Rocks can kill. Many rock ingredients have gone from objects of veneration and wonder in the ancient world to being dangerous and feared in our modern one. In ancient times metals like mercury and arsenic were famed for their poisonous potential but today we worry about a whole range of naturally toxic substances. One of the worst is lead.

Although we’re now used to hearing about lead poisoning from petrol and the benefits of lead-free fuel, lead mining has been polluting our planet’s atmosphere for over two thousand years. The virtual absence of free lead in our biosphere before the earliest metal-working societies is presumably why human beings, along with other terrestrial life forms, evolved no biochemical defences against the element. By contrast, our bodies possess very effective mechanisms for limiting the accumulation of other potentially dangerous, but naturally abundant metals, such as copper and iron. Lead atoms have a great affinity for sulphur atoms (as a result, the most widespread mineral of lead is lead sulphide, or galena). It's lead’s attraction to sulphur which is responsible for its toxicity. If lead is introduced to our systems – even at levels as low as a few parts per million – its atoms will combine with sulphur in the proteins within our bodies, disrupting them and causing a wide range of enzymes to malfunction.

Lead is a cumulative poison: its most serious effects do not appear until a critical amount (we're talking about a few grams) has entered the body. After lead is ingested, it appears first in the blood, but is soon distributed throughout the internal organs and the skeleton, where it may be retained for decades. Our bodies can’t eliminate it as a waste product, so it accumulates in the bones and tissues and clings to the red blood cells. Depending on how much they took in, those unfortunates who have ingested lead suffered a wide range of symptoms: headache, insomnia, jaundice and diarrhoea. That's just to start with. Then comes severe stomach pains, gout and extreme, even complete, constipation caused by paralysis of the intestinal tract. Finally come serious central nervous system disorders – deafness, blindness and paralysis – as well as imbecility, hyperactivity, dementia and, of course, death. Lead is not a human-friendly metal.

Today lead, mercury and arsenic, together with cadmium, are recognised to be the most lethal metallic pollutants. Others that can also be toxic are aluminium, chromium, copper, molybdenum, nickel and zinc, whilst problems are also encountered with rare elements like antimony, selenium, thallium and silver. But whilst metals may be harmful, often we can’t live without them. Although humans are 99 per cent oxygen, carbon and hydrogen with a bit of nitrogen, calcium and phosphorus, the remaining one per cent of us is made up of a huge variety of trace elements. The trace elements most essential for our health are chromium, cobalt, fluorine, iodine, iron, manganese, molybdenum, selenium and zinc. Many of these are the same elements that can be poisonous! That’s because elements that in small quantities do us good, in larger amounts are harmful.

Our survival as a species is inextricably linked with the rocks on our doorstep and, like rocks and minerals, the chemical elements required for human existence are unevenly spread. It is from rocks that we derive the essential nutrients for human existence - the trace elements that are so crucial for our good health. In some cases they are in the air we breathe, released as gases during volcanic eruptions. Water provides other essential elements, collected as it filters its way down through soil and rocks. But the main goodness of Earth comes from rocks themselves, which weather and rot, turn into soil, and feed plants and start a food chain that has humans at its endpoint.

Content last updated: 15/09/2004

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