Tepco employees not only contend with airborne particulate sources of radioactivity that occupy the air invisibly, but with permanent structures that emit radiation as well. The problematic reactors are actually fixed point sources of radioactivity with variable intensities from day to day, and they can quickly gobble up an employees available cumulative dose if time management near a reactor is not properly employed. The following is an example of how a desired dose rate may require a considerable distance from a source.
Shielding offers further reductions in exposure. An 16cm thick wall of concrete has the same effect as doubling ones distance from the source, resulting in a 75% reduction in dose.