Heavy metals
Metals with a relatively high density or a high relative atomic mass are called heavy metals. Metals from used and discarded devices and vehicles reach the environment, and some heavy metals occur naturally in the soil of particular areas.
Figure to be added
| Heavy metal | How it reaches the environment |
|---|---|
| Cadmium (Cd) | Industrial waste; waste from zinc plating; electroplating; making yellow-orange pigments |
| Arsenic (As) | Overuse of agrochemicals |
| Lead (Pb) | Burning leaded petrol |
| Mercury (Hg) | Burning coal; broken thermometers and pressure gauges; ship paints; industrial waste |
| Chromium (Cr) | Use as a pigment in paints, cement, paper and rubber |
Copper (Cu), manganese (Mn) and zinc (Zn) are also listed among the heavy metals.
Assignment 15.7
List the things and goods used at home. Note the heavy metals they contain and the harm those do to people and the environment.
For example: batteries (cadmium, lead, mercury) — harm the kidneys and nerves if they leak into water; thermometers and fluorescent lamps (mercury) — poisonous vapour when broken; old paints (lead, chromium) — harmful to children's brains.
Why heavy metals are so harmful
Heavy metals are elements: they cannot be broken down by bacteria or by the body, so they never go away. Taken in with water or food, the body cannot get rid of them easily, and they build up year after year in the kidneys, liver, bones and nerves. Small amounts at a time can therefore add up to serious damage. Cadmium, lead and arsenic are among the substances suspected in Sri Lanka's chronic kidney disease, discussed later in the chapter.
Heavy metals build up
Mercury, arsenic, chromium, cadmium, lead, copper, manganese and zinc come from industry, agrochemicals, fuels and discarded devices, and accumulate in the body.