containing compounds. Mercury secretion into bile was independent of the changes in bile flow produced by dehydrocholate (DHC) or hypertonic sucrose, but it was closely related to the rate of GSH secretion. Further, sex differences and individual variability in the biliary secretion of inorganic mercury were correlated with differing abilities to secrete GSH into bile. These results suggest that the biliary secretion of inorganic mercury is in large part dependent on the biliary transport of GSH.

Inorganic mercury secretion into bile as a low molecular weight complex

Previous studies on the biliary secretion of inorganic mercury have concluded that inorganic mercury in bile is bound almost exclusively to substances of high molecular weight (HMW). In contrast, our results showed that inorganic mercury in bile is bound predominantly to a substance of low molecular weight (LMW), which is most likely glutathione (GSH). The previously reported binding of inorganic mercury to bile proteins is now explained as a postsecretory in vitro artifact resulting from the rapid oxidation of endogenous GSH which occurs during the collection and storage of bile samples. Gel filtration on Sephadex G-75 of freshly collected bile from rats treated with 203HgCl2 or of control bile supplemented in vitro with 203HgCl2 showed that most of the mercury was in the LMW fraction. On Sephadex G-25, the biliary mercury peak co-eluted with the mercury-GSH standard. However, when bile was allowed to stand at room temperature, there was a time-dependent shift of the mercury towards the HMW fraction. The rate of this shift was proportional to the rate of oxidation of GSH in bile. When GSH oxidation was inhibited by collecting bile in EDTA at 4 degrees, the mercury remained associated with the LMW fraction. At a given GSH concentration in bile, the fraction of mercury bound to the HMW fraction was independent of mercury concentration, in the range of 0.05 to 5.0 microM HgCl2. These results suggest that the inorganic mercury was secreted into bile complexed with a LMW substance. This LMW substance has been tentatively identified as GSH.

Differences in the effects of selenite and biological selenium on the chemical form and distribution of mercury after the simultaneous administration of HgCl2 and selenium to rats

The interaction was compared between inorganic (HgCl2) mercury and selenite (Na2SeO3) vs. the interaction between inorganic mercury and biological selenium (a freeze-dried preparation of liver from rats treated with selenite). Organ concentrations of selenium were always significantly lower after biological selenium than after selenite. Biological selenium affected the organ distribution of inorganic mercury differently than selenite. Furthermore, the speciation of mercury was affected by this form of selenium. A mercury-selenide compound (presumably HgSe) accounted for a greater proportion of total mercury in tissues after selenite than after a dose of biological selenium. Administration of selenomethionine had a similar effect on the speciation of mercury to that seen after biological selenium. As the forms of selenium in selenomethionine or selenium deposited in the liver are most likely nearer than selenite to selenium present in food, our results suggest that, as far as the reaction of mercury with selenide is concerned, experiments with selenite overestimate the protective effect of dietary selenium against inorganic mercury and possibly against methylmercury.

The metabolism of arsenite and arsenate by the rat

Differences in distribution and metabolism of arsenite and arsenate were studied in rats and in rat liver and kidney slices and hepatocytes. Five minutes after i.v. administration of 4.8 nmol arsenite or arsenate to male Sprague-Dawley rats, blood levels of arsenic were only 10 percent of the initial dose. Blood arsenic levels then rose: by 4 hours about 67 percent of the initial dose of arsenite and 28 percent of the initial dose of arsenate were in the blood compartment. The predominant form of arsenic in the RBC was dimethylarsinic acid (DMA). Arsenite was rapidly distributed to both liver and kidney; arsenate was rapidly distributed to kidney only. After 4 hours of exposure to arsenite, liver slices had taken up six times more arsenic and kidney slices two times more arsenic than after exposure to arsenate. Isolated hepatocytes took up as much as 20 times more arsenic after arsenite exposure. DMA was found in the medium of the liver slices and hepatocytes exposed to arsenite, but very little DMA was found in the medium of the arsenate-exposed liver slices and hepatocytes. However, five times more DMA was found in the medium of the kidney slices exposed to arsenate than in the medium of the liver slices. Phosphate inhibited uptake and metabolism of arsenate by kidney slices. These studies indicate that inorganic arsenic is rapidly taken up by liver and kidney and methylated. Arsenite is methylated by both organs, whereas arsenate may be methylated by kidney only.

Biliary transport of glutathione and methylmercury

The hepatobiliary transport of glutathione (GSH) and methylmercury (MM) was investigated in male and female rats anesthetized with pentobarbital sodium. When bile flow was altered with either sodium dehydrocholate (DHC), hypertonic sucrose infusion, or by hypothermia, the absolute rates of GSH and MM secretion into bile were not affected, resulting in parallel concentration changes in the bile fluid for both GSH and MM. Indocyanine green and sulfobromophthalein (BSP), but not BSP-glutathione complex, inhibited the biliary secretion of free GSH. This inhibition was accompanied by a parallel inhibition of MM secretion into bile and occurred without any changes in liver GSH or MM levels. On the other hand, the intravenous administration of cysteine, GSH, and penicillamine was associated with an increase in the secretion rate of reduced sulfhydryl groups into bile and an increase in the biliary secretion rate of MM. The increased biliary secretion rate of MM after phenobarbital pretreatment was also associated with an increased rate of secretion of GSH into bile. In addition, sex differences and individual variability in the biliary secretion of MM were correlated with differing abilities to secrete GSH into bile. The results suggest the presence of a biliary transport system for GSH that determines the biliary secretion of MM.

Public health consequences of heavy metals in dump sites

Metals differ from most synthetic organic chemicals in that their clinical manifestations are well known and methods for their measurement in the body are generally well established. Since metals are ubiquitous, special care should be taken to identify the source, whether dump site or not. Isotopic ratios may be used for lead. Time of exposure may be highly variable so estimates will be necessary of integrated "dose-commitment." Transmission to man will follow many pathways. The contamination of children's hands and clothing by dust may be an important route. Because effects are so different, the chemical species (e.g., organic versus inorganic forms) of each metal must be identified. Exposure assessment requires identification of suitable indicator media, usually blood in the case of lead, urine with cadmium and inorganic mercury, and blood or hair with regard to methylmercury. Human head hair may have considerable potential, as it may provide a recapitulation of past exposures. The first health complaints associated with most metals are usually nonspecific. The complex social, political, and legal issues strongly indicate the need for objective tests for health effects. Most important is the identification and measurement of the critical effect, i.e., an effect that alerts the public health authorities that further exposure should cease. For example, in the case of lead, the critical effect is hematologic; with cadmium it is the presence in urine of abnormally high concentration of small molecular weight protein; and with mercury no early objective test has yet been devised.