Iodine in biology air should not exceed 1 mg/m³ (eight-hour time-weighted average). When mixed with ammonia and water, elemental iodine forms nitrogen triiodide, which is extremely shock-sensitive and can explode unexpectedly. Excessive iodine intake presents symptoms similar to those of iodine deficiency. Commonly encountered symptoms are abnormal growth of the thyroid gland and disorders in functioning, as well as in growth of the organism as a whole. Iodide toxicity is similar to (but not the same as) toxicity to ions of the other halogens, such as bromides or fluorides. Excess bromine and fluorine can prevent successful iodine uptake, storage and use in organisms,
Iodine be primordial. As such, iodine is monoisotopic and its atomic weight is known to great precision, as it is a constant of nature. The longest-lived of the radioactive isotopes of iodine is iodine-129, which has a half-life of 15.7 million years, decaying via beta decay to stable xenon-129. Some iodine-129 was formed along with iodine-127 before the formation of the Solar System, but it has by now completely decayed away, making it an extinct radionuclide that is nevertheless still useful in dating the history of the early Solar System or very old groundwaters, due to its mobility in the environment.