The average amount of cesium in the 141 samples was 2.2 becquerels per kilogram, but three cases had over 10 becquerels of cesium per kilogram, Kyodo News reported.
An average of about 64 becquerels of radioactive potassium, which is found naturally in the environment, was also found in the those who tested positive for cesium.
"The level of cesium is lower than that of potassium, and it definitely has no effect on the human body," said Hideaki Karaki, honorary professor of food safety at the University of Tokyo. "But we still need to know how cesium entered the infants' bodies."
The study was conducted by the Isotope Research Institute from November to January. Urine samples were taken from 2,022 infants and kids up to 7 years old. Some 1,881 infants and kids surveyed did not have cesium in their systems.