How lead gets into people

• Ingestion
• Inhalation
• Dermal (skin) absorption
• In utero exposure

Lead gets into people most frequently though ingestion and inhalation. Skin absorption and in utero exposure may also occur.

Ingestion

Diagram: absorption and storage of lead in the body

Ingestion is the most common route of exposure to lead in children and adults not occupationally exposed. In pre-school children, lead is frequently ingested through placing hands, toys and other objects and dust or soil on those objects into their mouths. This hand-to-mouth behaviour is normal in pre-school children, but in an environment contaminated with lead in soil, household dust and paint, it poses serious threats.

Estimates of the amounts of soil ingested by children vary from 4 to > 200 mg/d, however a suggested average intake is 60-100 mg/day which may increase one hundred fold in response to:

• normal toddler mouthing behaviour
• pica – the habitual of eating non-food objects
• other factors.

Children with 'pica'- the habitual of eating non-food objects - are at even greater risk as they may consume up to 20 grams of soil per day as well as objects that don't contain lead, in such a 'leaded environment'. Researchers in the US estimated that between 1 to 6% of preschool children have pica.

Children's gastrointestinal tracts absorbed 50% of ingested lead compared to adult absorption of 10-15%, making lead ingested through hand-to-mouth behaviour or dietary lead an exposure source of significant concern. Pregnant women may absorb much greater quantities of ingested lead, more closely approximating the 50% absorption rate of children.

In adults, ingestion may occur through eating, smoking, or nail-biting with lead contaminated hands, particularly after renovation or hobby activities. Other ways of ingesting lead include eating or drinking food contaminated with lead. Studies of gastrointestinal absorption indicate 10-15% of dietary lead is absorbed though this can rise up to 63% in fasting conditions.

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The Health Risk Assessment and Management of Contaminated Sites, 1991, South Australian Health Commission, Adelaide, p 106

Inhalation

Inhalation is another common route of exposure to lead. The amount of lead inhaled depends on the lead levels in air and the amount of air being inhaled by the individual. Generally adults inhale 15m³ of air daily while children of two inhale roughly 6m³ of air daily. Inhalation of lead from leaded petrol emissions is an important source of lead exposure providing a generalised low dose across the general population and a more concentrated dose to those in petrol refineries, depots and service stations and highly trafficked areas.

Lead inhalation is a particular hazard to workers in many lead industries, including mining, smelting, metal repair or foundry work, demolition and renovation activities that generate fumes and dusts. Lead particles and fumes are created through sanding, scraping or burning lead surfaces or welding and cutting lead or lead painted objects common in lead industry and bridge and building renovation work. Hobby activities that involve melting, burning, cutting or casting lead can also create respirable lead fumes and lead particles.

When particles are inhaled, depending on the particle size, lead is deposited in the upper and lower respiratory tracts. Larger particles stopping higher in the respiratory tract, trapped in the protective mucus lining of the nose, throat and upper respiratory passages. Only lead particles smaller than <1um in diameter and lead fumes are thought to reach the lower respiratory tracts. Clearance of the upper respiratory tract results in the swallowing and gastrointestinal absorption. Studies in the US and Australia have shown between 30-50% of inhaled lead is retained by the lungs (depending on particle size and breathing rates) and available for absorption. Over 90% of lead deposited in the lungs is absorbed so the rate of lead uptake is governed by the length of time lead is retained in the lungs.

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Dermal (skin) absorption

The ability of the skin to absorb certain organic lead compounds, such as tetraethyl lead found in petrol has been recognized since the 1940s. Recent laboratory research suggests inorganic lead compounds (e.g. lead nitrate, lead acetate and lead oxide) can be absorbed through the skin but in very small quantities. As a blood lead test is the most common detection method, additional research is needed on lead testing methods. Skin absorption may also pose a threat to workers in the construction trades and paint industry that are less likely to wear protective clothing to prevent lead dust from adhering to their skin

In utero exposure

As the placenta is an imperfect barrier between the mother and foetus for lead and other heavy metals, foetuses may be exposed to lead in utero if their mother is exposed to lead during her pregnancy.

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28 April 2003