Environmental context. Perchlorate is an emerging environmental contaminant that has a unique ability to interfere with normal iodine uptake by the human thyroid gland, and thus has the potential to adversely affect normal growth and development of infants and children. In the last decade, perchlorate’s environmental behaviour has been intensely studied in the United States, but has received little attention elsewhere. Recent evidence strongly suggests that perchlorate occurs at low levels naturally, and is ubiquitously present in the human diet. An atmospheric source for this natural occurrence is strongly implicated, and the naturally occurring isotopes of oxygen and chlorine offer considerable promise for unravelling the chemical mechanisms responsible.

Abstract. Salts of perchlorate (ClO₄^–) are widely used in solid rocket propellants, and in a variety of munitions, explosives, and pyrotechnics; it is an emerging environmental pollutant that has caused widespread water contamination in the United States and probably other locales worldwide. Perchlorate interferes with normal iodine uptake by the human thyroid, and may thus lead to a lowered production of key hormones that are needed for proper growth and development. Debate about ‘safe’ levels of perchlorate is being fuelled by considerable evidence of declining iodine intake in many western nations. With the advent of more sensitive analytical methods, perchlorate is being found as a nearly ubiquitous contaminant in water, beverages, fresh produce, and other sources of human exposure. Recent evidence, including isotopic forensics, makes a strong case for more widespread natural occurrence of perchlorate, outside of the long-established occurrence in caliches of the Atacama Desert in Chile. Many questions about this low-level occurrence remain, including the role of microbial metabolism in attenuating the concentrations typically found in surface- and groundwaters.