LEAD
Lead is a gray-white, soft metal with a low melting point, a high resistance to corrosion, and poor electrical conducting capabilities. It is highly toxic. In addition to its highly concentrated ores, lead is naturally available in all environmental media in small concentrations. From the atmosphere, lead is transferred to soil, water, and vegetation by dry and wet deposition. A significant part of lead particles from emissions sources is of sub micron size and can be transported over large distances. Larger lead particles settle more rapidly and closer to the source. Lead in soil binds hard, with a half-life of several hundred years. New depositions, primarily atmospheric, therefore contribute to increased concentrations. Atmospheric deposition is the largest source of lead in surface water, as well. Only limited amounts are transported to water from soil. Terrestrial and aquatic plants show a strong capability to bio accumulate lead from water and soil in industrially contaminated environments (WHO 1989). Lead can also be taken up by grazing animals, thus entering the terrestrial food chain.
Sources & uses
Mining, smelting, and processing of lead and lead containing metal ores generate the greatest part of lead emissions from stationary sources. In addition, the combustion of lead-containing wastes and fossil fuels in incinerators, power plants, industries, and household’s releases lead into the atmosphere. As a result of the extensive use of alkyl-lead compounds as fuel additives, vehicular traffic is the largest source of atmospheric lead in many urban areas, accounting for as much as 90% of all lead emissions into the atmosphere. High concentrations of lead in urban air have been attributed to vehicular emissions in various countries. Traffic-generated lead aerosols are mostly of the submicron size.Water distribution systems frequently contain lead pipes or lead solder, contaminating drinking water. Lead carbonate (“white lead”) was highly popular as a base for oil paints before its use was banned in most countries in the first half of the twentieth century. Lead-based paint and dust contaminated by such paint still represent significant sources of human exposure in several countries. Other lead-based products include food-can solder, ceramic glazes, crystal glassware, lead-jacketed cables, ammunition, and cosmetics.
Health Impacts of Exposure
The main pathways of lead to humans are ingestion and inhalation. Children up to about six years of age constitute the population group at the highest risk from lead exposure through ingestion: their developing nervous systems are susceptible to lead-induced disruptions; their intake of food is relatively high for their body weight; they are exposed to high intake from dust, dirt, soil, and lead-containing paint due to their hand-to-mouth behavior; and their absorption through the gut is very efficient. The main sources of lead exposure of children are dust and dirt; the role of dissolved lead in water supply systems, lead-based paint, and other sources varies across locations. The contribution of drinking water to exposure is highest in infants under one year of age and children under five years of age. Inhalation poses the highest risk of exposure to environmental lead in adults. Inhaled airborne lead represents a relatively small part of the body burden in children, but in adults it ranges from 15 to 70%. About 30–50% of lead inhaled with particles is retained in the respiratory system and absorbed into the body (WHO 1987. Lead affects several organs of the human body, including the nervous system, the blood-forming system, the kidneys, and the cardiovascular and reproductive systems. Of most concern are the adverse effects of lead on the nervous system of young children: reducing intelligence and causing attention deficit, hyperactivity, and behavioral abnormalities. The negative impact of lead exposure is generally stronger on verbal IQ than on performance IQ. (WHO 1995) Prenatal exposure to lead was demonstrated to produce toxic effects in the human fetus, including reduced birth weight, disturbed mental development, spontaneous abortion, and premature birth. Such risks were significantly greater at blood lead levels of 15 µg/dl and more (WHO 1995). High lead concentrations, generally due to occupational exposure or accidents, result in encephalopathy, a life-threatening condition at blood lead levels of 100 to 120 µg/dl in adults and 80 to 100 µg/dl in children (ATSDR 1990). An acute form of damage to the gastrointestinal tract known as “lead colic” is also associated with high lead levels.
Case study
The study was made on pregnant women visiting a tertiary care hospital (Qatar Hospital, Orangi Town, Pakistan) for prenatal care, who had been a resident of the city of Karachi for the past four years. A sample of peripheral venous blood from pregnant women and cord blood from the newborn were taken at delivery. Visits were made of 66 homes for three consecutive days to collect several samples from the household for the determination of lead exposure for the women and child of the family.
Among st the total number of subjects whose BLL was tested at Karachi, (n=524), 26.5% (n=139) were children (<16 years). The median BLL was 4 ug/dl (6.7-2.6) in children. The BLL increased with age, as higher levels were observed in age range 21-30 years of subjects and lower level in children with <10 years of age. Only 16% (n=22) children had desirable lead levels while most had either sub clinical (76%, n=106) or toxic lead levels (8%, n=11).The body intake of lead among pregnant women from different families ranged from 8.9 to 22.6 µg/kg body weight/week. The food was the most important source of lead intake. The food obtained from contaminated soils has a huge contribution in the cause. The IVBA of lead from food ranged between 29%–83% (mean = 62.37%). The contribution of lead by food was higher for families with a higher exposure to lead. The second most important source of lead exposure was respirable dust (PM4) intake.