What Are Heavy Metals?

The scientific world has no widely accepted definition for 'heavy metal'. However, it is generally accepted that the term 'heavy' refers to metals with a specific gravity that is at least 5 times the specific gravity of water. The specific gravity of water is 1 at 4°C (39°F). Simply stated, specific gravity is a measure of the density of a given amount of a solid substance when it is compared to an equal amount of water. In general terms, a 'heavy metal' has a specific weight higher that 8 grams per cubic centimetre (g/cm3).

We are particularly interested in the following toxic metallic elements having specific gravities greater than 8 or more times that of water: Cadmium, 8.65; Lead, 11.34; and Mercury, 13.546.

Beneficial Heavy Metals

In small quantities, certain heavy metals are nutritionally essential for a healthy life. Some of these are referred to as the trace elements (e.g., iron, copper, manganese, and zinc). These elements, or some form of them, are commonly found naturally in foodstuffs, in fruits and vegetables, and in commercially available multivitamin products (International Occupational Safety and Health Information Centre 1999). Diagnostic medical applications include direct injection of gallium during radiological procedures, dosing with chromium in parenteral nutrition mixtures, and the use of lead as a radiation shield around x-ray equipment (Roberts 1999). Heavy metals are also common in industrial applications such as in the manufacture of pesticides, batteries, alloys, electroplated metal parts, textile dyes, steel, and so forth. (International Occupational Safety and Health Information Centre 1999). Many of these products are in our homes and actually add to our quality of life when properly used.

Toxic Heavy Metals

Heavy metals become toxic when they are not metabolised by the body and accumulate in the soft tissues. Heavy metals may enter the human body through food, water, air, or absorption through the skin when they come in contact with humans in agriculture and in manufacturing, pharmaceutical, industrial, or residential settings. Industrial exposure accounts for a common route of exposure for adults. Ingestion is the most common route of exposure in children (Roberts 1999). Children may develop toxic levels from the normal hand-to-mouth activity of small children who come in contact with contaminated soil or by actually eating objects that are not food (dirt or paint chips) (Dupler 2001). (The same happens with domestic pets!)

Other common routes of exposure are during a radiological procedure, from inappropriate dosing or monitoring during intravenous (parenteral) nutrition, from a broken thermometer (Smith et al. 1997), or from a suicide or homicide attempt (Lupton et al. 1985). And from mercury-based dental amalgams.

Commonly Encountered Toxic Heavy Metals: (click on the links for more details information)

Cadmium

Lead

Mercury

Aluminium

Toxicity can result from any of these metals.

Although aluminium is not a heavy metal (specific gravity of 2.55-2.80), it makes up about 8% of the surface of the earth and is the third most abundant element and it is a toxic metal.

Exposure to toxic heavy metals is generally classified as acute, 14 days or less; intermediate, 15-354 days; and chronic, more than 365 days. Additionally, acute toxicity is usually from a sudden or unexpected exposure to a high level of the heavy metal (e.g., from careless handling, inadequate safety precautions, or an accidental spill or release of toxic material often in a laboratory, industrial, or transportation setting).

Chronic toxicity results from repeated or continuous exposure, leading to an accumulation of the toxic substance in the body. Chronic exposure may result from contaminated food, air, water, or dust; living near a hazardous waste site; spending time in areas with deteriorating lead paint; maternal transfer in the womb; or from participating in hobbies that use lead paint or solder. Chronic exposure may occur in either the home or workplace. Symptoms of chronic toxicity are often similar to many common conditions and may not be readily recognised. Routes of exposure include inhalation, skin or eye contact, and ingestion (WHO 1998; International Occupational Safety and Health Information Centre 1999; Roberts 1999; Dupler 2001; Ferner 2001).