of ion-thrust technology, particularly with smaller-scale space vehicles. according to the european space agency, "this small but potentially disruptive innovation could help to clear the skies of space junk, by enabling tiny satellites to self-destruct cheaply and easily at the end of their missions, by steering themselves into the atmosphere where they would burn up." in early 2021, the french group thrustme performed an in-orbit demonstration of an electric-powered ion thruster for spacecraft, where iodine was used in lieu of xenon as the source of plasma, in order to generate thrust by accelerating ions with an electrostatic field. medicine main article: iodine (medical use) elemental iodine elemental iodine is used as an antiseptic either as the element, or as the water-soluble triiodide anion i3− generated in situ by adding iodide to poorly water-soluble elemental iodine (the reverse chemical reaction makes some free elemental iodine available for antisepsis). elemental iodine may also be used to treat iodine deficiency. in the alternative, iodine may be produced from iodophors, which contain iodine complexed with a solubilizing agent (the iodide ion may be thought of loosely as the iodophor in triiodide water solutions). examples of such preparations include: tincture of iodine: iodine in ethanol, or iodine and sodium iodide in a mixture of ethanol and water. lugol's iodine: iodine and iodide in water alone, forming mostly triiodide. unlike tincture of iodine, lugol's iodine has a minimised amount of the free iodine (i2) component. povidone iodine (an iodophor). iodine-v: iodine (i2) and fulvic acid form a clathrate compound (iodine molecules are "caged" by fulvic acid in this host-guest complex). a water-soluble, solid, stable, crystalline complex. unlike other iodophors, iodine-v only contains iodine in molecular (i2) form. the antimicrobial action of iodine is quick and works at low concentrations, and thus it is used in operating theatres. its specific mode of action is unknown. it penetrates into microorganisms and attacks particular amino acids (such as cysteine and methionine), nucleotides, and fatty acids, ultimately resulting in cell death. it also has an antiviral action, but nonlipid viruses and parvoviruses are less sensitive than lipid enveloped viruses. iodine probably attacks surface proteins of enveloped viruses, and it may also destabilise membrane fatty acids by reacting with unsaturated carbon bonds. other formulations before the advent of organic chelating agents, salts of iodide were given orally in the treatment of lead or mercury poisoning, such as heavily popularized by louis melsens and many nineteenth and early twentieth century doctors. in medicine, a saturated solution of potassium iodide is used to treat acute thyrotoxicosis. it is also used to block uptake of iodine-131 in the thyroid gland (see isotopes section above), when this isotope is used as part of radiopharmaceuticals (such as iobenguane) that are not targeted to the thyroid or thyroid-type tissues. iodine-131 (usually as iodide) is a component of nuclear fallout, and is particularly dangerous owing to the thyroid gland's propensity to concentrate ingested iodine and retain it for periods longer than this isotope's radiological half-life of eight days. for this reason, people at risk of exposure to environmental radioactive iodine (iodine-131) in fallout may be instructed to take non-radioactive potassium iodide tablets. the typical adult dose is one 130 mg tablet per 24 hours, supplying 100 mg (100,000 micrograms) of ionic iodine. (the typical daily dose of iodine for normal health is of order 100 micrograms; see "dietary intake" below.) ingestion of this large dose of non-radioactive iodine minimises the uptake of radioactive iodine by the thyroid gland. diatrizoic acid, an iodine-containing radiocontrast agent as an element with high electron density and atomic number, iodine absorbs x-rays weaker than 33.3 kev due to the photoelectric effect of the innermost electrons. organoiodine compounds are used with intravenous injection as x-ray radiocontrast agents. this application is often in conjunction with advanced x-ray techniques such as angiography and ct scanning. at present, all water-soluble radiocontrast agents rely on iodine-containing compounds. others the production of ethylenediamine dihydroiodide, provided as a nutritional supplement for livestock, consumes a large portion of available iodine. another significant use is a catalyst for the production of acetic acid by the monsanto and cativa processes. in these technologies, which support the world's demand for acetic acid, hydroiodic acid converts the methanol feedstock into methyl iodide, which undergoes carbonylation. hydrolysis of the resulting acetyl iodide regenerates hydroiodic acid and gives acetic acid. inorganic iodides find specialised uses. titanium, zirconium, hafnium, and thorium are purified by the van arkel–de boer process, which involves the reversible formation of the tetraiodides of these elements. silver iodide is a major ingredient to traditional photographic film. thousands of kilograms of silver iodide are used annually for cloud seeding to induce rain. the organoiodine compound erythrosine is an important food coloring agent. perfluoroalkyl iodides are precursors to important surfactants, such as perfluorooctanesulfonic acid. the iodine clock reaction (in which iodine also serves as a test for starch, forming a dark blue complex), is a popular educational demonstration experiment and example of a seemingly oscillating reaction (it is only the concentration of an intermediate product that oscillates). although iodine has widespread roles in many species, agents containing it can exert a differential effect upon different species in an agricultural system. the growth of all strains of fusarium verticillioides is significantly inhibited by an iodine-containing fungistatic (aj1629-34ec) at concentrations that do not harm the crop. this might be a less toxic anti-fungal agricultural treatment due to its relatively natural chemistry. 125i is used as the radiolabel in investigating which ligands go to which plant pattern recognition receptors (prrs). biological role main article: iodine in biology the thyroid system of the thyroid hormones t3 and t4 comparison of the iodine content in urine in france (in microgramme/day), for some regions and departments (average levels of urine iodine, measured in micrograms per liter at the end of the twentieth century (1980 to 2000)) iodine is an essential element for life and, at atomic number z = 53, is the heaviest element commonly needed by living organisms. (lanthanum and the other lanthanides, as well as tungsten with z = 74 and uranium with z = 92, are used by a few microorganisms.) it is required for the synthesis of the growth-regulating thyroid hormones thyroxine and triiodothyronine (t4 and t3 respectively, named after their number of iodine atoms). a deficiency of iodine leads to decreased production of t3 and t4 and a concomitant enlargement of the thyroid tissue in an attempt to obtain more iodine, causing the disease known as simple goitre. the major form of thyroid hormone in the blood is thyroxine (t4), which has a longer half-life than t3. in humans, the ratio of t4 to t3 released into the blood is between 14:1 and 20:1. t4 is converted to the active t3 (three to four times more potent than t4) within cells by deiodinases (5'-iodinase). these are further processed by decarboxylation and deiodination to produce iodothyronamine (t1a) and thyronamine (t0a'). all three isoforms of the deiodinases are selenium-containing enzymes; thus dietary selenium is essential for t3 production. iodine accounts for 65% of the molecular weight of t4 and 59% of t3. fifteen to 20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of all iodine in the body is found in other tissues, including mammary glands, eyes, gastric mucosa, fetal thymus, cerebro-spinal fluid and choroid plexus, arterial walls, the cervix, and salivary glands. during pregnancy, the placenta is able to store and accumulate iodine. in the cells of those tissues, iodide enters directly by sodium-iodide symporter (nis). the action of iodine in mammary tissue is related to fetal and neonatal development, but in the other tissues, it is (at least) partially unknown. dietary intake the daily levels of intake recommended by the united states national academy of medicine are between 110 and 130 µg for infants up to 12 months, 90 µg for children up to eight years, 130 µg for children up to 13 years, 150 µg for adults, 220 µg for pregnant women and 290 µg for lactation. the tolerable upper intake level (ul) for adults is 1,100 μg/day. this upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone. the thyroid gland needs no more than 70 μg/day to synthesise the requisite daily amounts of t4 and t3. the higher recommended daily allowance levels of iodine seem necessary for optimal function of a number of body systems, including lactation, gastric mucosa, salivary glands, brain cells, choroid plexus, thymus, and arterial walls. natural sources of dietary iodine include seafood, such as fish, seaweeds (such as kelp) and shellfish, dairy products and eggs so long as the animals received enough iodine, and plants grown on iodine-rich soil. iodised salt is fortified with iodine in the form of sodium iodide. as of 2000, the median intake of iodine from food in the united states was 240 to 300 μg/day for men and 190 to 210 μg/day for women. the general us population has adequate iodine nutrition, with women of childbearing age and pregnant women having a possible mild risk of deficiency. in japan, consumption was considered much higher, ranging between 5,280 μg/day to 13,800 μg/day from dietary seaweed or kombu kelp, often in the form of kombu umami extracts for soup stock and potato chips. however, new studies suggest that japan's consumption is closer to 1,000–3,000 μg/day. the adult ul in japan was last revised to 3,000 µg/day in 2015. after iodine fortification programs such as iodisation of salt have been implemented, some cases of iodine-induced hyperthyroidism have been observed (so-called jod-basedow phenomenon). the condition seems to occur mainly in people over forty, and the risk appears higher when iodine deficiency is severe and the initial rise in iodine intake is high. deficiency main article: iodine deficiency in areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten, iodine deficiency gives rise to hypothyroidism, symptoms of which are extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures. iodine deficiency is the leading cause of preventable intellectual disability, a result that occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element. the addition of iodine to table salt has largely eliminated this problem in wealthier nations, but iodine deficiency remains a serious public health problem in the developing world today. iodine deficiency is also a problem in certain areas of europe. information processing, fine motor skills, and visual problem solving are improved by iodine repletion in moderately iodine-deficient children. precautions toxicity iodine hazards ghs labelling: pictograms signal word danger hazard statements h312, h315, h319, h332, h335, h372, h400 precautionary statements p261, p273, p280, p305, p314, p338, p351 nfpa 704 (fire diamond) 3 0 0 elemental iodine (i2) is toxic if taken orally undiluted. the lethal dose for an adult human is 30 mg/kg, which is about 2.1–2.4 grams for a human weighing 70 to 80 kg (even if experiments on rats demonstrated that these animals could survive after eating a 14000 mg/kg dose). excess iodine can be more cytotoxic in the presence of selenium deficiency. iodine supplementation in selenium-deficient populations is, in theory, problematic, partly for this reason. the toxicity derives from its oxidizing properties, through which it denaturates proteins (including enzymes). elemental iodine is also a skin irritant. direct contact with skin can cause damage, and solid iodine crystals should be handled with care. solutions with high elemental iodine concentration, such as tincture of iodine and lugol's solution, are capable of causing tissue damage if used in prolonged cleaning or antisepsis; similarly, liquid povidone-iodine (betadine) trapped against the skin resulted in chemical burns in some reported cases. occupational exposure people can be exposed to iodine in the workplace by inhalation, ingestion, skin contact, and eye contact. the occupational safety and health administration (osha) has set the legal limit (permissible exposure limit) for iodine exposure in the workplace at 0.1 ppm (1 mg/m3) during an 8-hour workday. the national institute for occupational safety and health (niosh) has set a recommended exposure limit (rel) of 0.1 ppm (1 mg/m3) during an 8-hour workday. at levels of 2 ppm, iodine is immediately dangerous to life and health. allergic reactions some people develop a hypersensitivity to products and foods containing