Dear Anil Shankhwar , it is an interesting question.
Heavy metal toxicity has proven to be a major threat and there are several health risks associated with it.
The toxic effects of these metals, even though they do not have any biological role, remain present in some or the other form harmful for the human body and its proper functioning.
They sometimes act as a pseudo element of the body while at certain times they may even interfere with metabolic processes.
Few metals, such as aluminium, can be removed through elimination activities, while some metals get accumulated in the body and food chain, exhibiting a chronic nature.
Various public health measures have been undertaken to control, prevent and treat metal toxicity occurring at various levels, such as occupational exposure, accidents and environmental factors.
Metal toxicity depends upon the absorbed dose, the route of exposure and duration of exposure, i.e. acute or chronic. This can lead to various disorders and can also result in excessive damage due to oxidative stress induced by free radical formation.
The answer to your question is not simple. Every single ‘heavy’ (or light, like aluminium) has its own mechanism of human toxicity. And metalloids like arsenic are usually also included in the term ‘heavy metals’.
Please look at my ResearchGate page for our publications on aluminium, lead, bismuth, arsenic, mercury, uranium, and some more. You will find there full texts of our review articles as well.
If there is anything else I can do for you, please do not hesitate to get in touch.
The toxicity of mercury has been known for centuries. Mad hatters in Victorian England got their names from the convulsions and loss of neuromuscular coordination symptomatic of chronic poisoning from the mercury used in the treatment of felt in hat manufacture. Inorganic mercury can be readily excreted and, while it is dangerous to those exposed to it occupationally. Organic mercury (methyl mercury) cannot be excreted easily and so may accumulate to toxic concentrations as a result of intermitted exposure over several years. It is a cumulative poison and since it can cross the barrier from blood vessels in the brain into the nervous tissue, it causes progressive and irreversible brain damage.
Human exposure to methyl mercury occurs only through the consumption of contaminated fish and seafood. This was brought to light by an outbreak of methyl mercury poisoning in the small Japanese coastal town of Minamata (Minamata disease). The only industry there was a factory that began producing vinyl chloride and acetaldehyde in 1952, both processes involving the use of mercury catalysts, large quantities of which were lost in washing the product and were discharged into the bay. The illness first appeared in 1953, and affected only fishermen and their family, but it was not diagnosed as metal poisoning derived from fish and seafood taken from Minamata Bay until 1956. In all 2000 cases were recognised; of these 43 died during the epidemic and over 700 of the survivors were left with severe permanent disabilities.
Cadmium and public health
Cadmium was claimed to be responsible for an outbreak of itai itai disease in a Japanese village on the Jintsu river. The painfull disease affected the bones and joints of old women, and resulted in a number of deaths. At the time, it was attributed to contamination of rice by cadmium in the effluent from a zinc smelter, but now it appears more likely to have been associated with malnutrition and vitamin deficiency.
In fact, there are no well-authenticated reports of cadmium contamination of sea food causing damage. High concentrations of cadmium (173 ppm) and zinc (57600 ppm) in oyster from the Derwent estuary in Tasmania caused nausea and vomiting in people who consumed them, but there were no further effects.
Copper in marine biota and public health
Bivalves growing in contaminated water have the capacity to accumulate copper. The concentration factor for oyster may be as high as 7500 and they may accumulate so much copper in their tissue systems that the flesh becomes green. This commonly happens to oysters in the River Fal and they have to be relaid in uncontaminated water for a year before they can be marketed.