Lead [Pb, 82]
177
The background Pb contents in barley and wheat grains are established at 0.2
and 0.5, respectively. Its contents in cereal grains from seven EU countries ranges in
0.007–0.013 mg/kg (Eriksson 2001a). Range of mean Pb amounts in grasses from
various countries is 0.4–4.6 mg/kg and in clover 1.3–8 mg/kg. Grass growing along
the high traffic road may contain Pb up to about 20 mg/kg.
24.6 HUMANS
The total amount of Pb in human body varies with age, occupation, and environment. It has been estimated that a 70 kg man contains an average 120 mg of Pb, of
which in the blood 0.2 mg/L, in the bone (in mg/kg) between 5 and 50, and in the
tissue 0.2–3 (Emsley 2011).
The main sources of Pb exposure are paints, water, food, dust, soil, kitchen utensils, and leaded gasoline. The majority of cases of Pb poisoning are due to oral
ingestion and absorption through the gut. Absorption of Pb from the gastrointestinal
tract is influenced by physiological factors (e.g., age, fasting, Ca and Fe status, and
pregnancy), and the physicochemical characteristics of ingested materials (e.g., particle size, mineralogy, solubility, and Pb species) (WHO 2011c).
Absorbed Pb is transferred to soft tissues, including the liver and the kidneys, and
to the bone tissue, where it accumulates with age. The principal vehicle for the transportation of Pb from the intestine to the various body tissues is the red blood cells,
in which Pb is bound primarily to hemoglobin. In the blood, approximately 99% of
the Pb is found in the erythrocytes, leaving about 1% in the plasma and the serum.
Concentration of Pb in the plasma is more significant than those in the whole blood
as a means of distribution to target organs, that is, the brain, lungs, the spleen, the
renal cortex, the aorta, teeth, and bones. Lead in blood has an estimated half-life of
35 days; in soft tissue, it is 40 days. Residence period of Pb in bone is up to 30 years,
with Pb concentrations in bone and teeth increasing as a function of age. The biological half-life of Pb may be considerably longer in children than in adults. Lead
binds to thiol groups and other ligands in proteins. Its toxicity has been attributed to
the inhibition of enzymes and interference with Ca, Mg, and Zn homeostasis. Leadinduced oxidative stress contributes to Pb poisoning for disrupting the delicate prooxidant/antioxidant balance that exists within mammalian cells. The mechanisms
for Pb-induced oxidative stress include the effect of Pb on membrane, DNA, and
antioxidant defense systems of cells. Antioxidant nutrients including, vitamin E, C,
B 6 , β-carotene, Zn, and Se, may have a beneficial role in Pb-induced oxidative stress
(Hsu and Guo 2002).
Absorption of Pb is higher in children than in adults. Children can absorb 40%–50%
of an oral dose of water-soluble Pb compared to 3%–10% for adults. Children who
are Fe or Ca deficient have higher blood Pb concentrations than children who are Fe
or Ca replete. Absorption of Pb may increase during pregnancy. More than 95% of
Pb is deposited in skeletal bone as insoluble phosphate. Autopsy studies have shown
that 90%–95% of the body’s burden is present in cortical bone and teeth. In adults,
80%–95% of the total body burden of Pb is found in the skeleton, compared with
about 73% in children. Lead can be transferred from the mother to the fetus and to
infants during breastfeeding (Concha et al. 2013; Leikin and Paloucek 2008).