The phasing out of leaded petrol seven years ago may cause a significant decline in crime in South Africa in another 11 years.
Or it may not. The broad societal effect of lead and a causal link between the metal and crime is still the subject of controversy and debate, and will be for years, as the scientific enquiry continues. The individual, clinical effect, however, has been in no doubt for some time.
In 2010 the World Health Organisation (WHO) created a booklet on childhood lead poisoning, drawing from hundreds of credible studies to put together an accessible guide to the effects of lead. Lead, it said, "attacks the brain and central nervous system to cause comas, convulsions and even death. Children who survive acute lead poisoning are typically left with grossly obvious mental retardation and behavioural disruption.
"At lower levels of exposure that cause no obvious symptoms and that previously were considered safe, lead is now known to produce a spectrum of conditions that cause loss of cognition, shortening of attention span, alteration of behaviour, dyslexia, attention deficit disorder, hypertension, renal impairment, immunotoxicity and toxicity to the reproductive organs. For the most part, these effects are permanent. They are irreversible and untreatable by modern medicine."
Like many other substances, the WHO had previously published guidelines on what was considered safe levels of lead consumption, measures echoed by the safety regulations of countries, including South Africa. But the body withdrew those guidelines entirely after reconsidering those levels in the light of studies that showed that any trace of lead, no matter how small, negatively affects IQ in children. Lead found a place on a very short list of natural materials considered to be anathema to humans.
The link between lead exposure and a propensity for violence has been largely settled too. Retrospective cohort studies strongly suggest that children become more violent when they have higher levels of lead in their system, but in 2003 a research team from a New Jersey medical school tackled the causal relationship directly by exposing cats to lead and measuring their threshold for aggression.
"The data of this study demonstrate that lead exposure enhances predatory aggression in the cat and provide experimental support for a causal relationship between lead exposure and aggressive behaviour in humans," the team summarised in its findings.
In South Africa, such findings are broadly accepted by experts and academics, and anecdotal evidence points to several sources of lead poisoning. Children in fishing villages (who smelt lead for sinkers), users of Ayurvedic or alternative Indian medicine, painted wood-product refurbishers and people near electronic repair operations that use solders have all been diagnosed with either lead poisoning or dangerously high levels of lead in their blood.
Isolated tests on schoolchildren have also shown alarmingly high levels of blood lead, with the sources unknown in most of the cases.
Yet the regulation of lead lags far behind the evidence of its danger. In South Africa, paint manufacturers may legally use lead up to 600 parts per million and dipstick testing has shown that even that limit is regularly breached.
In the United States the limit for lead in paint is 90 parts per million, similar to the regulatory levels set by the likes of Sri Lanka for children's toys.
Occupational health and safety limits are similarly high and similarly unlikely to be updated.
Neurotoxin can be linked to crime, say the experts
Depending on your definition of rigour, research findings on the effects of lead on humans date back either to the first century AD (when Pliny the Elder warned that lead fumes were dangerous) or to 1848, when lead poisoning in children by way of their toys was first documented.
But in the past 30 years the link between lead and behaviour has been almost completely settled and evidence of a causal link with crime has been growing for the past 15 years.
These are the findings of some of the most highly regarded peer-reviewed articles during that period:
- Lead exposure is associated with increased risk of antisocial and delinquent behaviour, and the effect follows a developmental course. – "Bone Lead Levels and Delinquent Behaviour", published in the Journal of the American Medical Association, 1996
- Although other social trends and government policies are often cited to explain the rise and fall in unwed pregnancy and crime rates over recent decades, the role of childhood lead exposure seems to be especially apparent. – "How Lead Exposure Relates to Temporal Changes in IQ, Violent Crime and Unwed Pregnancy", published in Environmental Research, 2000
- Elevated body lead burdens, measured by bone lead concentrations, are associated with elevated risk for adjudicated delinquency. – "Bone Lead Levels in Adjudicated Delinquents. A Case Control Study", published in Neurotoxicology and Teratology, 2002
- Lead exposure appears to be associated with increased withdrawn, easily frustrated, hyperactive and inattentive behaviour in early childhood; whereas by adolescence, many lead-exposed children may become aggressive, antisocial and delinquent. – "Neurodevelopmental Effects of Postnatal Lead Exposure at Very Low Levels", published in Neurotoxicology and Teratology, 2004
- Childhood lead exposure appears to be significantly related to adult violent crime. – "Environmental Policy as Social Policy? The Impact of Childhood Lead Exposure on Crime", published in The BE Journal of Economic Analysis & Policy, 2007
- Both clinical and econometric evidence suggest that lowered lead levels will lead to lower crime rates. – "Childhood Lead Poisoning: Conservative Estimates of the Social and Economic Benefits of Lead Hazard Control", published in Environmental Health, 2009
- Our findings along with others predict that prevention of children's lead exposure from lead dust now will realise numerous societal benefits two decades into the future, including lower rates of aggravated assault. – "The Urban Rise and Fall of Air Lead (Pb) and the Latent Surge and Retreat of Societal Violence", published in Environment International, 2012