Introduction

Clay is unique, especially from the perspective of medical geology, which is defined as the impacts of geologic materials and geologic processes on animal and human health (Selinus et al., Reference Selinus, Alloway, Centeno, Finkelman, Fuge, Lindh and Smedley2013). Gomes and Silva (Reference Gomes and Silva2007) provided an historic context of clay and medical geology. Their informative discussion includes an overview of some of the impacts that clays have on human health. Clay is the only natural material that can impact human health through all routes of exposure (Table 1). Moreover, these impacts can be harmful as well as beneficial. Even the absence of clay in the environment can result in health consequences. Most papers focus on either the impressive list of health benefits that can be attributed to clays (Carretero, Reference Carretero2002) or the health problems that can be caused by clays (Abrahams, Reference Abrahams, Selinus, Alloway, Centenoi, Finkelman, Fuge, Lindh and Smedley2013).

Table 1 Health impacts of clays by route of exposure

Note: Inhalation of clays may be used for beneficial drug delivery but for the purposes of this discussion this is not considered a natural process

Few papers (an exception being the broad discussion by Abrahams (Reference Abrahams2002) of the impact of soils on human health) provide an overview of both aspects of the impact of clay on animal and human health. The dual nature of clays can even be seen in some clay products. Porcelain, a ceramic material made by heating clay, was used to fashion false teeth (Fig. 1a). These teeth were healthier than the previous false teeth made from organic material such as wood, bone, horn, or even teeth taken from corpses (Staubach, Reference Staubach2005). They allowed the wearer to eat more healthily and have an improved self image. However, porcelain was also widely used to manufacture clay pipes (Fig. 1b), the use of these pipes has led to cancer of the mouth, lip, tongue, throat, larynx, and lung (Henley et al. Reference Henley, Thun, Chao and Calle2004).

Fig. 1 (a) A set of porcelain teeth made from clay are a benefit to health. (b) A porcelain pipe, the use of which may lead to various forms of cancer

This paper offers a brief overview of many of the positive and negative health impacts of clays that make clay one of the, if not the most, important natural materials impacting human health. The term clay is here considered in its broadest sense embracing hydrous aluminum silicates, sediments in the micrometer size range, and natural materials with plastic properties, and, where appropriate, references to clay-rich soils are included. The intention of this paper was not to provide a comprehensive, authoritative discussion of the many health impacts of clays, as many publications serve this purpose, some of which are cited herein. Rather, the goal of this paper was to provide a broad, though not exhaustive, overview of the subject.

The use of clay for medicinal purposes predates written records. Root-Bernstein and Root-Bernstein (Reference Root-Bernstein and Root-Bernstein1999) described a 1.5 to 2.1 million year old Homo habilis site in Zambia where these early humanoids apparently used kaolinite as a food supplement or as a detoxicant. Humans likely learned the beneficial effects of ingesting clays by observing various animal species ingesting clay-containing soils with absorbent properties, thus allowing the animals to eat potentially poisonous foods (Abrahms, 2013). The review starts by looking at the various routes of exposure by which clays can impact human health, looking first at the beneficial effects and then the deleterious effects.

Ingestion of Clay

Ingestion of clay, a form of geophagy, has been practiced for millennia and is still widely practiced today (Young, Reference Young2011; Henry & Cring, Reference Henry and Cring2013). Humanoids have been ingesting clay for at least two million years to ease indigestion and counteract poisons (Root-Bernstein & Root-Bernstein, Reference Root-Bernstein and Root-Bernstein1999). However, the possible benefits of eating clays, such as gaining nutrients, are far outweighed by the likely negative consequences such as tissue abrasion, intestinal blockage, anemia, exposure to pathogens and toxic trace elements, and potassium overdose (Abrahams, Reference Abrahams, Selinus, Alloway, Centenoi, Finkelman, Fuge, Lindh and Smedley2013). However, popular books such as The Clay Cure (Knishinsky, Reference Knishinsky1998) tout a wide range of ailments that can be addressed by eating various clays. Young (Young, Reference Young2011) takes a more balanced approach when discussing the urge to eat clay. She devoted thirteen pages to describing some of the health benefits of clays and nine pages to describing some of the health problems that can be caused by ingesting clays.

The practice of eating clay is widespread in Africa where, following childbirth or their monthly period, women seek out the ubiquitous termite mounds (Fig. 2), often consisting of red soil (Abrahams, Reference Abrahams, Selinus, Alloway, Centenoi, Finkelman, Fuge, Lindh and Smedley2013). The women are advised to eat the soil from near the top of the mound where it is richer in clay, the quartz and heavy minerals having rolled down the slopes. Clearly, the red soil is enriched in oxidized iron but also is rich in calcium and has abundant halloysite and meta-halloysite, which is similar to kaolinite, the principal ingredient in Kaopectate™ (Mahaney et al. Reference Mahaney, Zippin, Milner and Sanmugadas1999). These soils are also used by chimpanzees for geophagy. Studies indicate that, though calcium from the termite mound soils can be sorbed by the body, the ingested soil adsorbs iron and zinc from the body, potentially causing iron and zinc deficiency (Hooda et al. Reference Hooda, Henry, Seyoum, Armstrong and Fowler2002). However, if no benefits to eating this particular red soil occurred following childbirth or monthly periods, why this practice has been so widespread for millennia is difficult to understand.

Fig. 2 Termite mound. The red clay is commonly ingested by women after child birth and during monthly periods

Abrahams (Reference Abrahams, Selinus, Alloway, Centenoi, Finkelman, Fuge, Lindh and Smedley2013), in his comprehensive discussion on geophagy, described the possible health benefits of this practice. Soils, commonly with clays as the active constituents, can be used as a food supplement during times of famine and for detoxification. He states that soils may be the world’s oldest pharmaceutical with clay tablets (terra sigillata) being used in Europe for some 2000 years to treat a variety of issues including plague, stings, animal bites, ulcers, gout, dysentery, poisoning, etc. Obviously, some applications were more effective than others. Hunter (Reference Hunter1973) concluded that moderate ingestion of clays lacking large cation-exchange capacities could serve as a nutritional supplement for iron, copper, calcium, zinc, and manganese.

In their comprehensive review of clays used in health care and therapeutic products, Viseras et al. (Reference Viseras, Aguzzi, Cerezo and Lopez-Galindo2007) identified various clay minerals that are used in a range of commercial health care products. The clays may be the active ingredients or key inactive ingredients such as fillers, thickeners, gelling agents, etc. Many of these products are ingested to resolve intestinal problems such as indigestion and acid reflux. Aguzzi et al. (Reference Aguzzi, Cerezo, Viseras and Carmella2007) describes an important new health-related application of clay minerals: their use as drug delivery systems. The clays are used to delay or target drug release or to improve drug dissolution. Mbila (Reference Mbila2013) provided a useful discussion and a comprehensive list of medicines derived from soils.

Ingestion of clays can also cause a range of health problems. Abrahms (2013) reported that the adsorption properties of certain clays have led to iron-deficiency anemia, hypokalemia, caused by the absorption of potassium, and zinc deficiency. Abnormally large concentrations of potassium (hyperkalemia) and lead have also been reported. Ingestion of untreated (not sterilized) clay can also expose the consumer to parasites and pathogenic soil bacteria and fungi. Tooth-wear and potentially deadly intestinal blockages (Fig. 3) are additional health issues that can be caused by ingestion of clays.

Fig. 3 Intestinal blockage caused by ingestion of clay

Inhalation of Clay

Inhalation of airborne minerals including clays has impacted the heath of millions. Morman and Plumlee (Reference Morman and Plumlee2014) described common chronic diseases associated with or exacerbated by exposure to airborne dust, generally rich in clays. These include pneumoconiosis, asthma, and meningitis. In the 1930’s thousands of people living in the U.S. Dust Bowl died from ‘dust pneumonia’ ((Egan, Reference Egan2006): Fig. 4). Dubovsky (Reference Dubovsky1999) reported that South African farmers who were exposed to dust suffered from tuberculosis superimposed on silicosis. Silicosis, also known as Potter’s rot, can be caused by inhalation of clays and has the distinction of being the most widespread occupational lung disease. It affects people the world over but is more prevalent among workers in developing countries. This is particularly true in P. R. China where, in 2013, pneumoconiosis accounted for nearly 88% of all reported occupational diseases in China, of which about 35% were confirmed to be silicosis (Tse et al., Reference Tse, Dai, Chen, Liu, Zhang, Wong, Leung, Kromhout, Meijer, Liu, Wang, Yu, Shen and Chen2015). The occurrence of silicosis in China has resulted from prolonged exposures to high levels of clay-containing dust from a variety of industrial activities involving mining, rock drilling, and construction activities. Despite adoption of dust-control measures (e.g. increasing ventilation, wet processes, and the use of protective masks) these clay-related health problems are still on the increase. Respiratory problems from the inhalation of dust have been acknowledged since at least Ancient Greek times, but with industrialization the problem only worsened. Silicosis has no known cure; treatments instead focus on symptom-relief and reducing exposure to any lung irritants. Ross et al. (Reference Ross, Nolan, Langer, Cooper, Guthrie and Mossman1993) provided a comprehensive review of health issues associated with inhalation of mineral dusts. They specify a number of types of occupational pneumoconiosis experienced by workers exposed to clay dust including talc pneumoconiosis and kaolinosis.

Fig. 4 Children protecting themselves from dust pneumonia in the US ‘Dust Bowl’ in the 1930s. http://www.pbs.org/kenburns/dustbowl/legacy/

The presence of clay coating on quartz grains may play an important role in preventing or minimizing silicosis. Wendlandt et al. (Reference Wendlandt, Harrison and Vaughan2007), examining surface coating of quartz in bentonites, concluded that “clay coating on quartz grains reduces their cytotoxicity.” Earlier, Meldrum & Howden (Reference Meldrum and Howden2002) speculated that quartz grains liberated and not fractured during coal mining retain their clay mineral coating and would be less dangerous, as the amount of ‘free’ quartz surface, rather than the total amount of quartz present in respirable coal mine dust, may be the most relevant factor to the risk of coal worker’s pneumoconiosis.

No reports were found that suggest inhalation of clays alone could be beneficial.

Dermal Contact of Clay

Recent studies by Williams and co-workers (Haydel et al. Reference Haydel, Remenih and Williams2008; Londono et al. Reference Londono, Hartnett and Williams2017; Morrison et al. Reference Morrison, Misra and Williams2016; Williams et al., Reference Williams, Holland, Eberl, Brunet and Brunet de Courrsou2004) highlighted the remarkable effectiveness of using clay poultices to treat Buruli ulcer, a horrendous flesh-eating bacterium that is resistant to antibiotics. Their research is focusing on the crystallographic and chemical properties of the clays that result in effective antibacterial properties. The use of clays to treat a range of skin issues is not a recent innovation. Clay poultices were widely used in ancient times to stem bleeding, kill bacteria, and cure certain skin ailments. Clay poultices also found use in the military throughout World War I (Reinbacher, Reference Reinbacher2003).

Viseras et al. (Reference Viseras, Aguzzi, Cerezo and Lopez-Galindo2007) pointed out the many uses of clays in health care and therapeutic products and described their use in ointments, toothpaste, cleansing lotions, pastes, make-up, shampoos, anti-acne creams, anti-perspirants, and agents providing protection from the sun, grease, dust, water, etc. Carretero (Reference Carretero2002) reviewed the many topical applications of clays such as their use in dermatological protectors, cosmetics, and excipients followed by descriptions of their use in spas for geotherapy (mixed with water) and pelotherapy (mixed with salt water), mud baths, and paramuds (mixed with paraffin). The clays adsorb toxins from skin and provide heat to stimulate circulation for rheumatism treatment. This application of clays was also reviewed by Veniale et al. (Reference Veniale, Bettero, Jobstraibizer and Setti2007) who described pelotherapy as addressing muscle, bone, and skin pathologies and is now used for general wellness and relaxation. They pointed to the need for standardization and certification to ensure that the treatments do not produce non-beneficial effects.

Not all contact with clays is beneficial. Walking barefoot on volcanic clays can result in non-filarial elephantiasis, also referred to as non-filarial podoconiosis (Fig. 5). Davey et al. (Reference Davey, Tekola and Newport2007) referred to non-filarial podoconiosis as a non-infectious geochemical elephantiasis caused by exposure of bare feet to irritant alkalic clay soils (Fig. 6). This disease is found in at least 10 countries in tropical Africa, Central America, and northwest India, where such soils coexist with high altitude, high seasonal rainfall, and low income. Podoconiosis develops in men and women working barefoot on these soils, with signs becoming apparent in most patients by the third decade of life. Colloid-sized silicate particles appear to enter through the skin and are taken up into macrophages in the lower limb lymphatics causing the disease. This form of podoconiosis is unique in being entirely preventable and non-communicable. Primary prevention entails promoting use of footwear in areas of irritant soil.

Fig. 5 Non-filarial elephantiasis is a disease of the lymphatic vessels of the lower extremities that is caused by chronic exposure to irritant soils

Fig. 6 Clay-poor soils in the Maputaland region in South Africa. A shortage of essential elements may be a causative factor in Mseleni joint disease

Another problem caused by contact with clay was reported by Cross (Reference Cross1919). He described tunneling military troops who were greatly affected by sores which had incapacitated three to four thousand men. The clay in the tunnel walls was found to remove the natural greases from the skin which then dried and cracked leading to infections.

Powdered talc had been widely used as baby powder, an astringent powder used to prevent diaper rash, until some of the powdered talc was found to contain asbestos (Gordon et al. Reference Gordon, Fitzgerald and Millette2014). Subsequently, the suggestion was that the use of talcum powder on female genitalia could lead to ovarian cancer. Berge et al. (Reference Berge, Mundt, Luu and Boffetta2017) conducted a meta-analysis using 24 case-control studies and three cohort studies including 302,705 women with ovarian cancer. They found a weak but statistically significant association between genital application of talc and ovarian cancer. However, to be certain that the clay was the causative agent rather than impurities, the mineralogical purity of the clay would need to be determined.

All Routes of Exposure

Exposure to potentially toxic heavy metals such as arsenic, lead, mercury, cadmium, chromium, nickel, etc., in soils and clays can arise from ingestion, inhalation, or dermal contact (Morgan, Reference Morgan2013). However, health problems are most likely, though not exclusively, due to exposure to soils that have been contaminated by anthropogenic activities rather than natural soils.

Other Possible Health Impacts of Clays (Or the Lack Thereof)

Concluding Remarks

Medical geologists, whether their background is in clay mineralogy, crystallography, geochemistry, sedimentology, hydrogeology, etc., can play a critical role in assisting the medical/public health communities in addressing the myriad impacts that clays have on human health. Medical geologists can provide insights into the complex chemical, physical, and structural properties of these remarkable minerals, including their high adsorption, and cation exchange capacities. These specialists can identify associated minerals that cause health problems, such as the quartz and the asbestos that occurred with vermiculite in Libby, Montana (Schneider & McCumber, Reference Schneider and McCumber2004). Medical geologists can identify human pathogens that are hosted by clays (Lyles, Reference Lyles2018), or clays that inhibit the growth of human pathogens (Williams, Reference Williams2017). They can generate maps showing the distribution of clay types whose properties may impact human health. Medical geologists should also be involved in developing standards and certification for soils and clays used in geophagy to ensure that they are appropriate for that purpose. In all of these activities, medical geologists must collaborate with soil scientists, clay mineralogists, and biomedical/public health researchers to help maximize the health benefits of clays and minimize their negative health impacts.