The absorbed dose describes the amount of radiation absorbed by an object or person, i.e., the amount of energy that radioactive sources deposit in materials through which they pass. The units for absorbed dose are the radiation absorbed dose (rad), or better: the gray (Gy) unit.
One gray dose is equivalent to one joule radiation energy absorbed per kilogram of organ or tissue weight. Rad is the old and still used unit of absorbed dose. One gray is equivalent to 100 rads.
1 Gy = 100 rads
Equal doses of all types of ionizing radiation are not equally harmful. Alpha particles, for example, produce greater harm than do beta particles, gamma rays and X rays for a given absorbed dose. Alpha radiation is the most dangerous because it is easily absorbed by cells. Beta, gamma an X radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it (source: BBC).
To account for this difference, radiation dose is expressed as equivalent dose in units of sievert (Sv). The dose in Sv is equal to absorbed dose" multiplied by a radiation weighting factor (WR - see Table 1 below). Prior to 1990, this weighting factor was referred to as Quality Factor (QF).
Note that the heaviness of the alpha particle makes its penetration rates very low. Hence, it will not easily penetrate through the outer layers of the skin.
Table 1. Recommended Radiation Weighting Factors. source: Canadian Centre for Occupational Health and Safety
Now to your question:
Ionizing radiation has more energy than non-ionizing radiation. Ionizing radiation can cause chemical changes by breaking chemical bonds and damage living tissues.
Shorter wavelength UV begins to have enough energy to break chemical bonds. X-ray and gamma ray radiation, which are at the upper end of the EM spectrum, have high frequencies (~100 billion billion hertz). Radiation in this range has high energy and it can strip electrons from an atom or even break up the nucleus of the atom.
The process in which an electron is given enough energy to break away from an atom is called ionization. It results in the formation of two ions: the molecule with a net positive charge, and the free electron with a negative charge. Each ionization releases a large amount of energy which is absorbed by the surrounding tissues.
There are three main kinds of ionizing radiation:
Alpha particles, which include two protons and two neutrons; beta particles, which are essentially electrons; and gamma rays and x-rays are photons. Alpha particles, beta particles and neurons are not part of the EM as they are energetic particles as opposed to pure energy bundles (photons) (source: ARPANSA Australia).
So in the EM realm, frequency is proportional to energy and hence to tissue damage (Fig. 2) and hence to rads.
Fig. 2. EM spectrum. source: ARPANSA