Radiation is divided into two types: ionizing and nonionizing. Nonionizing types include light, radio waves, micro waves and radar. Nonionizing types are generally not tissue-harming.

Ionizing radiation produces immediate chemical effects, or ionization, on human tissue, and include gamma rays, X-rays, and particle bombardment. This type can be used for medical testing, sterilization, weapons, as well as many other uses.

Radiation sickness happens when humans or animals receive too much radiation. Exposure can be one large exposure (acute), or a series of smaller exposures (chronic). Radiation sickness is generally assosiated with acute exposure, and presents a series of characteristic symptoms in a specific order.

The degree of radiation sickness is dependent upon the dose and rate of exposure. Total body exposure of 100 roentgens causes radiation sickness. Total body exposure of 400 roentgens causes radiation sickness and death for half of exposed individuals. 100,000 rads causes almost immediate unconsciousness and death within an hour. In severe cases, death may occur within two to four weeks, those who survive six weeks after a single, large dose of ionization (such as the affect of a nuclear weapon explosion, or exposure to nuclear energy) may generally be expected to recover.

The severety of the syptoms and illness depends on the duration of the exposure, which type of radiation one has been exposed to, the amount of radiation and the body areas that were exposed. Symptoms do not usually occur directly after exposure, the best way to determine the severity of exposure is: the length of time between the exposure and the onset of symptoms, the severity of symptoms, and the severity of change in white blood cell count.

The symptoms of radiation sickness are vomiting (may include vomiting blood), diarrhea, skin burns, fatigue (can include fainting and weakness), dehydration, bleeding (from the nose, mouth, gums and rectum), open skin sores, ulceration (peeling and/or disintegration) of oral mucous membranes, the esophogus and internal tissues, inflammation, and hair loss.

The tissues most affected by ionization are those where the cells are rapidly reproducing. These tissues include blood forming tissues, the gonads, the skin, and tissues of the digestive tract.

Some tissues of the body are more affected by radiation than other tissues, these tissues are said to be more radiosensitive. The self renewal tissues are the most radiosensitive tissues of the body.