Adapted from ATSDR 1995.
Occurrence and Uses
Lead ores are found in many parts of the world. The richest ore is galena (lead sulphide) and this is the main commercial source of lead. Other lead ores include cerussite (carbonate), anglesite (sulphate), corcoite (chromate), wulfenite (molybdate), pyromorphite (phosphate), mutlockite (chloride) and vanadinite (vanadate). In many cases the lead ores may also contain other toxic metals.
Lead minerals are separated from gangue and other materials in the ore by dry crushing, wet grinding (to produce a slurry), gravity classification and flotation. The liberated lead minerals are smelted by a three-stage process of charge preparation (blending, conditioning, etc.), blast sintering and blast furnace reduction. The blast-furnace bullion is then refined by the removal of copper, tin, arsenic, antimony, zinc, silver and bismuth.
Metallic lead is used in the form of sheeting or pipes where pliability and resistance to corrosion are required, such as in chemical plants and the building industry; it is used also for cable sheathing, as an ingredient in solder and as a filler in the automobile industry. It is a valuable shielding material for ionizing radiations. It is used for metallizing to provide protective coatings, in the manufacture of storage batteries and as a heat treatment bath in wire drawing. Lead is present in a variety of alloys and its compounds are prepared and used in large quantities in many industries.
About 40% of lead is used as a metal, 25% in alloys and 35% in chemical compounds. Lead oxides are used in the plates of electric batteries and accumulators (PbO and Pb3O4), as compounding agents in rubber manufacture (PbO), as paint ingredients (Pb3O4) and as constituents of glazes, enamels and glass.
Lead salts form the basis of many paints and pigments; lead carbonate and lead sulphate are used as white pigments and the lead chromates provide chrome yellow, chrome orange, chrome red and chrome green. Lead arsenate is an insecticide, lead sulphate is used in rubber compounding, lead acetate has important uses in the chemical industry, lead naphthenate is an extensively used dryer and tetraethyllead is an antiknock additive for gasoline, where still permitted by law.
Lead alloys. Other metals such as antimony, arsenic, tin and bismuth may be added to lead to improve its mechanical or chemical properties, and lead itself may be added to alloys such as brass, bronze and steel to obtain certain desirable characteristics.
Inorganic lead compounds. Space is not available to describe the very large number of organic and inorganic lead compounds encountered in industry. However, the common inorganic compounds include lead monoxide (PbO), lead dioxide (PbO2), lead tetroxide (Pb3O4), lead sesquioxide (Pb2O3), lead carbonate, lead sulphate, lead chromates, lead arsenate, lead chloride, lead silicate and lead azide.
The maximum concentration of the organic (alkyl) lead compounds in gasolines is subject to legal prescriptions in many countries, and to limitation by the manufacturers with governmental concurrence in others. Many jurisdictions have simply banned its use.
The prime hazard of lead is its toxicity. Clinical lead poisoning has always been one of the most important occupational diseases. Medico-technical prevention has resulted in a considerable decrease in reported cases and also in less serious clinical manifestations. However, it is now evident that adverse effects occur at exposure levels hitherto regarded as acceptable.
Industrial consumption of lead is increasing and traditional consumers are being supplemented by new users such as the plastics industry. Hazardous exposure to lead, therefore, occurs in many occupations.
In lead mining, a considerable proportion of lead absorption occurs through the alimentary tract and consequently the extent of the hazard in this industry depends, to some extent, on the solubility of ores being worked. The lead sulphide (PbS) in galena is insoluble and absorption from the lung is limited; however, in the stomach, some lead sulphide may be converted to slightly soluble lead chloride which may then be absorbed in moderate quantities.
In lead smelting, the main hazards are the lead dust produced during crushing and dry grinding operations, and lead fumes and lead oxide encountered in sintering, blast-furnace reduction and refining.
Lead sheet and pipe are used principally for the constructon of equipment for storing and handling sulphuric acid. The use of lead for water and town gas pipes is limited nowadays. The hazards of working with lead increase with temperature. If lead is worked at temperatures below 500 °C, as in soldering, the risk of fume exposure is far less than in lead welding, where higher flame temperatures are used and the danger is higher. The spray coating of metals with molten lead is dangerous since it gives rise to dust and fumes at high temperatures.
The demolition of steel structures such as bridges and ships that have been painted with lead-based paints frequently gives rise to cases of lead poisoning. When metallic lead is heated to 550 °C, lead vapour will be evolved and will become oxidized. This is a condition that is liable to be present in metal refining, the melting of bronze and brass, the spraying of metallic lead, lead burning, chemical plant plumbing, ship breaking and the burning, cutting and welding of steel structures coated with paints containing lead tetroxide.
Routes of entry
The main route of entry in industry is the respiratory tract. A certain amount may be absorbed in the air passages, but the main portion is taken up by the pulmonary bloodstream. The degree of absorption depends on the proportion of the dust accounted for by particles less than 5 microns in size and the exposed worker’s respiratory minute volume. Increased workload therefore results in higher lead absorption. Although the respiratory tract is the main route of entry, poor work hygiene, smoking during work (pollution of tobacco, polluted fingers while smoking) and poor personal hygiene may considerably increase total exposure mainly by the oral route. This is one of the reasons why the correlation between the concentration of lead in workroom air and lead in blood levels often is very poor, certainly on an individual basis.
Another important factor is the level of energy expenditure: the product of concentration in air and of respiratory minute volume determines lead uptake. The effect of working overtime is to increase exposure time and reduce recovery time. Total exposure time is also much more complicated than official personnel records indicate. Only time analysis in the workplace can yield relevant data. The worker may move around the department or the factory; a job with frequent changes in posture (e.g., turning and bending) results in exposure to a great range of concentrations. A representative measure of lead intake is almost impossible to obtain without the use of a personal sampler applied for many hours and for many days.
Particle size. Since the most important route of lead absorption is by the lungs, the particle size of industrial lead dust is of considerable significance and this depends on the nature of the operation giving rise to the dust. Fine dust of respirable particle size is produced by processes such as the pulverizing and blending of lead colours, the abrasive working of lead-based fillers in automobile bodies and the dry rubbing-down of lead paint. The exhaust gases of gasoline engines yield lead chloride and lead bromide particles of 1 micron diameter. The larger particles, however, may be ingested and be absorbed via the stomach. A more informative picture of the hazard associated with a sample of lead dust might be given by including a size distribution as well as a total lead determination. But this information is probably more important for the research investigator than for the field hygienist.
In the human body, inorganic lead is not metabolized but is directly absorbed, distributed and excreted. The rate at which lead is absorbed depends on its chemical and physical form and on the physiological characteristics of the exposed person (e.g., nutritional status and age). Inhaled lead deposited in the lower respiratory tract is completely absorbed. The amount of lead absorbed from the gastrointestinal tract of adults is typically 10 to 15% of the ingested quantity; for pregnant women and children, the amount absorbed can increase to as much as 50%. The quantity absorbed increases significantly under fasting conditions and with iron or calcium deficiency.
Once in the blood, lead is distributed primarily among three compartments—blood, soft tissue (kidney, bone marrow, liver, and brain), and mineralizing tissue (bones and teeth). Mineralizing tissue contains about 95% of the total body burden of lead in adults.
The lead in mineralizing tissues accumulates in subcompartments that differ in the rate at which lead is resorbed. In bone, there is both a labile component, which readily exchanges lead with the blood, and an inert pool. The lead in the inert pool poses a special risk because it is a potential endogenous source of lead. When the body is under physiological stress such as pregnancy, lactation or chronic disease, this normally inert lead can be mobilized, increasing the lead level in blood. Because of these mobile lead stores, significant drops in a person’s blood lead level can take several months or sometimes years, even after complete removal from the source of lead exposure.
Of the lead in the blood, 99% is associated with erythrocytes; the remaining 1% is in the plasma, where it is available for transport to the tissues. The blood lead not retained is either excreted by the kidneys or through biliary clearance into the gastrointestinal tract. In single-exposure studies with adults, lead has a half-life, in blood, of approximately 25 days; in soft tissue, about 40 days; and in the non-labile portion of bone, more than 25 years. Consequently, after a single exposure a person’s blood lead level may begin to return to normal; the total body burden, however, may still be elevated.
For lead poisoning to develop, major acute exposures to lead need not occur. The body accumulates this metal over a lifetime and releases it slowly, so even small doses, over time, can cause lead poisoning. It is the total body burden of lead that is related to the risk of adverse effects.
Whether lead enters the body through inhalation or ingestion, the biologic effects are the same; there is interference with normal cell function and with a number of physiological processes.
Neurological effects. The most sensitive target of lead poisoning is the nervous system. In children, neurological deficits have been documented at exposure levels once thought to cause no harmful effects. In addition to the lack of a precise threshold, childhood lead toxicity may have permanent effects. One study showed that damage to the central nervous system (CNS) that occurred as a result of lead exposure at age 2 resulted in continued deficits in neurological development, such as lower IQ scores and cognitive deficits, at age 5. In another study that measured total body burden, primary school children with high tooth lead levels but with no known history of lead poisoning had larger deficits in psychometric intelligence scores, speech and language processing, attention and classroom performance than children with lower levels of lead. A 1990 follow-up report of children with elevated lead levels in their teeth noted a sevenfold increase in the odds of failure to graduate from high school, lower class standing, greater absenteeism, more reading disabilities and deficits in vocabulary, fine motor skills, reaction time and hand-eye coordination 11 years later. The reported effects are more likely caused by the enduring toxicity of lead than by recent excessive exposures because the blood lead levels found in the young adults were low (less than 10 micrograms per deciliter (μg/dL)).
Hearing acuity, particularly at higher frequencies, has been found to decrease with increasing blood lead levels. Hearing loss may contribute to the apparent learning disabilities or poor classroom behavior exhibited by children with lead intoxication.
Adults also experience CNS effects at relatively low blood lead levels, manifested by subtle behavioural changes, fatigue and impaired concentration. Peripheral nervous system damage, primarily motor, is seen mainly in adults. Peripheral neuropathy with mild slowing of nerve conduction velocity has been reported in asymptomatic lead workers. Lead neuropathy is believed to be a motor neuron, anterior horn cell disease with peripheral dying-back of the axons. Frank wrist drop occurs only as a late sign of lead intoxication.
Haematological effects. Lead inhibits the body’s ability to make hemoglobin by interfering with several enzymatic steps in the heme pathway. Ferrochelatase, which catalyzes the insertion of iron into protoporphyrin IX, is quite sensitive to lead. A decrease in the activity of this enzyme results in an increase of the substrate, erythrocyte protoporphyrin (EP), in the red blood cells. Recent data indicate that the EP level, which has been used to screen for lead toxicity in the past, is not sufficiently sensitive at lower levels of blood lead and is therefore not as useful a screening test for lead poisoning as previously thought.
Lead can induce two types of anaemia. Acute high-level lead poisoning has been associated with hemolytic anaemia. In chronic lead poisoning, lead induces anemia by both interfering with erythropoiesis and by diminishing red blood cell survival. It should be emphasized, however, that anemia is not an early manifestation of lead poisoning and is evident only when the blood lead level is significantly elevated for prolonged periods.
Endocrine effects. A strong inverse correlation exists between blood lead levels and levels of vitamin D. Because the vitamin D-endocrine system is responsible in large part for the maintenance of extra- and intra-cellular calcium homeostasis, it is likely that lead impairs cell growth and maturation and tooth and bone development.
Renal effects. A direct effect on the kidney of long-term lead exposure is nephropathy. Impairment of proximal tubular function manifests in aminoaciduria, glycosuria and hyperphosphaturia (a Fanconi-like syndrome). There is also evidence of an association between lead exposure and hypertension, an effect that may be mediated through renal mechanisms. Gout may develop as a result of lead-induced hyperuricemia, with selective decreases in the fractional excretion of uric acid before a decline in creatinine clearance. Renal failure accounts for 10% of deaths in patients with gout.
Reproductive and developmental effects. Maternal lead stores readily cross the placenta, placing the foetus at risk. An increased frequency of miscarriages and stillbirths among women working in the lead trades was reported as early as the end of the 19th century. Although the data concerning exposure levels are incomplete, these effects were probably a result of far greater exposures than are currently found in lead industries. Reliable dose-effect data for reproductive effects in women are still lacking today.
Increasing evidence indicates that lead not only affects the viability of the foetus, but development as well. Developmental consequences of prenatal exposure to low levels of lead include reduced birth weight and premature birth. Lead is an animal teratogen; however, most studies in humans have failed to show a relationship between lead levels and congenital malformations.
The effects of lead on the male reproductive system in humans have not been well characterized. The available data support a tentative conclusion that testicular effects, including reduced sperm counts and motility, may result from chronic exposure to lead.
Carcinogenic effects. Inorganic lead and inorganic lead compounds have been classified as Group 2B, possible human carcinogens, by the International Agency for Research on Cancer (IARC). Case reports have implicated lead as a potential renal carcinogen in humans, but the association remains uncertain. Soluble salts, such as lead acetate and lead phosphate, have been reported to cause kidney tumors in rats.
Continuum of signs and symptoms associated with lead toxicity
Mild toxicity associated with lead exposure includes the following:
- myalgia or paresthesia
- mild fatigue
- occasional abdominal discomfort.
The signs and symptoms associated with moderate toxicity include:
- general fatigue
- difficulty concentrating
- muscular exhaustibility
- diffuse abdominal pain
- weight loss
The signs and symptoms of severe toxicity include:
- paresis or paralysis
- encephalopathy, which may abruptly lead to seizures, changes in consciousness, coma and death
- lead line (blue-black) on gingival tissue
- colic (intermittent, severe abdominal cramps).
Some of the haematological signs of lead poisoning mimic other diseases or conditions. In the differential diagnosis of microcytic anaemia, lead poisoning can usually be ruled out by obtaining a venous blood lead concentration; if the blood lead level is less than 25 μg/dL, the anaemia usually reflects iron deficiency or haemoglobinopathy. Two rare diseases, acute intermittent porphyria and coproporphyria, also result in haeme abnormalities similar to those of lead poisoning.
Other effects of lead poisoning can be misleading. Patients exhibiting neurological signs due to lead poisoning have been treated only for peripheral neuropathy or carpal tunnel syndrome, delaying treatment for lead intoxication. Failure to correctly diagnose lead induced gastrointestinal distress has led to inappropriate abdominal surgery.
If pica or accidental ingestion of lead-containing objects (such as curtain weights or fishing sinkers) is suspected, an abdominal radiograph should be taken. Hair analysis is not usually an appropriate assay for lead toxicity because no correlation has been found between the amount of lead in the hair and the exposure level.
The probability of environmental lead contamination of a laboratory specimen and inconsistent sample preparation make the results of hair analysis difficult to interpret. Suggested laboratory tests to evaluate lead intoxication include the following:
- CBC with peripheral smear
- blood lead level
- erythrocyte protoporphyrin level
- BUN and creatinine level
CBC with peripheral smear. In a lead-poisoned patient, the haematocrit and haemoglobin values may be slightly to moderately low. The differential and total white count may appear normal. The peripheral smear may be either normochromic and normocytic or hypochromic and microcytic. Basophilic stippling is usually seen only in patients who have been significantly poisoned for a prolonged period. Eosinophilia may appear in patients with lead toxicity but does not show a clear dose-response effect.
It is important to note that basophilic stippling is not always seen in lead poisoned patients.
Blood lead level. A blood lead level is the most useful screening and diagnostic test for lead exposure. A blood lead level reflects lead’s dynamic equilibrium between absorption, excretion and deposition in soft- and hard-tissue compartments. For chronic exposures, blood lead levels often underrepresent the total body burden; nevertheless, it is the most widely accepted and commonly used measure of lead exposure. Blood lead levels srespond relatively rapidly to abrupt or intermittent changes in lead intake (e.g., ingestion of lead paint chips by children) and, within a limited range, bear a linear relationship to those intake levels.
Today, the average blood lead level in the US population, for example, is below 10 μg/dL, down from an average of 16 μg/dL (in the 1970s), the level before the legislated removal of lead from gasoline. A blood lead level of 10 μg/dL is about three times higher than the average level found in some remote populations.
The levels defining lead poisoning have been progressively declining. Taken together, effects occur over a wide range of blood lead concentrations, with no indication of a threshold. No safe level has yet been found for children. Even in adults, effects are being discovered at lower and lower levels as more sensitive analyses and measures are developed.
Erythrocyte protoporhyrin level. Until recently, the test of choice for screening asymptomatic populations at risk was erythrocyte protoporphyrin (EP), commonly assayed as zinc protoporphyrin (ZPP). An elevated level of protoporphyrin in the blood is a result of accumulation secondary to enzyme dysfunction in the erythrocytes. It reaches a steady state in the blood only after the entire population of circulating erythrocyles has turned over, about 120 days. Consequently, it lags behind blood lead levels and is an indirect measure of long-term lead exposure.
The major disadvantage of using EP (ZPP) testing as a method for lead screening is that it is not sensitive at the lower levels of lead poisoning. Data from the second US National Health and Nutrition Examination Survey (NHANES II) indicate that 58% of 118 children with blood lead levels above 30 μg/dL had EP levels within normal limits. This finding shows that a significant number of children with lead toxicity would be missed by reliance on EP (ZPP) testing alone as the screening tool. An EP (ZPP) level is still useful in screening patients for iron deficiency anaemia.
Normal values of ZPP are usually below 35 μg/dL. Hyperbilirubinaemia (jaundice) will cause falsely elevated readings when the haematofluorometer is used. EP is elevated in iron deficiency anaemia and in sickle cell and other haemolytic anaemias. In erythropoietic protoporphyria, an extremely rare disease, EP is markedly elevated (usually above 300 μg/dL).
BUN, creatinine and urinalysis. These parameters may reveal only late, significant effects of lead on renal function. Renal function in adults can also be assessed by measuring the fractional excretion of uric acid (normal range 5 to 10%; less than 5% in saturnine gout; greater than 10% in Fanconi syndrome).
Organic lead intoxication
The absorption of a sufficient quantity of tetraethyllead, whether briefly at a high rate or for prolonged periods at a lower rate, induces acute intoxication of the CNS. The milder manifestations are those of insomnia, lassitude and nervous excitation which reveals itself in lurid dreams and dream-like waking states of anxiety, in association with tremor, hyper-reflexia, spasmodic muscular contractions, bradycardia, vascular hypotension and hypothermia. The more severe responses include recurrent (sometimes nearly continuous) episodes of complete disorientation with hallucinations, facial contortions and intense general somatic muscular activity with resistance to physical restraint. Such episodes may be converted abruptly into maniacal or violent convulsive seizures which may terminate in coma and death.
Illness may persist for days or weeks, with intervals of quietude readily triggered into over-activity by any type of disturbance. In these less acute cases, fall in blood pressure and loss of body weight are common. When the onset of such symptomatology follows promptly (within a few hours) after brief, severe exposure to tetraethyllead, and when the symptomatology develops rapidly, an early fatal outcome is to be feared. When, however, the interval between the termination of brief or prolonged exposure and the onset of symptoms is delayed (by up to 8 days), the prognosis is guardedly hopeful, although partial or recurrent disorientation and depressed circulatory function may persist for weeks.
The initial diagnosis is suggested by a valid history of significant exposure to tetraethyllead, or by the clinical pattern of the presenting illness. It may be supported by the further development of the illness, and confirmed by evidence of a significant degree of absorption of tetraethyllead, provided by analyses of urine and blood which reveal typical findings (i.e., a striking elevation of the rate of excretion of lead in the urine) and a concurrently negligible or slight elevation of the concentration of lead in the blood.
Lead Control in the Working Environment
Clinical lead poisoning has historically been one of the most important occupational diseases, and it remains a major risk today. The considerable body of scientific knowledge concerning the toxic effects of lead has been enriched since the 1980s by significant new knowledge regarding the more subtle subclinical effects. Similarly, in a number of countries it was felt necessary to redraft or modernize work protective measures enacted over the last half-century and more.
Thus, in November 1979, in the US, the Final Standard on Occupational Exposure to Lead was issued by the Occupational Safety and Health Administration (OSHA) and in November 1980 a comprehensive Approved Code of Practice was issued in the United Kingdom regarding the control of lead at work.
The main features of the legislation, regulations and codes of practice emerging in the 1970s concerning the protection of the health of workers at work involved establishing comprehensive systems covering all work circumstances where lead is present and giving equal importance to hygiene measures, ambient monitoring and health surveillance (including biological monitoring).
Most codes of practice include the following aspects:
- assessment of work which exposes persons to lead
- information, instruction and training
- control measures for materials, plant and processes
- use and maintenance of control measures
- respiratory protective equipment and protective clothing
- washing and changing facilities and cleaning
- separate eating, drinking and smoking areas
- duty to avoid spread of contamination by lead
- air monitoring
- medical surveillance and biological tests
- keeping of records.
Some regulation, such as the OSHA lead standard, specifies the permissible exposure limit (PEL) of lead in the workplace, the frequency and extent of medical monitoring, and other responsibilities of the employer. As of this writing, if blood monitoring reveals a blood lead level greater than 40 μg/dL, the worker must be notified in writing and provided with medical examination. If a worker’s blood lead level reaches 60 μg/dL (or averages 50 μg/dL or more), the employer is obligated to remove the employee from excessive exposure, with maintenance of seniority and pay, until the employee’s blood lead level falls below 40 μg/dL (29 CFR 91 O.1025) (medical removal protection benefits).
Safety and Health Measures
The object of precautions is first to prevent the inhalation of lead and secondly to prevent its ingestion. These objects are most effectively achieved by the substitution of a less toxic substance for the lead compound. The use of lead polysilicates in the potteries is one example. The avoidance of lead carbonate paints for the painting of the interiors of buildings has proved very effective in reducing painters’ colic; effective substitutes for lead for this purpose have become so readily available that it has been considered reasonable in some countries to prohibit the use of lead paint for the interiors of buildings.
Even if it is not possible to avoid the use of lead itself, it is still possible to avoid dust. Water sprays may be used in large quantities to prevent the formation of dust and to prevent it from becoming airborne. In lead smelting, the ore and the scrap may be treated in this way and the floors on which it has been lying may be kept wet. Unfortunately, there is always a potential source of dust in these circumstances if the treated material or floors are ever allowed to become dry. In some instances, arrangements are made to ensure that the dust will be coarse rather than fine. Other specific engineering precautions are discussed elsewhere in this Encyclopaedia.
Workers who are exposed to lead in any of its forms should be provided with personal protective equipment (PPE), which should be washed or renewed regularly. Protective clothing made of certain man-made fibres retains much less dust than cotton overalls and should be used where the conditions of work render it possible; turn-ups, pleats and pockets in which lead dust may collect should be avoided.
Cloakroom accommodation should be provided for this PPE, with separate accommodation for clothing taken off during working hours. Washing accommodation, including bathing accommodation with warm water, should be provided and used. Time should be allowed for washing before eating. Arrangements should be made to prohibit eating and smoking in the vicinity of lead processes and suitable eating facilities should be provided.
It is essential that the rooms and the plant associated with lead processes should be kept clean by continuous cleaning either by a wet process or by vacuum cleaners. Where, in spite of these precautions, workers may still be exposed to lead, respiratory protective equipment should be provided and properly maintained. Supervision should ensure that this equipment is maintained in a clean and efficient condition and that it is used when necessary.
Both the toxic properties of organic lead compounds, and their ease of absorption, require that contact of the skin of workers with these compounds, alone or in concentrated mixtures in commercial formulations or in gasoline or other organic solvents, must be scrupulously avoided. Both technological and management control are essential, and appropriate training of workers in safe work practices and the use of PPE is required. It is essential that atmospheric concentrations of alkyl lead compounds in the workplace air should be maintained at extremely low levels. Personnel should not be allowed to eat, smoke or keep unsealed food or beverages at the workplace. Good sanitary facilities, including showers, should be provided and workers should be encouraged to practise good personal hygiene, especially by showering or washing after the work shift. Separate lockers should be supplied for working and private clothes.