Tap water in 16 municipalities across six prefectures has been found to contain a level of radioactive iodine higher than the maximum recommended for infants, prefectural government and other sources said.
However, in some of the municipalities where the standard was exceeded, radioactive iodine later returned to a safe level.
The central government recommends that babies younger than 1 year old avoid tap water containing radioactive iodine with more than 100 becquerels per liter, although the government and experts called on the public not to panic as drinking such water for a short period of time is not harmful to health.
No municipality had a level of radioactive iodine exceeding 300 becquerels per liter, the standard set for anybody age 1 and above. The highest level recorded was 298 becquerels per liter in Hitachi, Ibaraki Prefecture, on Wednesday.
Tap water was affected in four other municipalities in Ibaraki Prefecture and seven municipalities in Fukushima Prefecture, as well as Katsushika Ward in Tokyo, Matsudo in Chiba Prefecture, Kawaguchi in Saitama Prefecture and Utsunomiya, Tochigi Prefecture.