The mean concentration of total iodine in drinking water in the USA is 4 µg I L -1 , with a maximal reported value of 18 µg I L -1 . According to a survey conducted in 2005-2006 among 2362 U.S. residents, the median iodine level in tap water they consume was found to be about 4.55 µg I L -1 . found the following range of iodine concentration in tap water in 41 localities in Denmark: 1 -30.2 µg I L -1 .