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Naturally occurring elements are classified into major elements, minor elements, trace elements and noble gases ([[:File:UNN_Medical_Geology_Fig_3.png|Figure 3]]).

Naturally occurring elements are classified into major elements, minor elements, trace elements and noble gases ([[:File:UNN_Medical_Geology_Fig_3.png|Figure 3]]).

According to Nordberg, and Cherian<ref>Nordberg, M. and M. G. Cherian, 2010. Biological Responses of Elements. In: Selinus, O., Alloway, B., Centeno, J.A., Finkelman, R.B., Fuge, R., Lindh, U. and Smedley, P. (eds.) Essentials of Medical Geology. Elsevier, Amsterdam, 179-200.</ref> major elements are those elements that are needed in large quantities in the body; each makes more than 1% of the human body (oxygen alone making up to 65% of the body). They include hydrogen, oxygen, carbon and nitrogen. These elements form most of the DNA structures in the human body; they occur in most organs and tissues. In addition, they are the main building blocks of enzymes; they are also important constituents of the cell membrane, inter-cellular, intracellular fluids and are actively involved in metabolic processes, all in their ionic states. Hence, in the event of high intake of these elements, the body has many ‘sites’ where they can be used. Nature provides a good abundance of these elements as it has been observed that the concentration of these elements is fairly the same in similar rocks and show the least variance compared to the minor and trace elements. Thus, they rarely pose problems.<ref name=Setal2013 />

According to Nordberg, and Cherian<ref>Nordberg, M. and M. G. Cherian, 2010, Biological responses of elements, ''in'' O. Selinus, B. Alloway, J. A. Centeno, R. B. Finkelman, R. Fuge, U. Lindh, and P. Smedley, P., eds., Essentials of medical geology: Amsterdam, Elsevier, p. 179–200.</ref> major elements are those elements that are needed in large quantities in the body; each makes more than 1% of the human body (oxygen alone making up to 65% of the body). They include hydrogen, oxygen, carbon and nitrogen. These elements form most of the DNA structures in the human body; they occur in most organs and tissues. In addition, they are the main building blocks of enzymes; they are also important constituents of the cell membrane, inter-cellular, intracellular fluids and are actively involved in metabolic processes, all in their ionic states. Hence, in the event of high intake of these elements, the body has many ‘sites’ where they can be used. Nature provides a good abundance of these elements as it has been observed that the concentration of these elements is fairly the same in similar rocks and show the least variance compared to the minor and trace elements. Thus, they rarely pose problems.<ref name=Setal2013 />

Minor elements are those elements that are needed in moderate quantities; each has a concentration of 0.1% to 1.0% in the body.<ref name=Setal2013 /> They share in the functions of the major elements, but in smaller amounts.

Minor elements are those elements that are needed in moderate quantities; each has a concentration of 0.1% to 1.0% in the body.<ref name=Setal2013 /> They share in the functions of the major elements, but in smaller amounts.

Trace elements are those elements that have concentrations far below 0.1% in the body; they are needed in the body only in parts per million (ppm). They are essential to plant and animal lives only in trace amounts. Such trace elements are called essential trace elements.<ref>Christine, L. E., G. Sujuan, L. Yiming, L. Chaoke, J. Rongdi, S. H. Kathleen, C. Jingxiang, M. Feng, W. Yunpeng, Y. Po, S. Shuzhuang, W. U. Frederick, W. S. Charles, and C. H. Hugh, 2000, Trace Element Levels in Drinking Water and Cognitive Function among Elderly Chinese: American Journal of Epidemiology, 151(9), 913-920.</ref> They are highly variable in their abundance in various rocks (Table 1).

Trace elements are those elements that have concentrations far below 0.1% in the body; they are needed in the body only in parts per million (ppm). They are essential to plant and animal lives only in trace amounts. Such trace elements are called essential trace elements.<ref>Christine, L. E., G. Sujuan, L. Yiming, L. Chaoke, J. Rongdi, S. H. Kathleen, C. Jingxiang, M. Feng, W. Yunpeng, Y. Po, S. Shuzhuang, W. U. Frederick, W. S. Charles, and C. H. Hugh, 2000, Trace element levels in drinking water and cognitive function among elderly Chinese: American Journal of Epidemiology, v. 151, no. 9, p. 913–920.</ref> They are highly variable in their abundance in various rocks (Table 1).

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|+ Table 1: Average abundances of selected elements in bedrock, all values in ppm.<ref name=Khndare>Khandare, H. W., 2012, Medical Geology: An Emerging Field of Interdisciplinary Research on Geology and Human Health. International Journal of ChemTech Research CODEN (USA), 4(4), 1792-1796.</ref>

|+ {{table number|1}}Average abundances of selected elements in bedrock, all values in ppm.<ref name=Khndare>Khandare, H. W., 2012, Medical geology: An emerging field of interdisciplinary research on geology and human health: International Journal of ChemTech Research CODEN (USA), v. 4, no. 4, p. 1792–1796.</ref>

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! Element !! Ultrabasic Rock !! Basalt !! Granite !! Shale !! Limestone

! Element !! Ultrabasic Rock !! Basalt !! Granite !! Shale !! Limestone

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|+ Table 2: Diseases caused by the deficiencies and excessiveness/toxicity of some trace elements.<ref name=Khndare />

|+ {{table nubmer|2}}Diseases caused by the deficiencies and excessiveness/toxicity of some trace elements.<ref name=Khndare />

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! Element || Deficiency || Excessiveness/Toxicity

! Element || Deficiency || Excessiveness/Toxicity

However, some trace elements such as As, Cd, Pb, Hg and Rn have no/limited biological functions and are generally toxic to humans; they are called non-essential trace elements.<ref name=Setal2013 />

However, some trace elements such as As, Cd, Pb, Hg and Rn have no/limited biological functions and are generally toxic to humans; they are called non-essential trace elements.<ref name=Setal2013 />

From the above biological classification of elements, it can be seen why medical geology sets its eyes on the trace elements: they are the causes of most health problems sourced from geologic materials. They emanate from [[igneous]], sedimentary or metamorphic rocks, as well as industrial minerals. They contaminate groundwater through boreholes or through springs/streams. In addition, they can be released during volcanic eruptions.

From the above biological classification of elements, it can be seen why medical geology sets its eyes on the trace elements: they are the causes of most health problems sourced from geologic materials. They emanate from [[igneous]], sedimentary or metamorphic rocks, as well as industrial minerals. They contaminate groundwater through boreholes or through springs or streams. In addition, they can be released during volcanic eruptions.

===Geochemobiological Pathways===

===Geochemobiological Pathways===