Heavy Metal Contaminants in Food


  Heavy metals refer to metallic elements with a density greater than 5g/cm3 that exist in nature alone or forming compounds. In present, there are more than 40 kinds of identified heavy metals, in which the most common conclude mercury, copper, zinc, lead, etc.


Source of heavy metals

  Heavy metals are chemical substances distributed ubiquitously in the environment. They are found in agricultural chemicals, such as fertilisers and pesticides, which can leave residues in crops and natural environment if excessive use. Moreover, the pollution resulted from urbanization, industrialisation and other human activities have made ways for heavy metals to move into water sources directly or indirectly, then eventually enter the human body through the food chain.


Effects of heavy metals on human health

    In fact, the main hazard of heavy metals caused by food is usually chronic poisoning. As heavy metals enter human body via dietary intake of foodstuffs, these substances are absorbed, utilised and accumulated in the body. Long-term intake of excessive heavy metals damages the organs and their functions.


Table 1Potential Adverse Impacts of Different Heavy Metals on Health

Name of Metal

Element Symbol

Negative Health Impacts



Arsenic includes organic arsenic, trivalent inorganic arsenic and pentavalent inorganic arsenic. Excessive intake of arsenic can result in cardiovascular diseases and hypertension.


The toxicity of inorganic arsenic has the greatest impact on the human body. It can cause cancers for long-term consumption.



Excessive intake of cadmium can result in kidneys failure, bone softening, even bone fractures in severe cases.



The amount of lead intake determines the level of damage inflicted on the organs. It is particularly obvious hazards to the neurological, reproductive and immune systems.



Long-term exposure can cause numbness and impaired sensory perception, damage neurological system, and affect the brain and intellectual developments of foetuses and infants.



Tin induces gastrointestinal discomfort.


International concerns over hazards of heavy metal contamination

        The World Health Organization (WHO) conducted risk assessments of health-affecting chemicals, based on which it announced that 10 chemicals are of major public health concern. Four of these hazardous chemicals are heavy metals, namely arsenic (As), cadmium (Cd), lead (Pb) and mercury (Hg), which are more likely to flow from the natural environment into the food chain then affects a wide range of foods. Long-term eating of these contaminated foods is harmful to health.


In formulating the regulation of ‘Maximum Limits of Heavy Metal Contaminants in Food’ in Macao, besides of the 4 kinds of aforementioned heavy metals, tin (Sn) is also regulated. Due to its potential migration from tin-plated cans into food, it is hazardous to health for long-term consumption. The maximum limits of tin in canned beverages and canned foods for infants and young children are particularly covered within the scope of standard setting.


How can consumers reduce the risk of ingesting heavy metals?

  Although heavy metal contaminants in food are hardly noticeable, consumers do not need to worry too much. We can follow the dietary suggestions below to avoid and reduce intake of heavy metals through food consumption.

  • The farms in China that are dedicated to supply food to Macao are under strict supervision, inspection and quarantine by China government. They are located far away from pollution sources, like factories and sewage treatment plants, to reduce the risk of heavy metal contamination in foods. Thus, consumers should buy food from reputable shops in Macao and the food traders should not sell any food products of unknown origin.
  • Since most of the heavy metals adhere to the soil and outer surface of food, it is necessary to thoroughly wash food products with clear water before preparation, especially of seafood, such as shrimp and crab.
  • Avoid eating the head and viscera of seafood because the majority of heavy metals and natural toxins accumulate in these parts.
  • Pregnant women and children should eat an appropriate amount of fish and other seafood for their body to absorb beneficial nutrients. Nevertheless, large deep-sea fish should be avoided in order to reduce the risk of seafood poisoning, in which may contain highly toxic methylmercury.
  • Maintain a varied and balanced diet to prevent health hazards arising from excessive intake of any single type of contaminant.