But a cow that lives there and drinks water with low concentrations of iodine will eventually accumulate and concentrate these materials and become unsafe for consumption.
For example, the new EPA guide refers to International Atomic Energy Agency guidelines that suggest intervention is not necessary until drinking water is contaminated with radioactive iodine 131 at a concentration of 81, 000 picocuries per liter.
Tokyo residents were warned on Wednesday not to give tap water to babies less than a year old because levels of radioactive iodine - which can cause thyroid cancer - are twice the recommended safe level in some areas of the city.
By April 6th TEPCO had managed to staunch the leakage of highly contaminated water from one of the damaged reactors that had produced levels of radioactive iodine 7.5m times the legal limit in one sample of seawater.
Milk samples from Phoenix and Los Angeles contained iodine-131 at levels roughly equal to the maximum contaminant level permitted by EPA in drinking water, the data shows.