ACUTE EFFECTS

a. The effects of a radiation exposure may be grouped into two categories-- those appearing within days or weeks and those developing over a period of months to years. Effects appearing early result from massive cell-killing and as a group are associated into the acute radiation syndrome. In man, this rapid depletion of radiosensitive cells produces some effects within hours of the exposure and is usually resolved during the first month after the exposure. A convenient method of describing these effects is through the time course of their development.

b. Prodrome. Occurring in the first 24 to 48 hours after an exposure to ionizing radiation, nausea, vomiting, fatigue, and malaise are symptoms frequently expressed by the victim. As a group, these symptoms are rarely observed in individuals exposed to less than 50 rems but are almost always seen in those exposed to a dose greater than 200 rems. During this period of prodrome, the blood count will change in response to the radiation dose, the severity of the change depends on the actual dose. Fifty rems are considered a minimum dose for the appearance of changes in the blood count.

c. Acute radiation syndrome. After the prodromal period and a period of latency whose duration will depend upon the dose, occurs the period of manifest illness. Beginning toward the end of the first week following the exposure, a person receiving a modest dose of ionizing radiation will experience some continued nausea, diarrhea,inflammation of the throat, and loss of appetite. At a more severe dose (viz., usually greater than 200 rems), the individual will experience bloody diarrhea, fever, bruising (of the gums in particular), and hair loss. The loss of hair is a good indicator of this level of exposure. At doses beyond which survival is not usually possible (e.g., greater than 1,000 rems), the degree of development of the above symptoms will be more severe and will show no signs of dissipation. Death usually occurs during the second or third week following the exposure.

d. Although the signs and symptoms listed above will apply for the average human with a uniform exposure, there are several factors that will affect the individual's response to the radiation. The dose, type of radiation, area exposed, sex, age, and general health status are each important modifiers of the response. For example, factors relevant to the bone marrow injury are the dose, quality of the radiation, and the uniformity of the irradiation. This is a result of the location of the bloodforming organ within the body. Long bones, the sternum, costals, and cranium are all active sites of marrow production. Any single area not exposed will normally provide sufficient marrow reserve. Also, because active sites of marrow production change during development and aging, age becomes another important determinant. The intestinal lining, a second site critical for the development of radiation injury, can be affected by a highly localized dose. For this reason, radiation quality and depth of penetration are important to this type of injury. X-rays and gamma radiation have great penetrating ability and can affect vital organs.

e. The dose at which 50 percent lethality occurs is measured and quoted as the LD 50/30. For most mammals this dose is approximately 500 rems; for humans it is 350 to 450 rems. The outcome for a particular exposure cannot be accurately predicted. However, the LD 50 (50% of deaths) for man is believed to be somewhere between 200 and 500 rads. This is usually quoted as 450 rads.