The Concept of Critical Levels of Toxic Heavy Metals in Target Tissues

Abstract

The high reactivity of heavy metals with biological systems is well documented, although some disagreement remains on the precise dose-effect relationships involved. This represents a question of considerable importance, especially in attempts to assess the risks of exposure. The implicit assumption is usually made that a threshold concentration of specific metals exists in the most sensitive target organ, so that an increased frequency of functional lesions will be expected if this threshold is exceeded. The threshold for the metal defines its so-called critical level, and this review was written in order to examine the theoretical and practical difficulties in establishing such a level. Among these may be cited, for instance, the dependence of what constitutes the target tissue on the speciation of the metal, the changes in apparent critical level with rate and route of metal administration, the short half-life of some of the metals as well as their compartmentation in the tissues, and the considerable initiation delay frequently preceding the appearance of lesions. For these and other reasons a useful approximate value for a critical concentration has only been proposed so far for the total Cd concentration in the renal cortex of chronically exposed human adults.

Interactions between copper and cadmium modify metal organ distribution in mature tilapia, Oreochromis mossambicus

“…The present study examines Cu-Cd interactions in mature tilapia, paying particular attention to organs with diverse biological functions, because it is anticipated that differences in metal accumulation between organs will be related to their functions. Organ metal concentration or metalmetal interaction at this level may be a link to toxicity (Foulkes, 1990;Landrum et al, 1992). In keeping with this, it has recently been suggested that metal concentrations in the organs of fish, rather than the metal concentrations in the ambient water, could be used as a biomonitor for water pollution in natural freshwaters (Handy, 1992).…”

Baseline risk assessment of groundwater contamination at the uranium mill tailings site, near Gunnison, Colorado. Revision 2

“…The critical concentration of cadmium in the renal cortex below which no adverse effect would be anticipated has been estimated at about 200 pg/gram fresh weight (Foulkes, 1990;EPA, 1994a …”

Baseline risk assessment of ground water contamination at the uranium mill tailings site near Durango, Colorado. Revision 1

“…Progressive disruption of kidney function will lead to an increase in amino acids, glucose, phosphate, and protein in the urine. The critical concentration of cadmium in the renal cortex below which no adverse effect would be anticipated has been estimated a t about 200 pg/gram fresh weight (Foulkes, 1990;EPA, 1994a). However, the apparent critical level of cadmium in the kidney can vary substantially under different conditions of exposure; it can be affected by age, sex, and other factors influencing individual sensitivity to cadmium toxicity.…”