Environmental Lead from Leaded Gasoline
When these additives are used in gasoline, the atoms of lead that are liberated by the combustion of the tetraalkyl compounds must be removed before they form metallic deposits and damage the vehicle's engine. In order to convert
1920 1930 1940
FIGURE 15-4 The historical consumption of load in gasoline in the United States. [Source: C. E. Dunlop et al., "Past Leaded Gasoline Emissions as a Nonpoint Source Tracer in Riparian Systems," Environmental Science and Technology 34 (2000): 1211.]
the combustion products into volatile forms that can leave the engine in the exhaust gases, small quantities of ethylene dibromide and ethylene dichlo-ride are also added to the leaded gasoline. As a result, the lead is removed from the engine and enters the atmosphere from rhe tailpipe in the form of a mixture of the mixed dihalide PbBrCl and the dihalides PbBr? and PbC^-Subsequently, under the influence of sunlight, these compounds form PbO, which then exists in particulate form as an aerosol in the atmosphere for hours or days. Consequently, not all of it is deposited in the immediate surroundings of the roadway. As a result, PbO can enter the food chain at more distant sites if deposited on vegetables or on fields used by grazing animals. Furthermore, a small fraction of the ethylene dihalides are converted into dioxins and furans and enter the environment in these forms.
A high proportion of environmental lead in many parts of the world is that emitted from vehicles; it occurs in the environment mainly in inorganic form. The conversion to nonleaded gasoline in North America and Europe, the initial impetus for which was the interference of lead in exhaust gases with the proper functioning of catalytic converters, has had the welcome side effect of greatly decreasing the average amount of lead ingested by urban inhabitants. Indeed, the noted environmentalist Barry Commoner has called the elimination of lead from gasoline "one of the [few] environmental success stories." European scientists have traced the rise and fall of atmospheric alkylated lead by analyzing different vintages of a French red wine (Chateauneufdu-Pape) that used grapes grown near two busy auto routes. They found that the concentration of trimethyllead, PbR3 :—the degradation product of the tetramethyl compound— rose steadily to a maximum in the mid-1970s, which was followed by a steady decline to about one-tenth of the peak concentration by the early 1990s as the compound was phased out of gasoline. This pattern of usage is consistent with the variation in the U.S. consumption of lead for gasoline use; it is plotted in Figure 15-4 and shows a sharp rise from 1930 to 1970, followed by an even sharper decline thereafter-
Many countries still use leaded gasoline. In these areas, the air is the major source of lead ingested by humans, as it was in the past in North America and Europe. For example, in Mexico airborne lead from vehicular emissions is a major source of the elevated lead levels found in the
1950 1960 Year
1970 1980 1990
1950 1960 Year
1970 1980 1990
bloodstream of many Mexican children. Some of the gasoline-based lead enters the body directly from inhaled air, and some enters indirectly from food into which lead has been incorporated. Microorganisms do bioconcen-trate lead, but in contrast to mercury, lead does not undergo biomagnitication in the food chain.
Lead's Effects on Human Reproduction and Intelligence
Most ingested lead in humans is initially present in the blood, but that amount eventually reaches a plateau. Any excess enters the soft tissues, including the organs, particularly the brain. Eventually lead becomes deposited in bone, where it replaces calcium, because Pb2+ and Ca2+ ions are similar in size. Indeed, lead absorption by the body increases in persons having a calcium (or iron) deficiency and is much higher in children than in adults. A study in Mexico indicated that pregnant women can decrease the lead levels in their blood—and presumably in the blood of their developing fetus—by taking calcium supplements.
At high levels, inorganic lead (Pbz+) is a general metabolic poison. The toxicity of lead is proportional to the amount present in the soft tissues, not to that in blood or bone. Lead remains in human bones for decades; thus it can accumulate in the body. The dissolving of bone, as occurs with old age or illness such as osteoarthritis and advanced periodontal disease or in times of stress such as pregnancy and menopause, results in the remobilization of bone-stored lead back into the bloodstream where it can produce toxic effects. Excess lead may lead to the deterioration of bones in adults. Recently a correlation has been found between periodontal bone loss and blood lead levels in U.S. adults, particularly in those who smoke. Children exposed to environmental lead also have more dental cavities.
Although there is some evidence that too much lead can slightly increase the blood pressure of adults, the humans most at risk from Pb2+ even at relatively low levels are fetuses and children under the age of about seven years. Both these groups are more sensitive to lead than are adults, partly because they absorb a greater percentage of dietary lead and partly because their brains are growing rapidly. The metal readily crosses the placenta and thus is passed from mother to unborn child. Because of the immaturity of the fetus's blood-brain barrier, there is little to prevent the entry of lead into its brain. In addition, lead is transferred postnatally from the mother in her breast milk and/or from the tap water used to prepare formula for bottle-fed babies.
The principal risk to children from lead is interference with the normal development of their brains. A number of studies have found small but consistent and significant neuropsychological impairment in young children due to environmental lead absorbed either before or after birth. Lead appears to have deleterious effects on children's behavior and attentiveness, and possibly also on their IQs. This is illustrated in Figure 15-5, where a mental
12 18 Age at testing (months)
12 18 Age at testing (months)
FIGURE 15-5 The effect of prenatal exposure to lead on the mental development of infants. Lead exposure is measured by its concentration in the blood of the child's umbilical cord. [Source: D, Bellinger et al., "Longitudinal Analyses of Prenatal and Postnatal Lead Exposure and Early Cognitive Development/' New England Journal of Medicine 31 6 (1987): 1037-1043. Reprinted by permission of the New England Journal of Medicine.]
development score is plotted as a function of age for groups of young children differentiated by the amount of lead in their umbilical cord at birth. A study of children in a lead-smelting community (Port Pirie) in Australia indicates that children with a blood lead level of 300 ppb had an average IQ 4 to 5 points lower than those whose level was 100 ppb. This result is consistent with other studies that indicate an IQ deficit of about 2-3 points for each increase by 100 ppb of blood lead. Some studies indicate that prenatal exposure to lead—especially during the first trimester of pregnancy—has the greatest detrimental effect on the IQs measured in children in primary grades, some indicate it is the lead level at the age of two (when blood concentrations usually peak) that is predominant, and others that it is the concurrent lead level—even if lower than that in early childhood— that is the dominant factor. No threshold for the effects of lead upon IQ are apparent in the studies.
A survey in 1976-1980 of American children aged six months to five years found that about 4% of them had blood lead levels in excess of 300 ppb and that an additional 20% had levels over 200 ppb (see Figure l5-6a). These concentrations represent two of the cutoffs that had been proposed in the past as "safe" levels, but it appears that there may be no level at which lead does not produce a deleterious effect (i.e., there is no threshold) in young and unborn children. A second survey, in 1988-1991, indicated that blood lead levels in U.S. children had fallen substantially; less than 9% of those aged one to five years had blood lead levels greater than 100 ppb (see Figure 15-6b). The average American adult blood lead level fell from about 150 ppb in the 1970s to about 10-20 ppb today.
The effects of lead poisoning were known to the ancient Greeks, who realized that drinking acidic beverages from containers coated with lead-containing substances could result in illness. This information was not available to the Romans. Indeed, they sometimes deliberately adulterated overly acidic wine with sweet lead salts to improve the flavor. The concentration of lead in the bones of Romans is almost 100 times that found in modern North Americans. Some historians have hypothesized that chronic lead poisoning of upper-class Romans, from wine and other sources, contributed to the eventual downfall of the Roman Empire because of the metal's detrimental effects on the neurological and reproductive systems. The latter effects include dysfunctional sperm in males and an inability to bring the fetus to term in females. Indeed, in the past women who worked in the lead industry suffered higher-than-average rates of miscarriages and stillbirths. Due primarily to the contamination of beverages by lead from the distillation of alcohol in lead vessels, episodes
FIGURE 15-6 The distribution of blood lead levels in U.S. children aged one to five years (a) in 1976-1980 and (b) in 1988-1991. [Source: R. A. Coyer, "Results of Lead Research: Prenatal Exposure and Neurological Consequences," Environmental Health Perspectives 104 ;i996): 1050-1054.)
of colic and gout due to lead poisoning were recorded through the Middle Ages and even until recent times.
In summary, on an atom-for-atom basis, lead is not as dangerous as mercury. However, the general population is exposed to lead from a greater variety of sources and generally at higher levels than those associated with mercury. Overall, more people are adversely affected by lead, though on average to a lesser extent, than those fewer individuals exposed to mercury. Both metals are more toxic in their organic compound form than in the simple inorganic cation form. In terms of its environmental concentration, lead is much closer—within a factor of 10—to the level at which overt signs of poisoning become manifest than is any other substance, including mercury. Thus it is appropriate that society continues to take steps to further reduce human exposure to lead.
The concentrations of lead in blood samples are often reported in units of micrograms of Pb per deciliter of blood or in micromoles of lead per liter of blood. Calculate the value of the concentration in these units of a blood sample containing 60 ppb lead, assuming that the density of blood is one gram per milliliter.
100 120 J 40 160 180 200 220 240 260 280 >300
100 120 J 40 160 180 200 220 240 260 280 >300
Continue reading here: Environmental Sources of Cadmium
Was this article helpful?