The geochemistry of iodine and its application to environmental strategies for reducing the risks from iodine deficiency disorders (IDD). (CR/03/057N).
Medical intervention techniques such as salt iodisation have been successful in reducing the risks from iodine deficiency disorders (IDD) in many parts of the world. This project funded by the UK Department for International Development (DFID) addresses the perceived need for a better understanding of iodine geochemistry so that we can ensure that the small amounts of iodine that are available in the environment are used in the most efficient way. Futhermore, iodine added through environmental supplementation techniques (e.g. adding iodine to irrigation waters) needs to be managed in an effective way to ensure maximum use is made of the added iodine through a better understanding of its geochemical behaviour.
This project uses data from BGS case studies in Sri Lanka, China and Morocco plus an extensive bibliography of iodine geochemistry. From this we have created databases on the iodine content of soils, food and drinking water.
The geochemistry of iodine in soils has been studied in detail as this is the vital link between the food chain and the environment. A model for the iodine content of soils is discussed based on locational factors, the iodine fixation potential of the soil and the important pathway of iodine from the ocean-atmosphere- land-plant-man. Climate, topography and parent material all contribute to the iodine status of a soil which is determined by the interaction of numerous physical and chemical parameters. The iodine content of a soil depends not only on the supply of iodine, or lack of it, but the ability of the soil to fix the iodine, i.e. its iodine fixation potential. Organic matter plays a principal role in retaining iodine but other factors will contribute including soil texture, iron and aluminium oxides, clay minerals, water-logging, microbial activity and Eh and pH.
The atmophile nature of iodine is its most significant geochemical characteristic and the volatilisation of iodine from the soil-plant system is of far greater significance than previously believed. Direct adsorption of iodine on to plant leaves from the atmosphere can contribute to the total iodine content of plants and it is seen that, of the various plant parts, leaves tend to have the highest levels of iodine. The transfer ratio of iodine from soil to plant is low and with the exception of coastal zones it is suggested that most of the land surface is actually iodine-deficient in that locally grown crops alone cannot supply the recommended daily allowances to the local population. Industrialisation and development add adventitious iodine to the diet as well as the opportunity to eat foods from outside the local environment. Seafood is an external food source of major importance.
Soils are not a good indicator of an iodine-deficiency because of their heterogeneous nature and the complexity of factors that determine their iodine status. The level of iodine in surface waters is a much better indicator of an environment's iodine status.
Environmental solutions to reducing the risks from IDD require that either the existing natural iodine is managed more efficiently or iodine is added from an external source. Measures to manage the local iodine balance more efficiently include: changing the crops grown; using grazing livestock to concentrate iodine; improving the soil's ability to fix the iodine; making the soil's iodine more bioavailable without losing it through volatilisation; finding alternative more iodine-rich water supplies; and preventing removal of iodine by flooding.
Although better control of iodine in marginally iodine-deficient areas may help to reduce the risks from IDD it is not likely to be appropriate in regions of extreme poverty and harsh environmental conditions where the local population have little or no scope for controlling their own environment. In such instances, environmental intervention techniques such as adding iodine to irrigation waters may be more appropriate. These have been shown to bring substantial benefits to the local environment as well as being cost effective with some benefits over the conventional salt iodisation strategies. However, such additions of iodine to the environment need to be managed carefully so the benefits of the added iodine are not lost through volatilisation and rapid migration of iodine out of the soil system in a relatively short period of time.
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