Module 3: Toxicology - Section 12: Heavy Metals

OHM12.X: Lead - Part 1

INTRODUCTION:

Lead is one of the ancient metals. The use of lead has varied considerably throughout the years. The Egyptians used lead tools and vessels while the Romans drank wines and ciders sweetened with lead. Lead has also been used as a constituent of cosmetics and medicinal substances.

It is only in the last two decades that drastic measures have been taken to reduce the use of lead. The nature of exposure and toxicity has altered over the last century as people have become more familiar with the health outcomes associated with lead ¹.

Chemistry of Lead:

Lead is a bluish grey metal. Lead has two oxidation states in addition to its elemental state. Lead is found widely in the environment. Galena (PbS) contains 85% lead metal. Two other lead minerals are cerussite (PbCO₃) and anglesite (PbSO₁₂).

Uses and Occupational Exposure of Lead:

Lead is the most widely used non ferrous metal. Despite widely applied restrictions the metal has been used increasingly in industry. 23% of some 5.12 million tons of lead used worldwide is produced in the United States of America. Lead as an organic compound is used in petrol. Lead is used in the production of pigments, solder, plastics batteries and ammunition among others.

There has been a variable decrease in the manufacture of lead paints and increase in the production of lead storage batteries.

Hazardous exposure to lead may result from either spraying molten lead or lead paint, grinding and power sanding lead, pouring of leaded iron and steel, mixing of lead powders. The solubility of lead determines their toxicity.

Children are at high risk of developing lead poisoning from sources in the home environment ²

Lead Metabolism:

Five to ten percent of ingested lead is absorbed in adults and the rest is excreted in faeces. Children may absorb as much as fifty percent. In contrast as much as fifty percent of inhaled lead fume is absorbed. Lead salts of organic acids and organic lead can be appreciably absorbed through the skin.

Ninety five percent of lead in the bloodstream is bound to red blood cells and the remainder is bound in plasma. Ninety five percent of the body burden of lead resides in the cortex of bone as a storage depot. Blood lead exists in 3 compartments (blood, soft tissue and bone). Lead in blood has a half life of about 35 days, while lead in soft tissue and bone has on average half lives of 120 days and 90 days respectively.

Lead toxicity may occur within hours of ingesting one heavy dose of lead however more commonly it may occur weeks to months after repeated lesser exposures. Factors such as infection, prolonged heavy exercise or heavy alcohol intake can produce episodes of acute toxicity.

Lead is excreted primarily via the renal route and to a lesser extent the gastrointestinal tract. Urinary lead measurements provide a useful index of current airborne lead exposure.

Clinical Manifestations of Lead Poisoning:

A chart summarizing the clinical manifestations is available

• Gastrointestinal: Loss of appetite, increasing anorexia, dyspepsia and constipation followed by colic. The colic maybe severe and is either as a result of smooth muscle contraction or vagal irritation.
• Renal: Lead accumulates in the renal tubular lining cells as intra nuclear inclusions. Impairment of the proximal tubular function may result in amino aciduria, glycosuria and hyperphosphaturia. Prolonged exposure can lead to hypertension and chronic interstitial nephritis.
• CNS: Lead encephalopathy is a serious problem in childhood lead poisoning. Low level lead exposure can produce impaired psychological and cognitive performance. Impairments of long term memory verbal and visual spatial abstraction and psychomotor speed have been correlated with blood lead levels.
  Cranial nerve damage has been reported.
  Lead neuropathy is as a result of demyelination and direct axonal damage and is partially reversible. Lead neuropathy occurs in those muscles most heavily used. Lead neuropathy is primarily a motor neuropathy. There is a fine tremor, decreased muscle tone and atrophy.
• Haematological: Lead produces a normocytic normochromic anaemia in the latter stages.
• Fertility: Lead causes spontaneous abortion, altered spermatogenesis and sterility. The reproductive effects of lead have been well documented in animal studies. There are reports of increased stillbirths, premature rupture of membranes and congenital abnormalities related to lead exposure.

REFERENCES:

1. L Rosenstock, M Cullen. Chapter 30. Textbook of Occupational and Environmental Medicine.
2. http://www.cdc.gov/nceh/lead/factsheets/childhoodlead.htm
3. Occupational and Environmental Medicine: Joseph LaDou
4. Environmental and Occupational Medicine: W R Rom
5. Find the lead regulations under the OSHAct

Postgraduate Diploma in Occupational Health (DOH) - Modules 3: Occupational Medicine & Toxicology (Basic) by Profs Mohamed Jeebhay and Rodney Ehrlich, Health Sciences UCT is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License. Major contributors: Mohamed Jeebhay, Rodney Ehrlich, Jonny Myers, Leslie London, Sophie Kisting, Rajen Naidoo, Saloshni Naidoo. Source available from here. For any updates to the material, or more permissions beyond the scope of this license, please email healthoer@uct.ac.za or visit www.healthedu.uct.ac.za. Last updated Jan 2007.
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