|This article may contain inappropriate or misinterpreted citations that do not verify the text. (June 2009)|
The use of medicinal clay in folk medicine goes back to prehistoric times. Indigenous peoples around the world still use clay widely, which is related to geophagy. The first recorded use of medicinal clay goes back to ancient Mesopotamia. A wide variety of clays are used for medicinal purposes—primarily for external applications, such as the clay baths in health spas (mud therapy). Among the clays most commonly used are kaolin and the smectite clays such as bentonite, montmorillonite, and Fuller's earth.
- 1 Questions of nomenclature
- 2 Animal geophagy
- 3 Historical use
- 4 Preparation of clay
- 5 Medicinal properties of clay in modern research
- 5.1 Antibacterial properties
- 5.2 Trace minerals
- 5.3 External use
- 5.4 Internal use
- 6 Common medicinal clays
- 7 See also
- 8 Notes
- 9 References
- 10 Further reading
- 11 External links
Questions of nomenclature
|This section does not cite any references or sources. (June 2009)|
There are considerable problems with the exact nomenclature of various clays. No clay deposit is exactly the same and, typically, mineral clays are mixed in various proportions.
The overwhelming majority of clay mined commercially is for industrial uses, such as construction and oil drilling. Thus, the precise classification and chemical composition of these clays are somewhat secondary to their intended use. Bentonite clay, montmorillonite clay, and Fuller's earth are similar.
Sodium bentonite/calcium bentonite are the most commonly used medicinal clays today (sodium bentonite for external use, calcium bentonite for internal use), although there is no precise definition of what this term means. In fact, typically, "bentonite" refers to a wide spectrum of clays with a wide array of properties (such as a variety of colours). In alternative medicine, often this is used as more or less a catch-all term for medicinal clays. Another such term is "montmorillonite", which is often interchangeable with "bentonite". Bentonite is included in the United States Pharmacopeia, and the USP-grade bentonite is widely used in various pharmaceutical and cosmetic preparations as a compounding and suspending agent. It is not entirely clear where the source of USP-grade bentonite is located; it may be a mixture of various bentonites.
Most of the nomenclature currently used is discussed in a 2005 World Health Organization report BENTONITE, KAOLIN, AND. SELECTED CLAY MINERALS, as well as an entire section of the USGS website Minerals Information: Clays
Animals both in the wild and domesticated consume different types of earth and clay. Galen, the Greek philosopher and physician, was the first to record the use of clay by sick or injured animals in the second century AD. This type of geophagy has been documented in "many species of mammals, birds, reptiles, butterflies and isopods, especially among herbivores."
In particular, in Peru, Amazonian rainforest parrots of some 21 species gather at certain sites on cliff faces where bare soil is exposed, and eat the clayish soil.The soil they seek is highly specific, since they focus on a rather narrow band of exposed soil. What they seek is mostly clay that is less than 0.2 millimetre in particle diameter.
There is a large amount of anthropological and historical literature describing the medicinal use of clay around the world from the earliest times.
Some scholars believe that prehistoric ancestors such as Homo erectus and Homo neanderthalensis used ochres to cure wounds as well as paint caves. Ochres are a mixture of clay and iron hydroxides.
"The oldest evidence of geophagy practiced by humans comes from the prehistoric site at Kalambo Falls on the border between Zambia and Tanzania (Root-Bernstein & Root-Bernstein, 2000)." Here, a calcium-rich white clay was found alongside the bones of Homo habilis (the immediate predecessor of Homo sapiens).
Use by aboriginal peoples
Clay is used widely by indigenous peoples around the world, and is related to geophagy.
Ancient Egypt and Mesopotamia
The first recorded use of medicinal clay is on Mesopotamian clay tablets around 2500 B.C. Also, ancient Egyptians used clay. The Pharaohs’ physicians used the material as anti-inflammatory agents and antiseptics. It was used as a preservative for making mummies and is also reported that Cleopatra used clays to preserve her complexion.
The Ebers Papyrus of about 1550 BC (but containing the tradition going back many centuries earlier) is an important medical text from ancient Egypt. It describes the use of ochre for a wide variety of complaints, including for intestinal problems, as well as for various eye complaints.
This was a clay used in Classical Antiquity. It was mined on the island of Lemnos. Its use continued until the 19th century, as it was still listed in an important pharmacopoeia in 1848 (the deposits may have been exhausted by then).
if rubbed under the eyes, it moderates pain and watering from the same, and prevents the flow from the lachrymal ducts. In cases of haemorrhage it should be administered with vinegar. It is used against complaints of the spleen and kidneys, copious menstruation, also against poisons, and wounds caused by serpents.
Lemnian clay was shaped into tablets, or little cakes, and then distinctive seals were stamped into them, giving rise to its name terra sigillata—Latin for 'sealed earth'. Dioscorides also commented upon the use of terra sigillata.
Another physician famous in antiquity, Galen, recorded numerous cases of the internal and external uses of this clay in his treatise on clay therapy.
Galen... used as one of his means for curing injuries, festering wounds, and inflammations terra sigillata, a medicinal red clay compressed into round cakes and stamped with the image of the goddess Diana. This clay, which came from the island of Lemnos, was known throughout the classical world.
Other clays used in classical times
The other types of clay that were famous in antiquity were as follows.
- Terra chia, Terra cymolia (Cimolean earth): these were both white earths and considered of great value.
- Samian earth: Pliny in c. 50 AD (Nat. Hist.) details two distinct varieties, colyrium - an eye salve, and aster, which was used as a soap as well as in medicines.
- Terra sigillata strigoniensis (Strigian earth, derived from Silesia) - this clay, yellow in colour, appears to have been famous later in medieval times.
All the above seem to have been bentonitic clays.
- The earth which did not stain the hands was known as rubrica.
In medieval Persia, Avicenna (980-1037 CE), the 'Prince of Doctors', wrote about clay therapy in his numerous treatises.
- the terra sigillata,
- Egyptian earth,
- Samian earth,
- earth of Chios,
- Cimolean earth or pure clay (cimolite), soft earth, called al-hurr, green in color like verdigris, is smoked together with almond bark to serve as food when it will turn red and assume a good flavor; it is but rarely eaten without being smoked—also called 'Argentiera',
- earth of vines called ampelitis (Pliny XXXV, 56) or pharmakitis from Seleucia in Syria,
- Armenian earth (also known as the Armenian bole), salutary in cases of bubonic plague, being administered both externally and internally,
- earth of Nishapur.
A French naturalist Pierre Belon (1517–1564) was interested in investigating the mystery of the Lemnian clay. In 1543, he visited Constantinople where, after making enquiries, he encountered 18 types of different products marketed as Lemnian Earth (he was concerned about possible counterfeits). He then made a special journey to Lemnos, where he continued his investigation, and tried to find the source of the clay. He discovered that it was extracted only once a year (on 6 August) under the supervision of Christian monks and Turkish officials.
Preparation of clay
Clay gathered from its original source deposit is refined and processed in various ways by manufacturers. This can include heating or baking the clay, since the raw clay tends to contain a variety of micro-organisms
Too much processing, likewise, may reduce the clay's therapeutic potential. In particular, Mascolo et al. studied 'pharmaceutical grade clay' versus 'the natural and the commercial herbalist clay', and found an appreciable depletion of trace elements in the pharmaceutical grade clay. On the other hand, certain clays are typically heated or cooked before use.
Medicinal clay is typically available in health food stores as a dry powder, or in jars in its liquid hydrated state – which is convenient for internal use. For external use, the clay may be added to the bath, or prepared in wet packs or poultices for application to specific parts of the body. Often, warm packs are prepared; the heat opens up the pores of the skin, and helps the interaction of the clay with the body.
In the European health spas, the clay is prepared for use in a multitude of ways – depending on the traditions of a particular spa; typically it is mixed with peat and matured in special pools for a few months or even up to two years.
"The majority of spas … use artificial ponds where the natural ("virgin") clay is mixed with mineral, thermo-mineral, or sea water that issues in the vicinity of the spas or inside the spa buildings."
Medicinal properties of clay in modern research
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"...exhibits bactericidal activity against E. coli, ESBL [Extended-Spectrum Beta-Lactamases] E. coli, S. enterica serovar Typhimurium, P. aeruginosa, and M. marinum, and significantly reduces growth of S. aureus, PRSA, MRSA, and nonpathogenic M. smegmatis approximately 1,000-fold compared to cultures grown without added mineral products."
Falkinham et al. studied the antibiotic and antimicrobial activity of red clays from the Kingdom of Jordan (Jordan's Red Soil). The authors conclude that the antibiotic activity of Jordan's red clays is likely due to the proliferation of antibiotic-producing bacteria, that is induced by the clay.
Clays contain large amounts of trace minerals. It is common to see as many as 75 different trace minerals in Montmorillonite clays. Specific trace minerals that various clays possess vary very widely. Also, the amount of any particular trace mineral in any specific clay varies a lot among clays from different locations. For example, the amount of iron in various bentonite clays can vary from well below 1%, and up to 10%.
Mud baths are perhaps the most common use of clay and are commonly used at all health spas.
Many types of skin conditions have been treated by the application of medicinal clay. Montmorillonite has shown its effectiveness in this area. It has also been used as a base ingredient for tissue engineering. Clay is used in many dermatological over-the-counter remedies, such as in acne treatments (this information may not be mentioned on the label specifically).
Use in bandages
In April 2008, the Naval Medical Research Center announced the successful use of an updated Kaolinite-derived aluminosilicate nanoparticles infusion in traditional gauze known commercially as "QuikClot Combat Gauze". The earlier formulation had significant problems with heat generation in the wounds.
There are many over the counter remedies for internal use that contained clay before discontinuation. Examples include Kaopectate (Upjohn), Rheaban (Leeming Div., Pfizer), and Diar-Aid (Thompson Medical Co.). The labels on all of these showed the active ingredient to be Attapulgite, each tablet containing 600 (or 750 mg) of this component along with inert materials or adjuvants. However, since April 2003, attapulgite medication was discontinued due to lack of evidence according to the U.S. Foods and Drugs Administration.
Numerous medicines also use Kaolinite clay, which has long been a traditional remedy to soothe an upset stomach. Also, Kaolin is or has been used as the active substance in liquid anti-diarrhea medicines such as Kaomagma. Such medicines were changed away from aluminium substances due to a scare over Alzheimer's disease, but have since changed back to compounds containing aluminium as they are most effective.
Clays have been effective against the Candida albicans (in vitro), a yeast which is a causal agent of opportunistic oral and genital infections and rarely more serious infections. The influence of bentonite clay on the growth of Candida lipolytica has been studied. An unfavorable effect of bentonite on Candida lipolytica growth in vitro was observed  According to a 2009 study, bentonite clay acts very strongly against Candida. 
Heavy metal chelation
Oyanedel-Craver and Smith have studied sorption of four heavy metals (Pb, Cd, Zn and Hg) to 3 kinds of bentonite clay. The overall conclusion of the study was that the organoclays studied have considerable capacity for heavy metal sorption.
Irritable bowel syndrome
"[B]eidellitic montmorillonite is efficient for C-IBS patients (suffering from constipation-predominant irritable bowel syndrome)..."
Aflatoxins are naturally occurring mycotoxins that are produced by many species of Aspergillus, a fungus. Aflatoxins are toxic and among the most carcinogenic substances known. They cause Aflatoxicosis, which can afflict both animals and humans.
Bentonite clay has proven to show a very strong protective effect against Aflatoxicosis.
"The addition of bentonite or HSCAS [hydrated sodium calcium aluminosilicate] to the AF-contaminated diet diminished most of the deleterious effects of the aflatoxin. Pathological examinations of liver and kidney proved that both bentonite and HSCAS were hepatonephroprotective agents against aflatoxicosis."
Also, regarding chickens:
"The addition of sodium bentonite was significantly effective in ameliorating the negative effect of aflatoxicosis on the percentage and mean of phagocytosis."
Conversely, it has been found that prolonged exposure to bentonite in humans can actually have harmful effects 
Use during pregnancy
Use by the NASA space program
The effects of weightlessness on human body were studied by NASA in the 1960s. Experiments demonstrated that weightlessness leads to a rapid bone depletion, so various remedies were sought to counter that. A number of pharmaceutical companies were asked to develop calcium supplements, but apparently none of them were as effective as clay. The special clay that was used in this case was Terramin, a reddish clay found in California. Dr. Benjamin Ershoff of the California Polytechnic Institute demonstrated that the consumption of clay counters the effects of weightlessness. He reported that "the calcium in clay ...is absorbed more efficiently ... [clay] contains some factor or factors other than calcium which promotes improved calcium utilization and/or bone formation." He added, "Little or no benefit was noted when calcium alone was added to the diet."
Side effects and USFDA disproval of oral intake effectiveness
Substances discontinued such as kaolin and attapulgite were formerly considered gastric demulcents and diarrhea medication, until official studies by the USFDA disproved these views. Clays are classified as excipients and their main side-effects are that of neutral excipients, which is to impair and slow down absorption of antibiotics, hormones and heart medication amongst others by coating the digestive tract  and this slowed down absorption can lead to increased toxicity of some medication (e.g. citrate salts) which can become toxic if not metabolized quickly enough, which is one contraindication of attapulgite. Usual mild side-effects are nausea, slowed down absorption of nutrients from food (in excess dosage of medicinal clay) and constipation.
Common medicinal clays
- Bentonite-type clay has been used to treat infections, indigestion, and other medical problems by both applying wet clay topically to the skin as a poultice, and by ingesting it. Bentonite has been prescribed as a bulk laxative, and it is also used as a base for many dermatologic formulas. Dermatologically, it is used as part of a treatment for pruritis. Also, bentonite can be used as a therapeutic face pack for the treatment of acne/oily skin. Clearasil, an acne cream, uses bentonite as an agent to absorb excess sebum, clearing pores.
- Montmorillonite is the main constituent of bentonite.
- Attapulgite or palygorskite is a very absorbent clay, somewhat similar to bentonite. When used in medicine, it physically binds to acids and toxic substances in the stomach and digestive tract. For this reason, it has been used in several anti-diarrheal medications.
- Kaolin is not as absorbent as most clays used medicinally (it has a low shrink-swell capacity). Also, it has a low cation exchange capacity. This clay is also known as 'white cosmetic clay'. Clay, in the form of kaolin, was a common ingredient in western medicines such as Kaopectate Rolaids and Maalox, but is no longer present in them nor in similar preparations, as it is considered ineffective by the USFDA.
- Jared M. Diamond, "Evolutionary biology: Dirty eating for healthy living". Nature 400, 120-121 (1999)
- Myers, Jack. "Parrots that Eat Dirt". Highlightskids.com. Retrieved 1 November 2013.
- Olle Selinus, B. J. Alloway, Essentials of medical geology: impacts of the natural environment on public health. Academic Press, 2005 ISBN 0-12-636341-2, p. 446
- ASU research
- PAPYRUS EBERS, 1937 translation.
- Recipes for Treating the Eyes: Papyrus Ebers[dead link]
- Cited in Thompson CJS. The mystery and art of the apothecary, by C.J.S. thompson. London: John Lane; 1929. p. 44.
- Dr. H. Van Der Loos, The Miracles of Jesus, Leiden, the Netherlands, Brill, 1965. p. 82.
- Soranus' gynecology, Owsei Temkin (tr.), JHU Press, 1991 (reprint). pp. 239-240. ISBN 0-8018-4320-0
- L. Leclerc, "Traite des simples", II, 1881, pp. 421-427; for a general appreciation of this work see Baron Carra de Vaux, "Les penseurs de lslam", II, 1921, pp. 289-296 (original note in Laufer)
- Laufer, Berthold (1930). Geophagy (available online). Chicago: Field Museum Press.
- "Soil, including kaolinitic and montmorillonitic clays, contains considerable amounts of organic material, including many live microorganisms." from CDC.gov website Callahan GN. Eating dirt. Emerg Infect Dis [serial online] 2003 Aug. (accessed 16 June 2009)
- Mascolo, N.; Summa, V.; Tateo, F. (1999). "Characterization of toxic elements in clays for human healing use". Applied Clay Science 15 (5–6): 491. doi:10.1016/S0169-1317(99)00037-X.
- An example of this is the medieval 'Argentiera' clay, mentioned in this article.
- "Hot application is recommended in geotherapy, pelotherapy or paramuds in beauty therapy..." Carretaro MI, Gomes CSF, Tateo F. "Clays and human health." In: Bergaya F, Theng BKG, Lagaly G, editors. Handbook of Clay Science, Developments in Clay Science. Vol. 1. Elsevier Ltd; Amsterdam: 2006. pp. 717–741. ISBN 0-08-044183-1 p. 723
- Carretaro MI, Gomes CSF, Tateo F. "Clays and human health." In: Bergaya F, Theng BKG, Lagaly G, editors. Handbook of Clay Science, Developments in Clay Science. Vol. 1. Elsevier Ltd; Amsterdam: 2006. pp. 717–741. ISBN 0-08-044183-1, ISBN 978-0-08-044183-2 p. 724
- Haydel, S.; Remenih, C.; Williams, L. (2008). "Broad-spectrum in vitro antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens.". The Journal of antimicrobial chemotherapy 61 (2): 353–361. doi:10.1093/jac/dkm468. PMC 2413170. PMID 18070832.
- Haydel, S.; Remenih, C.; Williams, L. (2008). "Broad-spectrum in vitro antibacterial activities of clay minerals against antibiotic-susceptible and antibiotic-resistant bacterial pathogens.". The Journal of antimicrobial chemotherapy 61 (2): 353–361. doi:10.1093/jac/dkm468. PMC 2413170. PMID 18070832. (full text of the article)
- Falkinham Jo, 3.; Wall, T.; Tanner, J.; Tawaha, K.; Alali, F.; Li, C.; Oberlies, N. (2009). "Proliferation of antibiotic-producing bacteria and concomitant antibiotic production as the basis for the antibiotic activity of Jordan's red soils.". Applied and environmental microbiology 75 (9): 2735–2741. doi:10.1128/AEM.00104-09. PMC 2681674. PMID 19286796.
- US Patent 6962718 Table 1: Montmorillonite Components; Average Nutrient Content (accessed 29 Nov 09)
- A systematic review of contact dermatitis treatment and prevention. Saary J, Qureshi R, Palda V, DeKoven J, Pratt M, Skotnicki-Grant S, Holness L. J Am Acad Dermatol. 2005 Nov;53(5):845. Review.
- Guin, J. D. (2001). "Treatment of toxicodendron dermatitis (poison ivy and poison oak)". Skin therapy letter 6 (7): 3–5. PMID 11376396.
- Cytotoxic assessment of L-ascorbic acid/montmorillonite upon human dermal fibroblasts in vitro: MTT activity assay. Lee, Y.-H., Chen, B.-Y., Lin, F.-H., Lin, K.-Y., Lin, K.-F. 2008 Biomedical Engineering - Applications, Basis and Communications 20 (6), pp. 337-343
- Carretaro MI, Gomes CSF, Tateo F. "Clays and human health." In: Bergaya F, Theng BKG, Lagaly G, editors. Handbook of Clay Science, Developments in Clay Science. Vol. 1. Elsevier Ltd; Amsterdam: 2006. pp. 717–741. ISBN 0-08-044183-1 p. 722
- Preparation of biomimetic three-dimensional gelatin/montmorillonite-chitosan scaffold for tissue engineering. Zheng, J.P., Wang, C.Z., Wang, X.X., Wang, H.Y., Zhuang, H., Yao, K.D. 2007 Reactive and Functional Polymers 67 (9), pp. 780-788
- Rhee P, Brown C, Martin M, et al. (2008). "QuikClot use in trauma for hemorrhage control: case series of 103 documented uses". J Trauma 64 (4): 1093–9. doi:10.1097/TA.0b013e31812f6dbc. PMID 18404080.
- Rowe, Aaron. "Nanoparticles Help Gauze Stop Gushing Wounds". Wired.com. Retrieved 2012-10-07.
- "U.S. Patent 5079201". Freepatentsonline.com. 1989-09-19. Retrieved 2012-10-07.
- Maignan C and Pareilleux A, Influence of bentonite on the growth of Candida lipolytica. Comptes rendus hebdomadaires des seances de l'Academie des sciences. Serie D: Sciences naturelles 273(9):835-8, 1971 Aug 30
- Pareilleux A, Maignan C., Can J Microbiol. 1976 Aug;22(8):1065-71. [Metabolic activity of Candida lipolytica adsorbed to bentonite with hydrophobic chains] [Article in French]abstract and a link to full text
- M. Ghiaci, H. Aghaei, S. Soleimanian, M.E. Sedaghat S, "Enzyme immobilization: Part 1. Modified bentonite as a new and efficient support for immobilization of Candida rugosa lipase." Applied Clay Science, Volume 43, Issues 3-4, March 2009, Pages 289-295
- "American Heart Association: Chelation Therapy". Retrieved 2008-04-03.
- Immunization Safety Review Committee, Board on Health Promotion and Disease Prevention, Institute of Medicine (2004). Immunization Safety Review: Vaccines and Autism. Washington, DC: The National Academies Press. ISBN 0-309-53275-2.
- Oyanedel-Craver VA, Smith JA, "Effect of quaternary ammonium cation loading and pH on heavy metal sorption to Ca bentonite and two organobentonites". J Hazard Mater 2006; 137:1102-14.
- Ducrotte P, Dapoigny M, Bonaz B, Siproudhis L. "Symptomatic efficacy of beidellitic montmorillonite in irritable bowel syndrome: a randomized, controlled trial". Aliment Pharmacol Ther. 2005 Feb 15;21(4):435-44. available online
- M A Abdel-Wahhab, S A Nada, I M Farag, N F Abbas, H A Amra, "Potential protective effect of HSCAS and bentonite against dietary aflatoxicosis in rat: with special reference to chromosomal aberrations." Nat Toxins - 1998 (Vol. 6, Issue 5, Pages 211-8)
- I. K. IBRAHIM, A. M. SHAREEF, K. M. T. AL-JOUBORY, "Ameliorative effects of sodium bentonite on phagocytosis and Newcastle disease antibody formation in broiler chickens during aflatoxicosis". Research in Veterinary Science, Volume 69, Issue 2, October 2000, Pages 119-122
- Ubick, Suzanne. "Mud, Mud, Glorious Mud". Magazine of the California Academy of Sciences. California Academy of Sciences. Retrieved 2012.
- "Kaolin Information | Evidenced-Based Supplement Guide on MedicineNet.com - Are there any interactions with medications?". Medicinenet.com. Retrieved 2012-10-07.
- "Attapulgite Tablet Facts and Comparisons at". Drugs.com. Retrieved 2012-10-07.
- Bentonite from oregonstate.edu website
- Calamine from www.drugs.com website
- "Kaolin Information | Evidenced-Based Supplement Guide on MedicineNet.com - How does Kaolin work?". Medicinenet.com. Retrieved 2012-10-07.
- Johns T, Duquette M, "Detoxification and mineral supplementation as functions of geophagy." Am J Clin Nutr. 1991 Feb;53(2):448-56
- Ferrell, R. E. (2008). "Medicinal clay and spiritual healing". Clays and Clay Minerals 56 (6): 751–760. doi:10.1346/CCMN.2008.0560613.
- Williams, L. B.; Haydel, S. E.; Ferrell, R. E. (2009). "Bentonite, Bandaids, and Borborygmi". Elements 5 (2): 99–104. doi:10.2113/gselements.5.2.99. PMC 2895274. PMID 20607126.
- Retsas S. Medicinal Use of earths and minerals from Hippocrates to Sir Hans Sloane and beyond. Vesalius, XVIII, 2, 93-98, 2012.
- Michel Abehsera, The healing clay : the centuries-old health & beauty elixir rediscovered. Brooklyn, N.Y. : Swan House, 1979. ISBN 0-918282-10-1 OCLC 12094673
- Cindy Engel, Wild Health: Lessons In Natural Wellness From The Animal Kingdom. Houghton Mifflin Harcourt, 2003. ISBN 0-618-34068-8
- Knishinsky, Ran. "The clay cure – natural healing from the earth". Healing Arts Press. 1998.
- Laufer, Berthold (1930). "Geophagy" (available online). Chicago: Field Museum Press.
- W. Rudolph Reinbacher, "Healing earths: the third leg of medicine : a history of minerals in medicine with rare illustrations from 300 to 1000 years ago." Published by W. Rudolph Reinbacher, 2002
- Robert and Michele Root-Bernstein, "Honey, Mud, Maggots and Other Medical Marvels." Houghton Mifflin, 1997.
- "The Original Georgia Grown Gourmet White Dirt" Kaolin-type clay from Georgia, USA.
- "Parrots That Eat Dirt"- by Jack Myers
- French muck: Is this the new penicillin? - by Paul Rodgers (accessed 3 Nov 09)
- ABC News, Experts claim habit of eating dirt may be beneficial for some, October 4, 2005 (accessed 17 Dec 09)