The phrase at the end of the passage is useful because it indicates that studies have connected increased dosage with increased harm. The word “however” creates a shift to this assertion, thus the portion of the passage before the assertion must imply the opposite—that the radiation has a positive effect. Blank (i) can be susceptible to (vulnerable to), since if the doses make people less susceptible to something, the effect is positive. Blank (ii) continues this direction, so salubrious (healthful) would fit, and blank (iii) is being criticized by the passage’s argument, so it could be acts as a tonic (aid).