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Why use lead in paint

Why use lead in paint?
By James Mitchell Crow
Image
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.