While fluorine is used by plants to produce protective toxins and has "apparent beneficial intake" in humans. One study has indicated bromine to be necessary to collagen IV synthesis in humans. In terms of cell type, the body contains hundreds of different types of cells, but notably, the largest number of cells contained in a human body (though not the largest mass of cells) are not human cells, but bacteria residing in the normal human gastrointestinal tract. [2], Some elements (silicon, boron, nickel, vanadium) are probably needed by mammals also, but in far smaller doses. Metals differ from other pollutants in that they are neither created nor destroyed and occur naturally in the environment. Authors: Nielsen, Forrest H. USDA, ARS Source: turn cites Geigy Scientific Tables, Ciba-Geigy Limited, Basel, Switzerland, 1984. [7] This varies substantially by age, sex, and adiposity. The numbers in the table are averages of different numbers reported by different references. [8] Water is ~11% hydrogen by mass but ~67% hydrogen by atomic percent, and these numbers along with the complementary % numbers for oxygen in water, are the largest contributors to overall mass and atomic composition figures. This periodic table shows the percent composition of the average human body. Elements listed as "Possibly" or "Probably" essential are those cited by the National Research Council (United States) as beneficial to human health and possibly or probably essential.[10]. The composition of the human body is expressed in terms of chemicals: The composition of the human body can be viewed on an atomic and molecular scale as shown in this article. Almost 99% of the mass of the human body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus. Of the 94 naturally occurring chemical elements, 61 are listed in the table above. In terms of tissue type, the body may be analyzed into water, fat, connective tissue, muscle, bone, etc. Aluminium, the third most common element in the Earth's crust (after oxygen and silicon), serves no function in living cells, but is toxic in large amounts, depending on its physical and chemical forms and magnitude, duration, frequency of exposure, and how it was absorbed by the human body. Authors: Nielsen, Forrest H. USDA, ARS Source: Modern nutrition in health and disease / editors, Maurice E. Shils ... et al. Many investigators have considered the toxic effects of exposure to metals such as arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg), and there has been substantial expenditure on measures to reduce human exposure.