Geology Reference
In-Depth Information
Plants can take up lead from soil, thus providing a route of human
exposure. Careful research in recent years has established transfer
factors, terms the Concentration Factor (CF), where
CF ¼ D Concentration of lead in plant ð mg kg 1 dry wt
D Concentration of lead in soil ð mg kg 1 dry wt
The value of CF for lead is lower than for most metals and is typically
within the range 10 3 10 2 . Much higher values had been estimated
from earlier studies, which ignored the importance of direct atmospheric
deposition as a pathway for contamination. The direct input from the
air to leaves of plants is often as great, or greater than soil uptake. 24,25
This pathway may be described by another transfer factor, termed the
Air Accumulation Factor (AAF), where
AAF ð m 3 g 1 Þ¼ D Concentration of lead in plant ð mgg 1 dry wt
D Concentration of lead in air ð mgm 3 Þ
Values of AAF are plant dependent, due to differences in surface
characteristics, but values of 5-40 are typical. 25,26 Thus, a plant grown
on an agricultural soil with 50 mg kg 1 lead will derive 0.25 mg kg 1 dry
weight lead from the soil (CF ¼ 5 10 3 ), while airborne lead of 0.1 mg
m 3 will contribute 2.0 mgg 1 (Rmg kg 1 ) of lead (AAF ¼ 20 m 3 g 1 ).
Thus, in this instance airborne lead deposition is dominant. The air lead
concentration of 0.1 mgm 3 was typical of rural areas of the UK until
1985. Since that time, the drastic reduction of lead in gasoline has led to
appreciably reduced lead-in-air concentrations in both urban and rural
localities.
Human exposure to lead arises from four main sources: 2,27
(i) Inhalation of airborne particles. The adult human respires ap-
proximately 20 m 3 of air per day. Thus for an urban lead
concentration of 0.1 mgm 3 , intake is 2 mg per day. This is rather
efficiently absorbed (ca. 70%) and therefore uptake is around 1.4
mg per day in this instance.
(ii) Indigestion of lead in foodstuffs. The concentrations of lead in
food obviously vary between different foodstuffs and even be-
tween different batches of the same food. Typical fresh weight
concentrations (much of the weight of some foods is water) are
from 10 to 50 mgPbkg 1 . Thus a food consumption of 1.5 kg per
day represents an intake of around 50 mg per day and an uptake
(10-15% efficient) of around 6 mg per day.
(iii) Drinking water and beverages. Concentrations of lead in drink-
ing water vary greatly, related particularly to the presence or
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