Heavy Metal Toxicities in Soils, Crops, and Humans: Some Control Measures.
Umesh C. Gupta, Agriculture & Agri-Food Canada, 440 University Ave., Charlottetown, PE C1A 4N6, Canada and Subhas C. Gupta, Division of Plastic Surgery, Loma Linda Univ School of Medicine, CP 21126, 11175 Campus Street, Loma Linda, CA 92354.
Soil plays a vital role in sustaining human welfare and assuring future agricultural productivity and environmental stability. The environmental consequences of trace and toxic metals in soils are related to their bioavailability. Some soils are naturally enriched with minerals containing toxic metals due to the nature of their parent material. Dissolution of these minerals contributes to the heavy/toxic metal loading in the soil solution which may cause toxicities. For example, the selenium (Se)-rich soil in the San Joaquin valley in California and high arsenic (As) soils and ground water in Bangladesh are examples of pedogenic contamination. Likewise, mercury (Hg) emitted from natural or anthropogenic sources deposited on the soil or water will enter terrestrial and aquatic systems. Numerous factors are responsible for toxic soils which may produce unhealthy food for humans. In nature, trace element toxicities can occur in all living organisms. In some instances, the toxicities are a direct consequence of the organisms' position in the food chain and their environment. The major challenge is to develop accurate and sensitive methods to determine the bioavailability of toxic metals in soils. High levels of As are present in some agricultural soils because compounds containing As have been used extensively as pesticides in the past. Arsenic can be toxic to plants as well as to animals and humans. Some of the most serious As poisoning has been found in Bangladesh as a consequence of drinking water containing excessive amounts of As. In humans, As poisoning can cause lesions of the hands, feet and head. Cadmium (Cd) is not toxic to plants but it can accumulate in plants to levels which are toxic to humans and livestock.“Itai Itai,” a chronic Cd-poisoning disease in humans was found in the region located near the Kamioka mine in Japan caused by the high Cd content in the water. This disease is associated with excruciating pain, brittleness of bones and broken bones in more severe cases. Other Cd toxicity symptoms in humans include hypertension, cancer, and immune disorders. Coal-fired plants are probably the biggest source of Hg contamination. The atmosphere serves as a major medium of Hg transport and subsequent environmental loading. Mercury could be added to soils in toxic quantities through the application of sewage sludge, fertilizers, manures, and through organic mercurials as seed coat dressing. Toxicity symptoms in humans include altered sensation in the extremities, ataxia, and loss of vision from neuronal damage. Lead (Pb) occurs naturally in the environment. The common sources of Pb include pesticides, soil, dust, batteries, automobile exhausts and burning of coal. Toxicity symptoms in humans include arrested growth of bones. A wrist drop with neuropathy and growth arrests are also noted in humans. Soils bearing vegetation containing higher than 5 mg Se kg -1 are designated as toxic and are associated with livestock poisoning. Hair loss and nail deformities are the most common indicators of Se toxicity in humans. Plants are highly tolerant to zinc (Zn). Zinc toxicity In humans is rare but it can occur from its ingestion in large amounts, as from an acid drink or food in galvanized containers, or by inhalation of Zn oxide fumes. Symptoms of toxicity include vomiting, diarrhea and neurological damage. Measures to treat trace elements and metals' toxicity in humans include gastric lavage, hemodynamic support and chelating antidotes. Cadmium, Pb and Zn toxicity in soils and crops can be controlled to a certain extent by liming soils, but has generally not been found to reduce As and Hg toxicity to crops. Cutting smog-forming nitrogen oxides and sulfur oxides can substantially reduce Hg and other toxin pollution from coal-fired plants. For As, Hg and Se, plants genetically engineered to be hyper-accumulators of these elements can be used to some advantage in reducing toxicity in soils. Concerted efforts should be taken by soil scientists, agronomists and human and animal clinicians for joint efforts in studies to control heavy metal toxicities.