Jeff McMahon, Contributor:

Yes, David, I know. Very complicated to explain. EPA lumps these gamma and beta emitters together under one collective MCL, so if you’re seeing cesium-137 in your milk or water, the MCL is 3.0 picocuries per liter; if you’re seeing iodine-131, the MCL is 3.0; if you’re seeing cesium-137 and iodine-131, the MCL is still 3.0.

Here’s a somewhat historic EPA document that speaks directly to that issue:

Although not having a 4 mrem per year equivalent level specified in the current drinking water regulations as do tritium and strontium-90, the compliance monitoring scheme indicates that an iodine-131 level of 3 pCi/L is the MCL compliance level (presumably derived from the NBS Handbook); the ANPRM indicates that 700 pCi/L is the 4 mrem/year equivalent.

via Drinking Water Criteria Document for Beta and Gamma Emitting Radionuclides

Be advised that document won’t open in older browsers.

And here’s a current EPA faq that repeats the same 3 pCi/L MCL for iodine-131:

http://www.epa.gov/radiation/japan-faqs.html#rainwater

If we lump together the three radionuclides in that Hilo, Hawaii reading, we get 61 pCi/L. Alarming? How about that recent Boise rainfall sample: 468 pCi/L.

The reason we shouldn’t be alarmed, EPA says, is that these are short-term, temporary exposures and the MCL assumes long-term exposure.