Heavy metals in vegetables sold in some cities in the Visayas, Philippines

Every time we buy vegetables in the market, we do not doubt the quality of these farm products. We think they are clean, safe, nutritious and good for our health.

But the worsening environmental pollution due to the overuse and misuse of agricultural chemicals such as pesticides, the improper waste disposal, the manufacturing industry, and the transportation system may be affecting the quality of the food crops we eat everyday. Specifically, heavy metals most of which are toxic to humans at elevated concentrations, are starting to contaminate the vegetables we love to eat.

The scientific principle is simple: a contaminated soil will generally produce contaminated crops.

An interesting and very relevant student research conducted a few years ago revealed such alarming reality. Conducted to determine and compare the Pb, Cu and Zn contents of Alugbati (Basella rubra), Ampalaya (Momordica charantia), Kalabasa (Cucurbita maxima), Kangkong (Ipomoea aquatica), Pechay (Brassica rapa), and Talong (Solanum melongena) sold in markets in the cities of Baybay, Ormoc, and Tacloban (Leyte, Philippines), the study revealed that Ampalaya from Tacloban and Baybay contained excessive levels of Cu and may pose health problems to consumers. 

Likewise, Pechay from Baybay, Ormoc and Tacloban exceeded the safe level for Zn. All vegetable samples collected from the three cities were not contaminated with Pb. Cu and Zn levels varied with crop (vegetable) species and origin (production area). 

The results are very relevant in that they support and confirm the fear among consumers that some food crops sold in the local markets are not safe and may be one of the reasons for the various health problems experienced by many people.

The study was conducted in 2012 by Anna Luisa Ventulan, Christine Gay Cala, and Johannes Reiner Asio, all senior students at VSU Laboratory High School. The research adviser was Luz Geneston Asio of the Central Analytical Services Laboratory, Visayas State University, Baybay City, Leyte.

The arsenic contamination in rice

Arsenic (As) is a heavy metal that is well-known as a poison and a carcinogen. Its average concentration in the soil ranges from 5 to 6 mg/kg which is generally related to rock type and industrial activity.

Arsenic contamination of paddy soils is widespread and elevated arsenic levels in rice grains is now a hot issue in many parts of the world. Martha Rose Shulman wrote in the New York Times (15 April 2013) that “it is clear that the levels of inorganic arsenic in rice and rice products are high, and that we and especially children, babies and pregnant women should limit our intake of rice and rice products.”

According to Bogdan and Schenk (2012) in their recent study published in the highly respected Journal of Plant Nutrition and Soil Science (Wiley-VCH Verlag GmbH), flooded rice may contain high arsenic concentration compared to other grain crops. In fact, aside from arsenic-contaminated drinking water, rice is the largest food dietary source of inorganic arsenic. This is because the reducing environment in flooded rice fields causes the dissolution of arsenic and thus increases its availability to the rice plant. Meharg (2004) added that under paddy field conditions, inorganic arsenic introduced into the soil is inter-converted between the reduced inorganic species arsenite (the dominant type) and the oxidized species arsenate. Moreover, arsenite is taken up into the root by the highly efficient Si pathway and arsenate can be taken up via the phosphate transport system.

Bogdan and Schenk (2012) observed among other things that continuous arsenic supply in the soil resulted in a doubling of arsenic concentration in rice shoot and grains. They also found that arsenic was mobilized from the root and shoot to the rice grains where it accumulated.

References
Bogdan K. and M.K. Schenk. 2012. Arsenic mobilization in rice (Oryza sativa) and its accumulation in the grains. J. Plant Nutr. Soil Sci. 175: 135-141.
Meharg A.A. 2004. Arsenic in rice-understanding a new disaster for South-East Asia. Trends in Plant Science 9: 415-417.

Soil pollution and human health

People living in areas with fertile soils are better nourished than those living in degraded soils due to the higher quantity and quality of food in the former than the latter. Likewise, people living in polluted environments are more exposed to the ill effects of pollutants. The paths of environmental contaminants leading to humans are the following (Logan, 2000):

a) Soilàcropàhuman

b) Soilàlivestockàhuman

c) Soilàcropàlivestockàhuman

d) Soilàsurface watersàfishàhuman

e) Soilàgroundwateràhuman

f) Soilàairàhuman

g) Soilàhuman

The pathways a to e are indirect links between soil and human health and are relatively well-known. The pathways f and g are direct links and are little known and understood.

Direct links between soils and human health is geophagy

Humans ingest soil either involuntarily or deliberately. For the involuntary ingestion, every person ingests at least small quantities of soil. This is because any soil adhering to the skin of fingers may be inadvertently taken in by hand-to-mouth activity. This is especially true for children who like to play outdoors and for people working outside buildings or in the fields. Soil is also an important constituent of household dust and many foods such as fruits, vegetables and tubers crops usually contain some soil particles especially in poor countries. It is estimated that an average adult ingests soil at a rate of 10 mg per day.

Geophagy is the deliberate ingestion of soil by humans and animals. It is practiced by different peoples in all continents but is most common in the tropics particularly in Africa. This phenomenon was already known in the ancient world but the first detailed scientific report about it was written by the great German naturalist and founder of geography Alexander von Humboldt during his expedition of 1799-1804 to South America. Von Humboldt observed that eating soil was practiced by the indigenous Ottomac people in the Orinoco in Venezuela. The reasons for geophagy are still being debated until now but are known to vary from place to place. These include: soil as famine food to appease the pangs of hunger, as medicine and therapeutic (recent research has shown that clay adsorbs and detoxifies toxins and has antimicrobial action), cravings and good taste especially for pregnant women, as source of mineral nutrients to correct deficiencies, and an abnormal appetite for non-food substances. But excessive soil intake can lead to death of an individual due to the toxic effects of some mineral elements like Fe. This is likely to happen if the soil is contaminated with pollutants. Ingesting soil can also cause ingestion of eggs of parasitic worms and other disease-causing organisms (Abrahams, 2002; Dominy et al., 2004).

Another direct link between soil and human health occurs through inhalation. People inhale soil dusts inside their houses and by just walking in the street. The amount of inhaled dusts under normal conditions is generally low and thus is not harmful. But very dusty environments can cause lung problems. Also inhalation of even small amounts of the fibrous dust of serpentine and amphibole minerals commercially called asbestos is dangerous in that it can cause diseases and even cancer.

References

Abrahams, P.W. 2002. Soils: their implications to human health. The Science of the Total Environment 291: 1-32.

Dominy N.J., E. Davoust, and M. Minekus. 2004. Adaptive function of soil consumption: an in vitro study modeling the human stomach and small intestine. Journal of Experimental Biology 207: 319-324.

Logan, T.J. 2000. Soils and environmental quality. In: Handbook of Soil Science (M.E. Sumner, ed.). CRC Press, Boca Raton, pp: G155-G169.