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Courtesy of:
http://www.rsc.org/chemistryworld/News/2007/August/21080701.asp
Why use lead in paint?
Mattel, the world's biggest toy maker, has recalled millions of toys
that were coated with lead paint. Lead's poisonous properties have been
known for thousands of years, so why was lead ever added to paint, and
why is lead paint still being made?
What is 'lead paint'?
Any paint that relies on lead compounds for its colour. White lead, or
lead(II) carbonate (PbCO3), is a typical example, and was
once widely used to paint wooden surfaces in homes. Other lead
compounds, like vivid yellow lead chromate (PbCrO4), were
used as coloured pigments. As well as giving the paint its tint, lead
pigments are highly opaque, so that a relatively small amount of the
compound can cover a large area. White lead is very insoluble in water,
making the paint highly water-resistant with a durable, washable finish.
Lead carbonate can also neutralise the acidic decomposition products of
some of the oils that make up the paint, so the coating stays tough, yet
flexible and crack-resistant, for longer.
So what's the problem?
Lead is toxic, and as young children tend to chew things, they are
particularly prone to ingesting it. Not good for their vulnerable,
developing brains. It was increasingly recognised in the first half of
the 20th century that children were being poisoning with lead
paint, and its use in cots and toys had been phased out in the West by
the 1950s. However, lead-based decorative household paints were still
used for another couple of decades before this too ended due to health
concerns.
Why is lead toxic?
Lead can disrupt numerous crucial bodily functions, and hence has a wide
variety of symptoms, from vomiting to madness to death. It's known to be
a potent blocker of receptors of glutamate, a neurotransmitter crucial
for learning. It is also able to displace a series of other metals from
doing their normal job in the body - most significantly, calcium, iron
and zinc. A particular problem is that lead displaces the zinc from the
enzyme delta-aminolaevulinate dehydratase, which is crucial for the
biosynthesis of heme, the iron-binding part of the haemoglobin molecule
that carries oxygen around the blood. This results in cells around the
body being short of oxygen, causing a cascade of associated problems.
All lead paints have been banned then?
Almost. For the vast majority of uses, lead pigments have been replaced
with titanium dioxide, which is so safe it's also used in food
colourings as well as in sunscreen. In the EU, lead paint can now only
be used for the restoration and maintenance of works of art and
historical buildings. In the US, lead paint can be used in limited
industrial settings, such as to coat ships hulls.
How was the toxic paint found?
Mattel identified the toxic paint during routine safety checks. Lead in
paint - either in its liquid form, or as a dried coating on a product -
can be detected by a number of analytical techniques, said Allan Stewart
of testing and certification firm Intertek's Measurement Science Group
Lab on Teesside, UK. Atomic absorption and x-ray fluorescence
spectroscopy are both general methods that can reveal the presence of
lead, while inductively coupled plasma mass spectrometry can detect lead
in less than part-per-billion quantities, he added.
So what's happening in China?
China is gradually cracking down on the use of lead, banning leaded
petrol in 2000, for example. However, lead paint is cheaper than the
alternatives, which seems to have tempted certain Chinese manufacturers
to use it in preference to the non-toxic, and legal, replacements. In
response to a series of product safety scandals, which have included
toxic pet food and toothpaste, the Chinese government have appointed
legendary 'fixer' Wu Yi to the task. Yi, who as vice premier is the most
senior female official in the country, will head a new cabinet panel on
product safety, in an attempt to address the country's problems with
product safety.
James Mitchell Crow |