Lead is found deep within the earth. Ancient civilizations had almost no exposure to it until four or five thousand years ago, when lead was found as a by-product of silver smelting. Since then, it has been used progressively throughout history. In the Roman Empire, lead was widely used in water pipes and drinking and storage vessels. Many scientists and historians now feel that lead led to the downfall of the Roman Empire, with the ruling classes suffering decreased mental capacities, decreased birthrate, and shortened life span.

In the twentieth century, lead has been widely used in paint, some containing a high percentage of lead. This has been a problem especially with children, who are more sensitive to lead than adults because of their better absorption and smaller bodies. Lead has a mildly sweet taste, and children often suck on or eat the paint chips off of houses or out of the dirt, leading to many cases of lead poisoning. In the 1920s, tetraethyl lead was added to gasoline as an antiknock, higher-octane additive. This has probably been the most widespread and pervasive source of environmental contamination from lead to date. Other common uses for lead are as seals for tin cans, in pewter, in ceramics and pottery glazes, in insecticides, and more.

In recent years, however, there has been an attempt to decrease this environmental contamination. Cars are now using unleaded gasoline. This does not, of course, eliminate the problems of carbon monoxide and burned hydrocarbons, but it will help to decrease lead exposure in the future. In 1971, Congress passed the Lead Paint Act, limiting the use of lead in paints. This will also help, but not for many years to come, since many older homes still contain leaded paints. As they deteriorate, lead gets into the soil and does not degrade. In 1979, a law was passed decreasing the use of lead in food storage cans, though it is still present in some solders.

Bone analysis of very old skeletons indicates that modern humans have nearly 500-1,000 times more lead in our bones than did our ancient ancestors. Our total body content of lead nowadays is estimated at 125-200 mg. We can handle nearly 1-2 mg. daily with normal functioning, but the margin of safety is narrow. Luckily, most people's daily exposure is less than that, about 300-400 mcg.

Lead is a neurotoxin and commonly generates abnormal brain and nerve function. It passes into the brain and can also contaminate the in-utero fetus and breast milk. Most lead, though, is stored in the bones. With lead intoxication, "lead lines" are visible in the bones on X-rays. Some is also stored in the liver and soft tissues. Infants have very little lead, but our body concentrations usually increase with age.

Luckily, lead is not very well absorbed, usually less than 5 percent, though children absorb it at a higher rate. Many minerals, such as calcium and iron, interfere with further lead absorption. When lead gets into the blood, it does not stay long, either going into the bones and other tissues or being eliminated. Most ingested lead leaves via the feces; that which is absorbed or inhaled will usually be cleared by the kidneys, through perspiration.