Various types of toxic heavy metals, including lead, arsenic, cadmium, mercury etc. occur naturally in the environment – in the soil, in surface water, and in the atmosphere. These toxic elements can make their way into the food we eat through various routes. For example, heavy metal fumes released into the atmosphere can return by way of precipitation and contaminate the soil and surface water. The heavy metal residues can be directly taken-up by the cereals grown on the soil or washed into water bodies, where they are absorbed by fishes, thereby entering the human food chain. Thus, food is a major mode of entry of toxic metals into the human body, causing serious health problems over long-term exposure.
The metal contaminants of food that have been categorized under the FSSR are discussed below.
Lead can enter the human food chain as a result of human activity with the environment. For example, lead residues present in soil can contaminate vegetables. As for example, the recent Maggi incident, where the level of lead contamination was found to be higher than permissible limits, most probably arose from lead-contaminated soil where the main raw material i.e. wheat, is grown. Lead can also contaminate aquatic life such as fishes that are consumed as food.
- Health Hazards: Chronic exposure to lead through contaminated food affects the central nervous system (CNS) and the gastrointestinal (GI) system. Chronic exposure results in loss of concentration, confusion, depression, headaches etc. GI symptoms include stomach cramps, nausea and vomiting. It also causes toxicity to the developing fetus in utero, leading to premature and low birth weight babies.
Although copper is an essential element, it can be toxic to humans at high levels i.e. above the recommended permissible limits. Copper contamination of food can occur if the cooking utensils, including pots and pans are made of uncoated copper, especially if acidic foods are cooked. Copper can also enter the human food chain through the use of copper-containing fungicides.
- Health Hazards: Chronic exposure to copper can cause liver failure.
Arsenic can enter humans through consumption of shellfish, as well as through vegetables grown in fields where fertilizers had been applied. Arsenic poisoning arising from contaminated ground water, used for drinking purposes, is a huge problem in West Bengal, where over 2 lakh people are suffering and has been referred to as the “the biggest arsenic calamity in the world” (British Medical Journal, 1996; 313: 7048). Recently arsenic poisoning has also been reported from 30 districts in UP (Times of India, July 14, 2014).
- Health Hazards: Chronic exposure to arsenic through contaminated drinking water can cause dermatitis and can lead to various types of cancer, including cancer of the skin, bladder and lungs. It can also affect memory, reproductive function, and cause cardiovascular diseases.
The use of tin cans for packaging foods and beverages can lead to contamination with tin. Therefore, avoiding canned food would be a good idea to reduce the risk of tin poisoning. Tin can also enter food through the use of tin-containing organo-pesticides.
- Health Hazards: Studies in humans and animals have indicated that chronic exposure to tin and its compounds through food can cause GI effects, liver damage, reproductive effects, developmental effects, as well as cancer.
Zinc Although zinc occurs normally in small amounts in nature, it largely enters the environment as a result of human activities such as mining and smelting operations. Disposal of waste containing zinc can lead to seepage into groundwater, which can cause contamination of drinking water with zinc. Health Hazards: Prolong oral exposure to zinc causes decreased copper absorption, leading to copper deficiency. Copper-sensitive enzymes such as superoxide dismutase fail to function properly and anemia has also been reported.
Cadmium can enter the body through food crops via contaminated soil and water. Kidney of food animals such as goats and sheep can contain high levels of cadmium. This metal can also be found in molluscs and crustaceans. Cadmium can also be present in chocolate-based sweets and candies.
- Health Hazards: Chronic exposure to cadmium through food can cause hypertension, kidney damage, bone lesions (osteoporosis & osteomalacia), as well as birth defects.
Mercury can enter the body through food in the form of methyl mercury, which can accumulate in large sea-fish such as swordfish. Reducing intake of sea-fishes in the diet is an effective way of reducing exposure to this mercury compound.
- Health Hazards: Mercury has toxic effects on the GI system, CNS, immune system, respiratory system, renal system, as well as the eyes and skin.
Chromium can be generated by industrial processes and manufacturing activities. It can contaminate groundwater by poor storage, leakage and improper disposal practices, as a result of which the drinking water can become contaminated.
- Health Hazards: Chronic oral exposure to chromium can cause kidney damage, liver damage, GI effects, cardiovascular effects, as well as hematological changes. Animal studies have also indicated that chromium can cause developmental defects in utero.
Nickel is a naturally occurring metal that can enter the human food chain from environmental contamination. While short-term exposure can cause allergic reactions, long-term exposure can lead to reproductive failure and developmental problems, as evidenced from animal studies.
- Health Hazards: Oral exposure to nickel can cause GI effects such as stomach aches, hematological effects (increased red blood cells), as well as renal effects (proteinuria).
Maximum Permissible Limits
The standard limits of the above-mentioned metal contaminants have been defined under the existing regulations. Moreover, the recently proposed standard limits of metals in new food articles have also been shared by FSSAI for comments & suggestions.
It should be noted that with respect to the levels of chromium in gelatin, an amendment to the existing regulations has been made. The recent FSS (Contaminants, Toxins and Residues) (Amendment) Regulations 2015 proposed by FSSAI on June 5, 2015 has indicated that the maximum permissible limit of chromium in gelatin should not be more than 10 ppb.
Analytical Testing of Metal Contaminants in Food
All the above metal contaminants i.e. Lead, Copper, Arsenic, Tin, Zinc, Cadmium, Mercury, Chromium and Nickel in foods can be tested by Atomic Absorption Spectrometry (AAS). AAS is an analytical technique that measures the concentration of various elements. The technique is so sensitive that it can measure down to parts per billion (ppb) i.e. 1 part in 109 in a sample. The technique uses wavelengths of light that are specifically absorbed by an element. AAS has many applications in different areas of chemistry, one of which is for the testing of metal contaminants in food. A variation of AAS, called vapour generation AAS can detect even miniscule levels of mercury, including methyl mercury in fish. Another variation is the flameless-AAS.
An even more sensitive analytical method is the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), which measures down to parts per quadrillion (ppq) i.e. 1 part in 1015, and therefore, a million times more sensitive than AAS! ICP-MS can estimate elements having an atomic mass of 7 – 250 i.e. Lithium to Uranium, and sometimes even higher. Therefore, it is possible to measure all the above metals by ICP-MS also. Another method that can be used is Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). Besides these highly sensitive methods, conventional colorimetric methods that measure single elements are also available at the disposal of the analytical chemist.
Therefore, it is evident that since both standards and highly sensitive analytical methods are available, regular testing of raw food materials could prevent entry of toxic metals through food, thereby averting untoward health problems and helping us to lead more healthy and productive lives.
**Important – As per the latest FSSAI amendment, Zinc has been excluded from the list of metal contaminants as mentioned earlier under FSS (Contaminants, Toxins & Residues) Regulations, 2011.