Sometimes the smallest of things can impact human health beyond all proportion to their size. Trace elements are an example of this. Trace elements in an amount that is either above or below certain thresholds in soil, animal feed, or the human diet can have important impacts on the health of both livestock and people.
Everything around us—solid, liquid and gas—is made up of some combination of the one hundred or so elements that are part of creation. Some elements, like carbon, oxygen, and hydrogen, are rather abundant. Others, such as zinc, copper, or mercury, are much less abundant and are called trace elements, because they are found in trace amounts. We are all exposed to trace elements through our food, water and air, and they can significantly impact our health if exposure levels are too low or too high.
All living things require certain elements in order to function. An element that is required by a living thing is labeled an essential element. An element is considered essential when it is part of at least one molecule among the thousands that are part of an organism’s regular metabolism. Too little of an essential element can affect the health of living things, and can even result in death. Elements such as hydrogen, carbon, oxygen, nitrogen, potassium and phosphorus are required in large amounts. Other elements, called essential trace elements, are required only in very small amounts (on the order of micrograms (one millionth of a gram) or milligrams (one thousandth of a gram) per day). Examples include iron, zinc, and selenium. Another class of elements is called nonessential trace elements. These are elements that are neither present in large amounts, nor required by living things; lead, mercury and cadmium are examples. Some of these are at times consumed by humans and other animals and can be toxic in small amounts. Finally, some elements are difficult to classify; they are not considered essential, but organisms do seem to benefit from the presence of these elements in their diet. For example, selenium is required by mammals but not by plants, yet there are reports that plants benefit from some selenium.
Sources of Trace Elements
Trace elements in the environment can occur naturally or can arise from human activities. Trace elements are naturally found in rock, and are released into soil as the rock breaks down into soil. The amounts of specific elements vary from place to place; in some places, trace elements are present in higher concentrations, while in others they are truly rare. Volcanic eruptions, windblown dust or smoke from natural fires can move these elements around in the air, after which they are deposited in soil or water. There are a number of human sources. Trace elements are sometimes added to fertilizers as micronutrients (e.g. zinc, copper, and/or iron), and other times are found as a contaminant in fertilizers (e.g. cadmium). They are frequently a contaminant in coal, oil, or gas, and are added to the atmosphere when these are burned. Industrial processes, such as mining or smelting, frequently release trace elements into water (as a result of leaching) or air. For many years, the combustion of leaded gasoline resulted in the release of significant amounts of lead into air and soil near roadways. While leaded fuel is now banned in most countries, it was still in use in over 60 countries as recently as 2005 (Taylor and Gethin-Damon, 2012). Finally, certain waste products can contain trace elements. Batteries can contain lead or cadmium; solar panels can contain cadmium; some household paints contain lead; and fluorescent light bulbs contain mercury. Disposal of these products can result in their release to soil, air, or water.
Once in the environment, trace elements begin to cycle. In water, trace elements can be accumulated by aquatic plants, small animals, and ultimately, fish. Elements in soil can be accumulated by plants to different degrees, which may in turn become part of the human diet, or be consumed by grazing animals (which also may be eaten by humans). Fertilizing with manure or compost can recycle trace elements back into soil.
Trace Elements and Human Health
Depending on the dose, exposure to non-essential trace elements can either cause harm or have no effect. Essential trace elements are of concern because harm can be caused to humans and other living things if they are exposed to too little or too much of these in their diet. A number of diseases have been attributed to deficiencies of specific trace elements. For example, an iron deficiency can result in anemia, and an iodine deficiency can cause goiter.
Iron (Fe) is probably the best known essential trace element. It has many roles in the human body. One role is to deliver oxygen to all cells of the body. In that role, Fe is a part of hemoglobin, a molecule found in red blood cells. A lack of dietary iron results in insufficient production of hemoglobin, which in turn causes anemia, with symptoms of pallor, fatigue, and weakness. About half of all cases of anemia are caused by lack of iron.
While any trace element has the potential to cause harm if too much is ingested, several are of particular interest in specific regions of the world; the rest of the article will consider a number of these case studies.
Arsenic (As) is a non-essential trace element that causes significant problems in Bangladesh and parts of Southeast Asia. Unfortunately, soils, rock and groundwater in these regions naturally have quite high amounts of As present in the environment. In parts of Bangladesh, drinking water is high in As because it is collected from shallow tube wells (these are drilled to avoid water contaminated with bacteria).
The World Health Organization (WHO) has set a standard of 10 ppb for drinking water (maximum uptake of ~ 130 μg/day for an adult). Over 50 million people are estimated to be drinking water that has As levels in excess of these standards. Aside from drinking water, a second route of exposure is consumption of plants that have accumulated As from soil or from contaminated irrigation water. Consuming too much As can lead to acute poisoning. Chronic effects include skin, lung, and bladder cancer.
Dealing with an environment naturally high in As can be challenging. It can help to drill deeper wells, to avoid shallow, As-contaminated water. Also, while it is difficult to remove As from water, specialized filters containing iron hydroxide (rusted iron) can absorb dissolved As. An example of a simple sand and iron-hydroxide-based filter is accessible at www.who.int/household_ water/resources/Bipin.pdf. Alternatively, water can be distilled to purify it. Finally, different plant species, and even different varieties within a species, vary in the degree to which they accumulate different elements. For a number of species, it may be known which varieties can be grown to limit As accumulation in food crops .
Like As, mercury (Hg) is a non-essential trace element. It is more toxic than As; the maximum recommended intake is ~ 30 μg/day for adults. It is especially toxic to children, because it affects the brain and nervous system—especially when those organs are still developing. It accumulates in fat tissue and can be passed along to children through breast milk. There is little Hg found in plant tissue; because it collects in fat tissue, it is more likely found in fish – especially older (larger) fish and top predators.
In parts of South America and in Africa near Lake Victoria, small-scale gold mining and refining is a major cause of human exposure and environmental release of Hg. Gold ore can be purified by mixing it with Hg. The gold dissolves in the liquid Hg, which is then separated from the remaining ore. The Hg is then burned off into the atmosphere, leaving a pure gold nugget behind. As a result of these activities, small-scale miners are at risk of Hg poisoning and the local environment can become severely contaminated with Hg. A simple retort can be constructed to (safely) recover much of the Hg during this process, reducing human and environmental exposure. Following is a link to the first of four short YouTube videos that show how to make and use a simple retort: www.youtube.com/watch?v=KL9deyMSzs8 . In addition to preventing environmental Hg contamination, the Hg is recovered and can be reused, actually saving money since less Hg will need to be purchased by small-scale gold refiners.
Selenium (Se) is a unique element because it is not essential for plants, but is essential for animals. The recommended daily intake is 20 μg per day for children and 50-70 μg per day for adults. While Se is needed for survival and good health, it can become toxic if too much is ingested (45 μg per day for children and 400 μg per day for adults). The amount ingested depends on what plants are consumed (plants vary in the amount of Se they accumulate) and where they are grown, since the amount of Se in soils varies from one region to another. The east coast of North America (including soils in the part of Florida where ECHO is located), northern Europe and large parts of China and sub-Saharan Africa are known to have soils low in selenium (amounts below 0.5 mg/kg). Plants grown on these soils will also tend to be low in Se. Selenium is important for the body’s immune response. A deficiency of Se has been linked to an increased speed of progression of HIV/ AIDS, and to higher death rates from this disease. In livestock, low Se can result in white muscle disease; animals with this disease may be stiff or in pain when walking, or even be unable to walk.
Problems resulting from Se-deficient soils can be prevented in a few different ways. Sometimes Se is added to chemical fertilizers in order to boost its concentration in plants. In areas with low Se soil, livestock feed is sometimes supplemented with Se, and livestock are occasionally given injections of Se to avoid deficiency. Recently, a goat born on ECHO’s farm had trouble standing up. It was given an injection of supplemental Se in case Se was deficient, and the goat recovered soon after.
Soils of the Canadian and American Prairies, and parts of the Amazon and China, are high in Se. Plants of the Brassicaceae family (cabbage, kale, etc) accumulate much more Se than many other types of plants. Brazil nuts also tend to be high in Se. When certain plants (e.g. the leguminous Astragalusbisulcatus) with an ability to accumulate high amounts of Se are consumed by livestock, it can result in a disease called the ‘blind staggers.’ In humans, Se toxicity can result in selenosis; symptoms include hair loss, garlicky breath, and birth defects.
Trace elements can and do impact human health, either because levels are too high, resulting in toxicity, or because levels (or consumption) of essential elements are too low to maintain health. Issues involving trace elements are often site specific (e.g. low selenium or high mercury in parts of Africa, or high arsenic in parts of Bangladesh). Methods for reducing negative effects are also site-specific.
All links were accessed in January, 2013.
Martel, Owen. 2009. How to Make and Use a Simple Retort Tube. www.youtube.com/watch?v=KL9deyMSzs8
Murcott, Susan, Tommy Ka Kit Ngai, Roshan Raj Shrestha, and Bipin Dango. 2005. Kanchan™ Arsenic Filter (KAF) –Research and Implementation of an Appropriate Drinking Water Solution for Rural Nepal. www.who.int/household_water/resources/Bipin.pdf
Taylor, Robert, and Zac Gethin-Damon. 2012. Countries where Leaded Petrol is Possibly Still Sold for Road Use As at 17th June 2011. The Lead Education and Abatement Design Group. www.lead.org.au/fs/fst27.html
WHO. Arsenic Fact Sheet N°372. 2012. www.who.int/mediacentre/factsheets/fs372/en/
WHO. Chemical hazards in drinking-water: Selenium. Assessed from www.who.int/watersanitationhealth/dwq/chemicals/selenium/en/
WHO. Mercury and Health Fact Sheet N°361. 2012. www.who.int/mediacentre/factsheets/fs361/en/
Berkelaar, E. 2013. Trace Elements and Human Health. ECHO Development Notes no. 119