Why dose rate matters

Problem

Why can a person receive $100\ \text{mSv}$ spread over the course of a year without measurable harm, while the same dose of $100\ \text{mSv}$ received in a few minutes can be clinically significant? The total dose is the same: what makes the difference?

Solution

Total dose versus dose rate. The quantity that matters for biological effects is not just the total accumulated dose, but also the dose rate, i.e. the dose per unit time. The same amount of energy deposited in tissue produces very different effects depending on how quickly it is released.

Dose spread over time. If the $100\ \text{mSv}$ arrive over an entire year, cells have time to repair DNA damage using their own enzymatic mechanisms between exposures, and the body can replace the few cells that are irreparably damaged. The damage stays diluted and its effects are negligible.

Concentrated dose. If the same $100\ \text{mSv}$ arrive within a few minutes, a large number of cells are damaged simultaneously: the repair systems are overwhelmed because they cannot keep pace. The result is acute damage to many tissues at once, with possible clinical effects.

Analogy. It is like sun exposure: a few minutes a day for weeks does not burn, but the same total exposure concentrated into a single day causes sunburn. What matters is the rate at which the damage accumulates relative to the capacity for cellular repair.

$\ev{\text{dose rate matters: slow dose} \Rightarrow \text{time for cellular repair}}$

Collegamenti

Argomenti: Fisica nucleare Concetti: Decadimento radioattivo