Sources, Pathways, and Health Effects of Iodine in the Environment

This chapter focuses on the source, pathways, exposure routes, benefits, and biosynthesis of iodine and its health consequences. Iodine is a heavy, greyish-black crystalline solid with a characteristic shinning metallic property. Iodine plays a vital role in human, plant, and animal life. Apart from its use in curing thyroid dysfunction, iodine in its liquid or solid form is also useful in many ways for scientific purposes such as disinfectant in water treatment, cleaning products for household, and base antiseptics in wound cleaning and for sterilizing skin for surgical procedures. Iodine is distributed in the natural environment through interaction between the marine, atmosphere, and terrestrial environment. It occurs in relative abundance in the marine environment, in which the ocean is known as the reservoir of iodine. It escapes from the oceans as iodide in sea spray or as iodide and methyl iodide produced by marine organisms. It is deposited on land (soil, plants, and water) and atmosphere through volatilization, wet and dry deposition, and sorption and absorption processes. Humans get exposed to iodine majorly via the ingestion of food. World Health Organization (WHO) recommends daily iodine consumption of 75–150 μg depending on age groups. This value represents an accumulation of 90% from food intake and 10% from water. Iodine is an essential element that is required in minute quantity in the human body for the biosynthesis of the thyroid hormones. In the thyroid hormones, iodine is of concern in normalizing human body growth, maturation and development, cell and tissue growth, and body metabolism. It is also essential for the development of the fetal nervous system. This explains why pregnant and lactating women require more iodine than other adults. In the same manner, dairy cows require more iodine than other cows because they release about 10% of their iodine intake during the lactation period. However, deviation from the daily consumption of iodine (either an optimum decrease or increase) could result in several health conditions. At a deficient level, it could result in brain damage, leading to mental retardation and cretinism in children and other thyroid dysfunctions such as goiter, hypothyroidism, and hyperthyroidism. At the excessive and toxic level, it mostly affects the thyroid hormone secretion that could result in hyperthyroidism and autoimmune thyroiditis, especially in susceptible individuals. It is worth noting that of all health issues caused by iodine, brain damage, especially in children, is the most devastating, which poses a serious health concern because it could lead to irreversible health conditions. Also, reported cases of health issues caused by iodine deficiency far outweigh those caused by iodine excess. Globally, efforts have been made to combat the issue of iodine deficiency through the implementation of the universal salt iodination programs, which promote household usage of iodized salts and other iodine supplementation in iodine-deficient areas. This program has been found to be successful as the number of countries which have sufficient iodine intake level has increased from 43 countries in the year 2004 to 135 countries in the year 2019.