The rocks of Earth's crust are rich in minerals and metals. As mentioned in newspapers, some vegetables contained excessive amounts of ‘heavy metals’. Do ‘heavy metals’ mean that these metals are heavy?
Heavy metals generally refer to the metals with a density greater than 5g/cm3. Although arsenic (As) is not a metal, it is categorised as a heavy metal because it has obvious biotoxicity and similar characteristics as heavy metals. Common heavy metals include cadmium (Cd), mercury (Hg), lead (Pb), arsenic (As), copper (Cu), zinc (Zn), chromium (Cr), nickel (Ni) and so on.
During the natural process of soil formation, the content of heavy metals is different along of the changes of terrain and climate. Additionally, some natural phenomena, like volcanic eruptions and mountain fires, increase heavy metal pollutions to our environment. Moreover, when the groundwater containing heavy metals is used for irrigation for a long time, heavy metals accumulate in the farmland with large quantity, causing the crops to comprise heavy metals.
In the past, with the rapid of industrial development, manufacturing factories, like electroplating factory and chemical plant, caused heavy pollution since they illegally discharged the untreated wastewater directly into rivers and streams. The exhaust gas emissions contaminated the soil with heavy metals. Likewise, massive quantities of industrial and domestic wastes were dumped and buried into the landfill together. Under the normal conditions, soil can reduce the bioavailability of heavy metals through sedimentation, adsorption and redox reaction; so, it can decrease the crops’ uptake of these metals from soil. Nevertheless, when the total concentration of heavy metals in soil is too high, especially of man-made pollution, the crops and environments are contaminated significantly.
The presence of heavy metals in our living environment is inevitable; still, a small amount of them can be found in uncontaminated soil. Even if the soil contains a particularly high concentration of heavy metals by its special geological features, the quantity of heavy metals existing in the soil is not equal to the quantity of bioabsorption. Furthermore, some heavy metals, like cooper (Cu) and zinc (Zn), are the micronutrients for plants. When the crops absorb to a level which is sufficient to harm human health, the plants have suffered from obvious toxic symptoms, such as yellowing, withering and decreasing the yield. It is easily noticeable. As for arsenic (As), chromium (Cr), nickel (Ni), mercury (Ag) and lead (Pb), they exist in soil and are not easily absorbed by the roots of plants. Even if the crops absorb them, it is unlikely that they are transferred to edible parts, such as stems, leaves and fruits. Only cadmium (Cd) is prone to be absorbed. Even accumulating with high concentration of cadmium, there is no symptom in the appearance of the plants.
How can we ensure to buy the safe vegetables in Macao?
All fruits and vegetables imported to Macao are subject to inspect and quarantine by the Municipal Affairs Bureau (IAM). Vegetables imported from Mainland China come from the registered vegetable farms for supplying to Hong Kong and Macao. The relevant authorities implement strict supervision of the cultivation, production and processing procedures. Before the vegetables enter the retail market, the IAM performs pesticides and other tests, like heavy metals, to safeguard public safety. Therefore, citizens do not need to be unduly worried about the vegetable safety as long as they purchase fruits and vegetables from reliable retailers. Regarding to the unknown sources of fruits and vegetables, we must have a correct attitude to buy a right food, and reject any suspected food for ensuring food safety. Finally, vegetables are a crucial part of healthy diet. It is good to maintain a balanced diet and eat a wide variety of vegetables and foods.
004/DIR/DSA/2018
