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Introduction
Soils are important for human health in a number of ways. Approximately 78% of the average per capita calorie consumption worldwide comes from crops grown directly in soil, and another nearly 20% comes from terrestrial food sources that rely indirectly on soil (Brevik 2013a). Soils are also a major source of nutrients, and they act as natural filters to remove contaminants from water. However, soils may contain heavy metals, chemicals, or pathogens that have the potential to negatively impact human health. This article will summarize some of the more important and direct relationships between soils and human health.
Quality Food Production and Food Security
Quality food production and food security have several components, including the production of sufficient amounts of food, adequate nutrient content in the food products, and the exclusion of potentially toxic compounds from the food products (Hubert et al. 2010). Soils play a major role in all of these areas of quality food production and security.
Influence of Soils on Crop Yield and Food Security
Food security is achieved when all people have access to sufficient, safe, and nutritious food (Food and Agriculture Organization of the United Nations, 2003). Food security is central to human health (Brevik 2009a; Carvalho 2006), and the ability to produce nutritious crops in sufficient amounts depends on soil properties and conditions. In particular, soils that have well-developed structure, sufficient organic matter, and other physical and chemical properties conducive to promoting crop growth lead to strong yields and are thus important for food security (Reicosky et al. 2011; Brevik 2009b). Soil degradation, which includes soil erosion and loss of soil structure and nutrient content, decreases crop production and threatens food security (Brevik 2013b; Pimentel & Burgess 2013; Lal 2009) (Figure 1). Soils that contain substances such as heavy metals, which may be toxic to humans, can pass those substances on to humans through crop uptake, leading to unsafe foods that compromise food security (Hubert et al. 2010; Brevik 2009a).
Figure 1: Soil degradation along the top of the hill has left the soils unable to support strong plant growth. Soil degradation over large areas may threaten food security.
Photo by Gene Alexander, USDA NRCS
Human Nutrient Supply from Soils
A mere 11 elements constitute 99.9% of the atoms in the human body. These are typically divided into major and minor elements. The four major elements, H, O, C, and N, make up approximately 99% of the human body, and seven minor elements, Na, K, Ca, Mg, P, S, and Cl, make up another 0.9% of the body (Combs 2005). Approximately 18 additional elements — called trace elements — are considered essential in small amounts to maintain human life. However, human health experts do not universally agree on the exact number and identity of these trace elements. Out of the approximately 29 elements considered essential for human life, 18 are either essential or beneficial to plants and are obtained from soil, and most of the other elements can be taken up from the soil by plants (Brevik 2013a). The exceptions include H, O, and C, which plants obtain from air and water (Kirkby 2012). Therefore, soils that provide a healthy, nutrient-rich growth medium for plants will result in plant tissues that contain most of the elements required for human life when the plants are consumed (Combs 2005; Committee on Minerals and Toxic Substances in Diets and Water for Animals, National Research Council, 2005).
Negative Health Effects
Heavy Metals
Exposure to heavy metals through soil contact is a major human health concern. Arsenic is a metalloid, but it is commonly grouped with the heavy metals. The heavy metals of greatest concern for human health include: As, Pb, Cd, Cr, Cu, Hg, Ni, and Zn (Fergusson 1990). Heavy metals enter soils naturally through the weathering of rocks, but they have also been introduced into soils through human activity. Heavy metals are the by-products of mining ores, and they are present in mine spoils and in the immediate surroundings of metal processing plants. Heavy metals are released into soils from landfills that contain industrial and household wastes and from sewage sludge that comes from wastewater treatment plants. E-wastes, or wastes associated with electronic appliances, are an increasing source of Pb, Sb, Hg, Cd, and Ni in the soil (Robinson 2009). Urban soils are particularly susceptible to significant accumulations of heavy metals from automobile exhaust, coal burning, erosion of metal structures, and refuse incineration. In agricultural settings, the use of fertilizers, manures, and pesticides has also contributed to the accumulation of heavy metals in soils (Senesi et al. 1999). Arsenic has been used in pesticides, and the build-up of arsenic in orchard soils is problematic since it may persist for decades (Walsh et al. 1977). The heavy metals with the most toxicity in humans, including Cd, Pb, Hg, and As, are those with no biological function that disrupt enzymatic activities commonly affecting the brain and kidneys (Hu 2002).
Organic Chemicals
Organic chemicals have been deposited into the soil both naturally and anthropogenically, and many of the organic chemicals deposited into the air and water eventually end up in the soil. Soil contamination with organic chemicals is a serious problem in all nations (Aelion 2009). A large amount of these organic chemicals come from the agricultural application of herbicides, insecticides, and nematicides (Figure 2). Soil pollution with organic chemicals is not limited to farming areas. Soils in urban areas are also polluted with organic chemicals as a result of industrial activities, coal burning, motor vehicle emissions, waste incineration, and sewage and solid waste dumping (Leake et al. 2009). Both farming and urban areas have soil contamination that includes a complex mixture of organic chemicals, metals, and microorganisms caused by municipal and domestic septic system waste, farm animal waste, and other biowastes (Pettry et al. 1973). A more recent health concern includes pharmaceutical waste derived from antibiotics, hormones, and antiparasitic drugs used to treat humans and domestic animals (Albihn 2001).
Figure 2: The application of agro-chemicals to crops is one source of organic chemicals in the soil system.
Photo by Jeff Vanuga, USDA NRCS
The most common types of organic chemicals found in soil include polyhalogenated biphenyls, aromatic hydrocarbons, insecticides, herbicides, fossil fuels, and the by-products of fossil fuel combustion (Burgess 2013). These organic chemicals are highly diluted in the upper layers of the soil, and they form chemical mixtures used in reactions involving microorganisms. We have very little toxicological information about the health effects of these chemical mixtures (Carpenter et al. 2002). Studies of the health effects of low concentrations and mixtures of these chemicals in soil have been very limited (Feron et al. 2002). Due to the very long half-lives of many organic chemicals, they are referred to as "persistent organic pollutants." These persistent organic pollutants are organic chemicals that resist decomposition in the environment and bioaccumulate as they move up the food chain. An example of this is 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), which was shown to disrupt the hormonal systems of raptors (Vega et al. 2007).
Airborne Dust
Airborne dust can impact human health, especially when the particles are less than 10 microns in size (Monteil 2008). The main direct health effect of inhaled dust is irritation of the respiratory passages and diseases, such as lung cancer. However, airborne dust can carry additional materials, such as pathogens, harmful gases, organic chemicals, heavy metals, insects, pollen, and radioactive materials, that can cause other health problems (Bartos et al. 2009). Humans can breathe airborne dust containing toxicants into the lungs, where the toxicants may enter the bloodstream. Cultivation for agricultural production and deflation (wind erosion) from unpaved road and work sites and denuded fields can introduce dusts into the atmosphere. Airborne dust from Africa is a significant health concern for North American soils. Clouds of dust from the Sahara and Sahel deserts follow the trade winds across the Atlantic Ocean, and African dust has been linked to elevated levels of Hg, Se, and Pb in North American soils (Garrison et al. 2003). The number of asthma cases in the United States more than doubled between 1980 and 2000, and asthma rates have also increased in the Caribbean (Brevik 2013a). Airborne dust from Africa has been tentatively linked to increased asthma in North America (Monteil 2008).
Soil Pathogens
Figure 3:
A ringworm infection is evident on a woman's torso. This infection is caused by the fungus Trichophyton rubrum, found in soils.
Photo by Lucille K. Georg/CDC
Water Quality and Soil
A major way that soils influence human health is through their ability to remove contaminants from water. Contaminants are removed by soil through physical capture as the water moves through pore spaces, through chemical sorption to solid surfaces, and through biodegradation carried out by microorganisms living in the soil (Helmke & Losco 2013).
Improper sewage sanitation is a problem for approximately 40% of the world's population, and millions of people die each year from water-borne diseases (Massoud et al. 2009). Humans can take advantage of the purifying abilities of soil to address wastewater issues. Well-designed, properly maintained, and functioning on-site sewage treatment systems are highly effective at reducing the risk of water-borne diseases in areas with low population densities (Massoud et al. 2009). Twenty-five percent of the households in the United States use on-site sewage disposal systems, such as septic systems, to deal with their wastewater (Katz et al. 2011). The use of soils to address groundwater contamination issues has been one of the fastest growing areas in soil science research in recent years (Brevik & Hartemink 2010).
References and Recommended Reading
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