Biological Effects of Radiation and Radiation Protection
Core concepts
Damage begins with ionization of water and DNA. Effects are deterministic (threshold dose, severity increases with dose: erythema, cataract, sterility, acute radiation syndrome) or stochastic (no threshold, probability increases with dose: cancer, heritable effects). Whole-body acute picture: prodromal symptoms may appear near 100 rad (1 Gy); hematopoietic syndrome about 100-500 rad; gastrointestinal about 500-1000 rad; central nervous system above roughly 1000 rad. The LD50/60 for humans without medical support is about 400-450 rad.
Protection framework
ALARA governs every decision: no dose is so small that it need not be reduced. External exposure is controlled by time, distance and shielding; internal exposure by containment and contamination control.
Know the 10 CFR 20 limits: occupational total effective dose equivalent 5 rem/y; lens of the eye 15 rem/y; skin and extremities 50 rem/y; individual member of the public 100 mrem/y; a minor 10 percent of the occupational limit; a declared pregnant worker 500 mrem over the entire gestation. Area postings: radiation area 5 mrem/h, high radiation area 100 mrem/h at 30 cm.
Traps
Mixing rem, mrem and Sv (1 Sv = 100 rem, so 5 rem = 50 mSv). Asserting a threshold for stochastic effects. Forgetting that eye and extremity limits differ from whole-body. Confusing dose with dose rate - 50 mR/h for 2 hours is 100 mrem.
Worked example
A radiographer accumulates 140 mrem in one month. At that pace the annual total is 140 x 12 = 1,680 mrem = 1.68 rem, about one third of the 5 rem limit, and the correct action is still to apply ALARA to reduce it further.