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Learn about the harmful effects of lead, mercury, cadmium, and arsenic, their sources, health impacts, and ways to mitigate exposure in this detailed guide by Dr. Sherin Bakhashab.
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Introduction • Heavy metals are toxic to human health • Most common heavy metals are lead(Pb), mercury(Hg), cadmium(Cd) and arsenic(As) • Indoor concentration of heavy metals is generally less than their outdoor concentration • They are mainly produced by industrial activities, and deposit slowly in the surrounding water and soil
Properties of heavy metals • They occur near the bottom of the periodic table • Have high densities • Toxic in nature • Nondegradable Note: Arsenic is not actually a metal but is a semimetal i.e. its properties are intermediate between those of metals and nonmetals.
Transport phenomenon • Water • Food • Air • Adsorption or absorption onto various materials Example: Over half of the heavy metal input into Great Lakes is due to deposition from air.
Toxicity of heavy metals • Mercury is highly toxic in vapor form but lead, cadmium and arsenic are more toxic in their cationic form • Toxicity arises from strong affinity of the heavy metal cations for sulfur • Medicinal treatment for heavy metal poisoning is done by chelation therapy by administering compounds known as chelates Example : British Anti-Lewisite(BAL), ethylene diamine tetra acetic acid(EDTA).
Toxicity of trace heavy metals NIOSH: National Institute for Occupational Safety and Health
Mercury • Most volatile of all metals • Highly toxic in vapor form • Liquid mercury itself is not highly toxic, and most of that ingested is excreted
Sources of Mercury • Elemental mercury is employed in many applications due to its unusual property of being a liquid that conducts electricity • Used in electrical switches, fluorescent light bulbs and mercury lamps • Emission of mercury vapor from large industrial operations • Unregulated burning of coal and fuel oil • Emissions from mercury containing products :batteries, thermometers, etc. • Mercury amalgams: dental fillings
Health effects • Skin burns • Irritation of nose and skin • Rashes • Excessive perspiration • Damage to the kidneys • Damage to vision • Minamata disease: sever neurological disease • Dysfunctions of the central nervous system • Loss of hearing and muscle coordination • Sever brain damage • Death
Lead • Has a very low melting point of 327 degrees C • Romans used it in water ducts and in cooking vessels • Analysis of ice-core samples from Greenland indicate that atmospheric lead concentration reached a peak in roman times
Sources of lead • Commonly used in the building industry for roofing and flashing and for soundproofing • Used in pipes • When combined with tin, it forms solder, used in electronics and in other applications to make connections between solid metals • Lead is also used in ammunition Note: Lead shots have been banned in United States, Canada, Netherlands, Norway and Denmark • Lead is used in batteries and sinkers in fishing Ammunition
Sources (contd.) • Used in paints Lead chromate is the yellow pigment used in paints usually applied to school buses. Lead is also used in corrosion-resistant paints and has a bright red color • Used in ceramics and dishware The leaching of lead from glazed ceramics used to prepare food is a major source of dietary lead, especially in Mexico • In the past, lead salts were used as coloring agents in various foods • Lead is used in some types of PVC mini-blinds
Health effects • At high levels, inorganic lead is a general metabolic poison • Lead poisoning effects the neurological and reproductive systems • Lead breaks the blood-brain barrier and interferes with the normal development of brain in infants
Health effects(contd.) • Lead is observed to lower IQ levels in children • Lead is transferred postnatally from the mother in her breast milk • At elevated levels, lead poisoning would eventually result in death
Facts about lead poisoning • The human groups most at risk of lead poisoning are fetuses and children under the age of seven • Episodes of lead poisoning were recorded through the middle ages and even until recent times • A recent study in Mexico indicated that pregnant women can decrease the lead levels in their blood and presumably in the blood of their developing fetus by taking calcium supplements.
Cadmium • Cadmium lies in the same subgroup of the periodic table as zinc and mercury, but is more similar to zinc • Coal burning is the main source of environmental cadmium • Burning of wastes containing cadmium is an important source of the metal in the environment • Cadmium is most toxic in its ionic form unlike mercury Note: Mercury is most toxic in vapor form and lead, cadmium and arsenic are most toxic in their ionic forms.
Sources of Cadmium • Cadmium is used as an electrode in “NiCad” batteries • Cadmium is used as a pigment in paints(yellow color) • It is also used in photovoltaic devices and in TV screens • Cigarette smoke • Fertilizers and pesticides Note: The greatest proportion of our exposure to cadmium comes from our food supply- seafood, organ meats, particularly kidneys, and also from potatoes, rice, and other grains.
Health effects • Severe pain in joints • Bone diseases • Kidney problems • Its lifetime in the body is several years • Areas of greatest risk are Japan and central Europe • In very high levels it poses serious health problems related to bones, liver and kidneys and can eventually cause death.
Arsenic • Arsenic oxides were the common poisons used for murder and suicide from roman times through to the middle ages • Arsenic compounds were used widely as pesticides before the organic chemicals era • Arsenic is very much similar to phosphorous
Sources of Arsenic • Pesticides • Mining, melting of gold, lead, copper and nickel • Production of iron and steel • Combustion of coal • Used as a wood preservative • Herbicides • Tobacco smoke • Wallpaper paste and pigments in wallpaper
Health effects • Birth defects • Carcinogen: Lung cancer results from the inhalation of arsenic and probably also from its ingestion. Skin and liver cancer, and perhaps cancers of the bladder and kidneys, arise from ingested arsenic • Gastrointestinal damage • Severe vomiting • Diarrhea • Death
Recent studies on arsenic exposure • Arsenic emitted from a copper-melting plant in Bulgaria has been shown recently to have produced a three-fold increase in birth defects in new born children in that area • Most daily exposure of arsenic by north American adults is due to food intake, especially of meat and seafood
Recent studies..(contd.) • Recent studies have shown that about 1% of Americans consume drinking water that has arsenic levels of 25 ppb or more, and in Utah and California water supplies have been found to contain as much as 500 ppb • Scientists have estimated that there is a one-in-a-thousand lifetime risk of dying from cancer induced by normal background levels of arsenic ( this equals the risk estimate due to tobacco smoke and radon exposure ).
General sources of heavy metals in residential houses • Infiltration from outside, along with the dust carried on shoes and clothes • Indoor sources include old-lead and latex based paints, domestic water supply, burning of wood, and tobacco smoke • Pesticides and fungicides are major sources of arsenic and mercury indoors
Methods for measurement of trace metals • Most common method of collecting particulate matter is through filters • Identification and concentration of individual trace metals like lead, cadmium, arsenic, mercury and chromium is determined by Atomic absorption spectrophotometer X-ray fluorescence
References • Krishnamurthy, S. 1992. Biomethylation and environmental transport of metals. Journal of Chemical Education 69(5) • Colin Baird.2000. Environmental Chemistry, W.H. Freeman and Company, New York.
