The bane of lead

SummaryFood commonly contaminated with lead poses a serious health problem. Lead accumulates in bone tissue and can be mobilised during pregnancy and breast feeding, disturbing normal brain development of the foetus and young child. The action taken by developed nations to cut lead pollution is slowly having a desired effect, but developing countries are lagging behind.

Lead is a heavy metal that has been mined and used for thousands of years, and poisoning humans in the process. Although lead poisoning is one of the oldest known work and environmental hazards, that only small amounts of lead is necessary to cause harm is a much later finding. There is actually no known amount of lead that is too small to not cause harm in some way.

Chronic lead poisoning is the biggest danger because of its long half-life when bound to bone tissue and a particular concern is children. Lead mobilised from bone stores in pregnant and breastfeeding women can disturb brain development of the foetus already in the womb and cause permanent impairment in early life of a child. The classic signs and symptoms in children are loss of appetite, abdominal pain, vomiting, weight loss, constipation, anemia, kidney failure, irritability, lethargy, learning disabilities, and behavioural problems. Slow development of normal childhood behaviours, such as talking and use of words, and permanent mental retardation can both be seen depending on the extent of lead exposure.

In adults, there is an association between blood lead concentration, elevated systolic blood pressure and chronic kidney disease at relatively low blood lead levels. Inorganic lead is also classified as a probable carcinogen in humans.

Environmental exposure

Abnormally high rates of death and illness among children were seen at the beginning of 2010 in Bukkuyum and Anka areas of Zamfara state in northern Nigeria. Investigations by the joint Environment Unit of the UN Office for the Coordination of Humanitarian Affairs and the UN Environment Programme revealed that the cause of the health problems was acute lead poisoning from the processing of lead-rich ore used in the gold extraction process in homes and compounds in the affected areas. More than 18,000 people were affected and 200 children reportedly died as a result of the poisoning.

In 2010, lead poisoning was detected in Central China’s Hunan province that left 19 villagers hospitalised. The patients came mainly from two villages in Guiyang county. The city’s environment protection department implicated and shut down a scrap lead recycling plant in Guiyang. This followed a case in 2009 where as many as 254 children younger than 14 were found to have excessive blood-lead levels in Jiahe county, which borders Guiyang county. Four of them, from three villages, were diagnosed with lead poisoning. A local lead smelter blamed for the massive lead-pollution incident in Jiahe was shut down.

Dietary exposure

Several women taking Ayurvedic medicines during pregnancy were detected with dangerous lead poisoning, US researches reported in August 2012, after investigating cases associated with the use of such traditional drugs. During a year-long survey, the New York City investigated six cases of lead poisoning associated with the use of 10 oral Ayurvedic medications made in India. Ayurvedic medicine is a Hindu system of traditional medicine native to India and a form of alternative medicine sometimes containing high levels of lead.

Lead can leach out of ceramic cups and plates when the glaze is improperly fired or when the glaze has broken down because of wear from daily usage, particularly after repeated use in a microwave or dishwasher. Chips and cracks in ceramic ware also allow leaching of lead. When lead is released into food and drink from ceramics, hazardous levels can contaminate food substances and expose children and adults to toxic levels. Acidic juices pose a particular problem in promoting leaching of lead. In 2003, US authorities investigated a case of lead poisoning in a boy aged 20 months. The child was home full-time and consumed all meals and beverages using ceramic dinnerware. The ceramic dinnerware was manufactured in France and released levels of lead that were ten times higher than allowed.

There is still some debate about the real impact of low dose lead exposure from the general food supply or the immediate environment. It has been proposed that lead exposure in children is correlated with neuropsychiatric disorders such as attention deficit hyperactivity disorder and antisocial behaviour. Elevated lead levels in children have been shown to be correlated with higher scores on aggression and delinquency measures. A correlation has also been found between prenatal and early childhood lead exposure and violent crime in adulthood. Countries with the highest air lead levels have been found to have the highest murder rates, after adjusting for confounding factors.

Although the facts are there, the above correlations might be a little simplistic. However, it is likely that lead is one of several contributors in a multifactorial model for deviant behaviour and such findings cannot be easily dismissed. What is clear is that even low levels of lead exposure can affect brain development as evidenced by a lower IQ score.

Common lead sources

So what are the most common lead sources? The general public can be exposed to lead via food, water, air, soil and dust. Food is the major source in adults not affected by occupational exposure, although for children ingestion of soil and dust can also be important contributors. Lead used for soldering of food cans and the making of ceramic household goods and water pipes or lead added to paint and petrol can easily contaminate food directly or indirectly by leaking into the environment and finding its way into crops. Surprisingly, some herbal remedies still have lead added deliberately. A particular risk group is hunters and their families with high consumption of game meat because of the common use of lead ammunition.

Particularly high lead levels have been identified in game meat and offal, seaweed and dietary supplements like the herbal remedies mentioned above. However, because the presence of lead is so widespread in food, foods consumed in higher amounts like bread and rolls, tap water and potatoes contribute the most to lead exposure for the general public. It is thus very difficult for individuals to reduce their lead exposure from food. Collective remedial actions as outlined below can gradually reduce lead exposure.

Mode of action

The primary cause of lead’s toxicity is its interference with a variety of enzymes. Lead mimics other metals that take part in biological processes and interact with many of the same enzymes, thus interfering with the enzyme’s ability to catalyze its normal reactions. Among the essential metals with which lead interacts are calcium, iron, and zinc.

Lead also interferes with the release of neurotransmitters, chemicals used by neurons in the brain to send signals to other cells. Glutamate is one such neurotransmitter important in many functions including learning, with lead blocking its activation of NMDA receptors. The targeting of NMDA receptors is thought to be one of the main causes for lead’s toxicity to neurons

Remedial measures

International and European health-based guidance values for lead exposure have been amended several times as new information has come to light. In 2010, EFSA  concluded that existing recommendations were no longer appropriate and that, as there was no evidence for a threshold for a number of critical endpoints including developmental neurotoxicity and adult nephrotoxicity, it would not be proper to issue a guidance value. This conclusion was confirmed by JECFA in 2010, while also expressing a concern that there was potential at current levels of exposure for lead to affect neurodevelopment in infants, children and the foetus of pregnant women.

Internationally, control measures are now in place to regulate lead in a number of products. A range of preventative measures also exist at national and local levels. Recommendations by health professionals for lowering childhood exposures include banning the use of lead where it is not essential and strengthening regulations that limit the amount of lead in soil, water, air, household dust, and products. A 1978 law in the US restricts the use of lead in paint for residences, furniture, and toys to 0.06% or less. Legislative control measures have been taken to remove lead from paint, petrol, food cans and water pipes in Europe since the 1970s. Leaded petrol was banned in the European Union from 2000 with exemptions possible until 2005.

Unfortunately lead remediation will take time to filter through the environment and many developing countries are lagging behind in making the efforts necessary. The lead problem will remain for decades to come and continued vigilance will be necessary. However, in an encouraging sign FDA, in an ongoing survey of 285 of the most important food products in the U.S. food supply, found that dietary intake of lead by a 2-year-old child had dropped more than 90 percent since 1979. Similarly, EFSA in a recent survey covering a shorter time period found that dietary exposure had been reduced by close to 25% between 2003 and 2010 in the overall population.