Lead, easy to shape and a good conductor, is one of the most serious of environmental pollutants. It has adverse effects as it accumulates in the body, especially for children and pregnant women. It can lead to behaviour disorders, anemia, mental retardation and permanent nerve damage. Most lead accumulates in bone and kidney

Cars have been getting cleaner in important respects for decades. Engine compression ratios were raised, allowing engines to burn fuel more cleanly and efficiently. However, those high compression engines needed high octane petrol (to avoid untimely explosions in the cylinder). Before the 1940s, making high octane petrol was considered costly in energy. Then it was discovered that using lead as an additive to petrol allowed high compression engines to run on relatively low octane fuel. Lead was widely seen as an environmental boon in its day

Man-made sources of lead include lead smelting and refining, the combustion of leaded fuel, the production of storage batteries, the manufacture of alkyl lead and lead points and the application of lead-based pesticides. Lead pipes, lead-glazed earthenware and flaking lead points are possible sources of lead in the domestic environment. The predominant source of atmosphere lead appears to be from the use of 'antiknock' agents in petrol. Lead pollutes the air (above quiet roads the concentration may be in the range 0.25 - 1.2 �g/m3; busy roads 2.5 - 4.5 �g/m3; and congested roads up to 50 �g/m3); it also pollutes fresh water (1-10 �g/litre; this figure may be much higher in areas with lead pipes and soft, slightly acidic water), sea water (0,01 - 0,3 �g/litre) and food (an average of 0,2 �g/kg)

Food is the major source of lead intake in adults who are not occupationally exposed or have high concentration of lead in drinking water. The contribution of airborne lead to the total daily absorption as compared to average dietary intake is more difficult to estimate, as it depends upon the concentration, particle size and solubility of the lead. Some scientists suggest that airborne lead is much more dangerous and that about 50% of it may be absorbed on inhalation

Although symptoms of clinical lead poisoning in adults do not appear at levels in the whole blood below about 80 �g/100g, the inhibition of certain enzymes involved in the synthesis of haem can be shown to occur at levels now present among urban populations. The developing nervous system of children is particularly at risk. An estimated 1.7 million children in the United States have unacceptably high levels of lead in their blood. Young children in urban and industrial areas are much more prone to lead poisoning than other sectors of the population. Such children may ingest lead from paint, roadside dust, vehicle exhaust emissions and industrial pollution. In New York and Chicago 1-2% of children in low income areas were shown to have blood levels indicative of lead poisoning, whereas a further 25% had levels above 40 �g/100g. A study in London showed that 40.9% of children living within 100-400m of a lead factory had blood lead levels in excess of 40 �g/100g

According to a scientist at London University, pollutants like lead are already affecting the intelligence of one in 10 British children, and as much as 90 percent of children in some African countries

1. Lead is a neurotoxin and accumulates in the environment, so it is patently foolish to spray it around the streets. Lead contributes to the high rate of osteoporosis, the brittle-bone disease that bends the backs, shortens the stature and breaks the hips of many older women. At high levels, lead in the blood of a pregnant woman can lead to a miscarriage or premature birth and at lower levels it can hold back the development of an unborn baby's nervous system and brain. Soft (acidic) water in areas with lead plumbing can be hazardous to bottle-fed babies. Childhood is a risky time because exposure to lead during the first four years can damage nerve cells and retardation. Students with high levels of lead are more likely to be distracted and easily bored. Hyperactivity in a young child often turns out to be a symptom of lead poisoning, and hyperactive children are more likely to become delinquent

2. In the Philippines, a nationwide drop in children's IQ, five points on average, is blamed on lead in the air

3. No other toxic chemical pollutant has accumulated in man to average levels so close to the threshold for overt clinical poisoning

1. In reducing the amount of lead in petrol, manufacturers needed to restore octane levels with other fuel additive. One way was to substitute volatile organic compounds (VOCs). Some of these are carcinogenic and all react with NOx to create ozone, and are greenhouse gases. Other new fuel additives, especially oxygenates, are expensive and have been appearing as contaminants in water supplies. The other disadvantage of reducing lead, and hence octane ratings, of petrol is that car manufacturers have lowered compression ratios and hence fuel efficiency. Older cars have to be de-tuned so that they can burn unleaded fuel, and so they are also less fuel efficient. It terms of pollution it means that unleaded petrol contributes to greater carbon dioxide emissions for the entire car fleet, and increased VOC emissions for cars burning lead-free petrol without catalytic converters (and for those with inefficient catalytic converters)

2. The possibility of a causal relationship between lead absorption and mental retardation has been suggested, but has also been disputed