Letter to the Editor: The Potential Impact of the Climate Crisis on Global Iodine Status

Dear Editor:

The long-standing problem of iodine deficiency disorders (IDDs) has been greatly improved in recent times, principally through human intervention through iodization of table salt. ¹ However, problems associated with IDDs may persist principally in “risk” groups such as pregnant mothers and their neonates and young women of childbearing age. A topic receiving less attention in terms of iodine and associated thyroidal status is its life cycle in the marine environment, which is the most important source of global iodine supply. ² A role for iodine assuming increasing importance is the contribution to global warming of gaseous iodine released into the upper troposphere and lower stratosphere. ³ This can arise from natural volatilization of iodine from seawater or marine algae or from anthropometric ozone (O₃ ⁻) pollution. ⁴

Atmospheric iodine has been shown to drive new marine particle formation, leading further to Cloud Condensation Nuclei increases, cloud brightening, and ultimately higher cloud reflectance, partially negating global warming. ⁵ The principal iodine speciation in macroalgae and surface water is iodide (I⁻) with iodate (IO₃ ⁻) predominating in the deep ocean. It has been known for some time that iodine is released at the interface between water and air, the marine boundary layer, and provides a source of volatile iodines (HOI⁻ and I₂). This release comes from seawater itself and algae, principally macroalgae (seaweeds), particularly when stressed by exposure to atmospheric O₃ ⁻ at low tides. ^5,6 O₃ ⁻ of natural or anthropogenic origin reacts with I⁻, the major speciation of iodine in seawater and seaweeds, to undergo the reaction leading to gaseous iodine production and associated reduction in harmful O₃ ⁻ as shown hereunder: O 3 − + H + + I − > H O I + O 2 , H + + H O I + I − > I 2 + H 2 O .

I^– in surface water or seaweed reacts with atmospheric O₃ ⁻ to produce gaseous I_2. Since O₃ ⁻ is one of the major air pollutants, its reaction with I^– is the most important factor facilitating transfer of volatile iodine compounds from the ocean into the atmosphere. ² Such transfer is also supported by the increased ocean temperatures and increased seawater acidity derived from CO₂, which favors reduced I^– over oxidized IO₃ ⁻. ^3,7 Evidence for the multiple climatic effects increasing global iodine bioavailability is provided by melting of the Arctic and Antarctic sea ice shield and retreat of alpine glaciers ^8,9 as well as dust clouds arising from deserts, ¹⁰ which have been shown to encapsulate volatile iodine species.

Studies show evidence of a threefold increase since 1950 in atmospheric iodine deposition in Europe ⁹ with atmospheric iodine levels in the Northern hemisphere higher presumably as a consequence of O₃ ⁻ levels produced in the more industrialized North. ² A review of glacial cycles over the Arctic Ocean demonstrated that climate change involving sea ice retreat and anthropogenic O₃ ⁻ induced iodine emissions will produce greatly increased biogenic gaseous iodine emissions. ¹¹ These findings were calculated on the basis of changes in biogenic iodine release in previous earth warming cycles, which in preindustrial times did not involve the additive effects of anthropogenic O₃ ⁻, which would result in an even greater change. ¹¹

From historic data these workers hypothesized a near future scenario with the highest iodine levels in 127,000 years on the basis of lower Arctic ice core iodine values 3.8 and 5.1 μgm⁻² per year in the last glacial cycle increasing to a maximum of 9.6 μgm⁻² per year when the sea ice retreated. The consequence of such increases in global iodine levels, both atmospheric and deposited in rain, remains unknown. The contribution to respiratory intake of increased atmospheric iodine is more difficult to assess, although higher urinary iodine levels occurred in those communities living adjacent to an abundant seaweed growth over which higher volatile iodine levels were observed.

The calculated daily iodine intake based on atmospheric I₂ levels recorded in the seaweed abundant area would range from 3.8 to 19.6 μg/day. ⁶ This effect, together with increased iodine deposition in rain would be expected to be greater in an atmosphere enriched in iodine. Therefore, the generation of volatile iodines, by reducing atmospheric O₃ ⁻ pollution can alleviate the damaging effects of global warming ¹² and could have a paradoxically beneficial consequence of improvements in iodine status and thyroidal health.