Lead poisoning is a condition caused by increased levels of blood serum lead levels. The average person has less than 10 micrograms per decaliter, or 10 parts per million, ppm, of lead in their blood serum. People who have been exposed to an unusual amount of lead will have lead serum levels higher than 20 ppm - most clinical symptoms of lead poisoning begin at around 100 ppm. The symptoms of lead poisoning include reduced IQ and, in extreme cases, seizure.
Outside of occupational hazards, the majority of lead poisoning occurs in children under age twelve. The main sources of poisoning are ingestion of lead contaminated soil (this is less of a problem in countries that no longer have leaded gasoline) and lead based paints. This is particularly a problem in older houses where the sweet tasting lead paint is likely to chip.
Once in the body, lead is biologically inactive – its toxic properties come from lead's ability to mimic other biologically important metals, the most notable of which are iron or zinc. Lead is able to bind to and interact with the same proteins and molecules as iron, but after replacing iron, those molecules function differently and fail to carry out the same reactions.
Most lead poisoning symptoms are thought to occur by interfering with an essential enzyme Delta-aminolevulinic acid dehydratase, or ALAD. ALAD is a zinc binding protein which is important in the biosynthesis of heme, the cofactor found in hemoglobin. Genetic mutations of ALAD cause the disease porphyria, a disease which was highlighted in the movie The Madness of King George.