"What is the threat to Tokyo residents from contaminated tap water?"
Officials said the tap water showed 210 becquerels per litre of iodine-131 - more than twice the recommended limit of 100 becquerels per litre for infants. The recommended limit for adults is 300 becquerels. (Becquerels is a measure of radiation emmitted whereas millisieverts is a measure of dosage on the body).
Professor Richard Wakeford says that infants who drank water contaminated at the Japanese limit of 100 becquerels for a year would effectively receive a dose of 0.4 millisieverts of radiation.
Adults who drank water contaminated at the Japanese limit of 300 becquerels for a year would effectively receive a dose of 0.3 millisieverts of radiation.
In comparison, the average person is exposed to 2.5 millisieverts from natural background radiation in a year.
In theory, drinking water contaminated at 210 becquerels for a year would cause a very small additional risk of cancer - but in practice nothing more than you could expect to get from normal background levels of radiation.
Is it safe to bathe or wash food in contaminated tap water?
Yes, because only small amounts of water would be ingested into the body from these practices.
So, be calm now... :D...