Monday, 9 November 2015

Lead (Part 2): Even at volumes 5000 times smaller than a pea, the metal lead is a potent poison

In Part 1, I had looked at the work that Clair Patterson and Arthur Holmes did in establishing an age of the Earth. In that endeavour Patterson developed analytical methods to measure minute quantities of lead in samples and radically changed the way we treated lead poisoning of our environment.  We must be grateful to Patterson for his dedication and personal sacrifice over a period of 20 years when he fought hard to have lead removed from auto-fuel ensuring that effects of this poison are minimized.

Lead metal has been known to be a potent poison for a few millennia.  How strong a poison lead is can be inferred from the recommended maximum exposure to it for children. There is no lower threshold of lead in the body that can be considered harmless but the current maximum lead concentration in blood (BLL) is recommended to be less than 5 microgram per decilitre (5 micg/dL).  A microgram is a millionth of a gram.

I have a particular problem with these obscure units as they do not tell me a lot about how much lead is considered harmful.  It is better to convert them to something that one can understand.  A child has about 3 litre of blood and if you use the density of lead as 11.342 gram/cc then
the maximum amount of permitted lead is 0.15 mg which is equal to a lead ball bearing of diameter 0.3 mm. 
Taking a pea to be of diameter 5 mm, 
the maximum recommended volume of lead in a child is ~5000 times smaller than the volume of a pea!  

Lead does funny things to the human body - we shall look at the poisonous effects of lead later but it is an eye-opener to see how recommended lead levels have been changing over the past 50 years or so.  
In the USA, for children, they were (in units of micg/dL):

1960 -   60 
1975 -   30 
1985 -   25       20 (World Health Organisation - WHO)
1990 -   10
2006 -     5      10 (for adults)
There is current feeling that the levels should be further reduced to 2 microgram per decilitre of blood. The rationale is as follows:
  • There is sufficient scientific evidence that children suffer from cognitive and behavioural deficits even at BLLs less than 10 μg/dL.
  • Lead toxicity is irreversible and its effects persist for a lifetime.
  • A level of 2 μg/dL provides a tangible goal; a goal of zero, while defensible scientifically, does not and, as a result, would tend to be ignored. 2 μg/dL, so to speak, is ... technically attainable.
  • Appropriate analytical methodology is well developed, available, and comes at reasonable cost.
There is really no need to delay.  Since the sale of leaded petrol was stopped and lead from paint was eliminated, lead levels in the environment have come down significantly.  But these levels are still worryingly high - lead contamination of top soil is of serious concern in urban areas.  

TOXIC EFFECTS OF LEAD:  There is hardly any part of the body that is not affected by lead poisoning but the main worry is the nervous system - both in children and adults.  Children are especially vulnerable and lead causes irreversible damage to their nervous system. 
Lead ions are not too different from calcium ions and lead readily replaces calcium in bones and teeth. Lead exposure can cause weakness in fingers, wrists and ankles.
Increase in blood pressure, anemia, severe stomach pains (colic), muscle weakness, serious damage to kidneys, miscarriage in pregnant women, damage to organs responsible for sperm production are some of the well established effects of lead exposure.

However, the most serious effect of lead in the environment is on a child's physical and mental growth.  On exposure to lead, a child may develop cognitive problems, increased aggressiveness in behaviour, lowered IQ,  poor concentration. 

Relative to an average IQ score of 100, studies have found that a child may lose 7.5 IQ points at 10 micg/dL BLL, rising to 17 lost IQ points at 50 micg/dL

 Studies have linked serious crime rates in a large number of countries to corresponding average lead levels in children with a time lag of about 20 years.  
"A Dose-Response relationship between preschool blood lead and IQ later in life is clearly established. Temporal analyses show a corresponding population-dose-response relationship between preschool blood lead trends and population mental retardation prevalence, crime, and unwed teen pregnancy rates. Average USA preschool blood lead rose by almost 500% from the 1930s through the 1960s. Mental Retardation prevalence then rose by 500% with a 12-year lag; age-15-17 unwed teen pregnancy rates rose by 500% with a 17-year lag; the Index crime rate rose by 500% with a 19-year lag; and the violent crime rate rose by 500% with a 23-year lag."

Lead in paint and petrol has now been banned in almost all countries and the crime rates quoted above have declined in a way that follows the trends in removal of lead from the environment (figures 3 to 13)

In the next blog I shall talk about the distribution of lead in our environment and how it can find its way into the human systems. 

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