Why are government recommendations about radiation exposure made on an amount-per-year scale?

Answer 1

They aren't always. There are exposure limits for other time periods as well. Brazil, for example, has a 5-year limit of exposure (which is only twice the 1-year limit of exposure), and often workers around nuclear reactors have guidelines for shorter periods in addition to the yearly ones. The issue is that radiation damage is cumulative. A single medical X-ray is not likely to cause any significant adverse effects. However, the technician goes and hides behind a lead shield, because while you might have one or two a year, he's sitting there with the thing two hundred and fifty days a year, multiple X-rays per day. Basically, the body can recover from a single moderate dose better than from a continuous lower dose.