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Toxicity of metals and semimetals depends on several factors related to the metal bearing material as well as to the organisms and environment in the ecosystem:

  • Dose. All substances, whether synthetic or natural, may cause harm to people, animals, plants, microbes and their environment if the dose is high enough. The dose-effect may be enhanced by bioaccumulation. Bioaccumulation can make any substance toxic but many metal ions are highly water soluble and easily excreted with little toxic effect.
  • Residence time/life time in organism and ecosystem. As many metal ions are highly water soluble they are easily excreted with little toxic effect. Water soluble organometals will normally be excreted and are unlikely to cause problems.
  • Speciation (chemical form) and the potential of a given species to react with receptor biological molecules.
    • Different ions have different toxicities. For example:
      • Cr(3+) ions have very little toxicity while chromate(2-) ions (CrO42-) contribute to lung cancer but have little effect when ingested. Also the cation associated with the chromate is important and should not be ignored. It affects solubility and may be toxic itself. Thus, lead chromate may be the most toxic inorganic form of chromate - but this will depend on route of exposure, particle size etc.
      • Arsenic is a metalloid (semi-metal). The pure element seems to have low toxicity. Arsenic trioxide, arsenite, arsenate and arsine are the best known toxic forms.
    • Radio-isotopes are a special case since they may be harmful externally and at a distance by causing physical damage within the organism if the radiation emitted is high.
    • Insoluble forms of any substance may be a problem if inhaled in particle form less than 7 micrometers aerodynamic diameter and more so if less than 100 nm aerodynamic diameter. {Duffus}. Stable solid forms of any substance may be a problem. No evidence of absorption of particles through the skin or even from the gut.
  • Bioavailability. An immobilised species is trapped in some way so that it is not bio-available, i.e. not accessible to uptake into or onto a living organism, including any external interaction which might have adverse effects. Therefore immobilization of metal ions and metalloids will improve water quality. The potential to react with receptor biological molecules means that a species that is toxic like chromate(2-) ions will not cause any harm if it is immobilized (for example bound to a solid compound so large that it cannot be inhaled).
    • Note 1: radio-isotopes are a special case since they may be harmful externally and at a distance.
    • Note 2: some chemical species are absorbed and concentrated in specific organs. See bioaccumulation.
  • Route of exposure. Inhalation of CrO42- may cause lung cancer - but the ions have little effect when ingested (again, the cation matters and should not be ignored - although it often has been in the past.)

See also

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