I've heard talk of this safety threshold for ionizing radiation, whereby if the dose is sufficiently low enough, it would produce no harmful side-effects. Can someone explain to me the reasoning behind this? The way I see it, every single ionizing particle that passes through a cell has a chance of damaging it. The cell-repair mechanism is imperfect, so even a single particle can damage a cell beyond repair, causing cancer. It's not as if at low radiation doses the particles won't have enough energy to penetrate a cell or something right? So no matter how low the dose, there will always be a risk.