The rad does not account for the kind of damage done, only for how much radiation goes in. Neutrons are more dangerous then radiation of the same energy and intensity. To take into account this fact the REM was derived. To find the dose in REMS the dosage in rads is multiplied by a conversion factor that reflects the effectiveness of the kind of radiation causing the damage. 1 Doses in REMS are additive whereas doses in rads are not.
Radioactivity can ionize molecules.
The ionized molecule will then chemically react with adjoining molecules to form new compounds. These fragments and new compounds can alter cellular processes and functions. Even gamma rays can excite electrons out of orbit, leaving behind an ion. If radiation passes through a normal body cell the damage is said to be somatic . If the radiation passes through a sex cell the chromosomes may be damaged.