Ecological effects - trdoer40198

size distribution of the particles. The lung retains a very minor fraction of

particles over 0.5 um in mean maximal external diameter. The larger particles

are cleared by the respiratory tract and then swallowed. Those particles less than

0.5 um are efficiently absorbed (WHO 1995).

Distribution in the body occurs in a similar manner regardless of the route of

absorption. Lead is distributed to both soft tissue and bone, although distribution

is not homogenous. Three pools of lead have been identified: blood, bone, and

soft tissues. This compartmentalization and distribution to these compartments

forms the basis for the biokinetic models for lead. Blood is the compartment in

which lead is most often measured as a marker of recent exposure (due to the

short half-life of lead in blood), although lead in blood is also derived from lead

stored in tissues. Human bone has at least two, possibly three, kinetically distinct

lead compartments with differing abilities to mobilize lead to the blood. Lead in

bone may contribute as much as 50 percent of blood lead, so bone itself is a

significant source of lead. The fraction of lead in bone increases with the age of

the person, therefore this is more of a concern for adults. In the body, about

94 percent of the adult body burden of lead is localized in the skeleton, about

4 percent is in the blood, and 2 percent is in soft tissue. In children, only about

73 percent of lead in the body is in the bone. Mobilization of lead during

pregnancy and lactation will elevate blood lead concentrations and can be of

concern for fetal exposures. Lead is efficiently transferred across the placental

membranes. The lead concentration in human umbilical cord blood is 85 to

90 percent that of maternal blood, and lead accumulation in fetal tissues is

proportional to maternal blood lead levels (World Health Organization 1995).

Absorbed lead is eliminated through urinary and fecal excretion. The

unabsorbed gastrointestinal lead and the airborne lead that was swallowed are

also eliminated in feces. Based on estimates of first-order elimination of half-

lives for lead in blood, a constant lead intake rate over the course of months is

required to maintain a steady-state blood-lead level. Exposures of 1 day/week are

sufficient to maintain these steady-state conditions (USEPA 1994).

Ecological effects

The effects of metals in soils are very much dependent upon the availability

of the metal from the soil matrix. Lead seems to be tightly bound by most soils,

and substantial amounts must accumulate before it affects the growth of higher

plants (Eisler 1988). Plants readily accumulate lead in soils with low pH or low

organic content. Lead has very high residence time in forest litter. Estimates

range from 220 years to 500 years (as summarized in Eisler 1988). Lead

toxicosis has been observed in plants from lead concentrations ranging from

0.005 to 33,000 mg/L. Effects include growth stimulation (at low levels), growth

inhibition, leaf yellowing, abscission, inhibition of mitosis and chlorophyll

synthesis, loss of turgor pressure and death.

Eisler (1988) reviewed the potential effects of lead contamination to wildlife

for the U.S. Fish and Wildlife Service. Lead toxicity in water fowl through the

ingestion of lead pellets is well documented. Several accidental lead poisoning

D46

Appendix D Toxicological Profiles