An Introduction to Environmental Chemistry

The Chemistry of Continental Waters 171

Box 5.5 Essential and non-essential elements

Many substances that might be considered

toxic are in fact essential to life, for example

the heavy metals Co, Cu, Fe, Se, Zn. Thus,

there is a relationship between concentration

of the substance and the health response it

invokes. An element is essential to life when

a deficient intake of the element consistently

results in impairment of a life function from

optimal to suboptimal (Fig. 1a). Moreover,

when physiological levels of this element, but

not of others, are supplemented or restored,

the impairment is cured and optimal health is

restored (Fig. 1a). By contrast, non-essential

elements do not produce a positive health

response. An organism may tolerate low

concentrations of some non-essential

elements (Fig. 1b). Even dangerous poisons

such as arsenic oxide (As 2 O 3 ) can be tolerated

to some extent if the amount taken (dose) is

increased in small degrees. The so-called

‘arsenic eaters’ of Styria, a mountainous

Austrian state, are said to have taken six

times the minimum fatal dose of arsenic

oxide without ill effect. These people

maintained that their curious diet

supplement improved their personal

appearance and increased their energy for

ascents of their Alpine homeland! Of course

if concentration exceeds the organism’s

tolerance threshold, impairment is observed

(Fig. 1b). In both essential and non-essential

elements, excessive concentrations will result

in toxicity and ultimately death (Fig. 1). The

16th century Swiss scientist Paracelsus

recognized this with his comment ‘Tis the

dose that maketh the poison’.

Death

Deficiency

Optimal health

(a) Essential

Health response

X

ToxicityDeath

(b) Non-essential

Tolerance

Health response

X

ToxicityDeath

Fig. 1Health–response curves for essential and non-essential elements (X).

mental concerns revolve around both the amounts of mercury used and its acci-

dental release into the environment. On average 1.3 kg of Hg^0 is used for each

kilogram of gold recovered, although much of this mercury is contained and

reused. However, many hundreds of tonnes of mercury have been released into

the Amazon Basin during the 1980s and 1990s. This practice is widespread not