Chernobyl, 10 years after: health consequences.

On April 26, 1986, reactor no. 4 at the Chernobyl (Ukraine) nuclear power plant exploded. Over the next 10 days, considerable quantities of radionuclides were discharged into the atmosphere (table 1). The passage of the radioactive cloud over Europe led to varying degrees of contamination according to region (figures 1 and 2); the most contaminated regions were in southern Belarus, northern Ukraine, and the Bryansk and Kaluga regions of Russia. The heavier particles (e.g., fuel elements) were deposited less than 100 km from the plant, but the more volatile fission products (such as cesium and iodine) were able to travel great distances (1). The biologic effects of ionizing radiation are fairly well known, especially the carcinogenic potential. These effects are a function of the amount of energy absorbed by tissues per unit time (dose and dose rate). At high doses and high dose rates, above dose thresholds that vary for different organs and tissues, ionizing radiation can cause tissue destruction (table 2). Below these thresholds, or at low dose rates, the biologic damage is compatible with cell or tissue survival (causing, for example, DNA mutations or chromosomal alterations) and can be repaired. The effects of doses below 200 mSv at low dose rates are still little understood (8). The existence of a threshold dose below which there is no effect remains controversial. Humanity, it must be remembered, is continuously exposed to natural radiation at a mean rate, worldwide, of 2.4 mSv/year, or approximately 170 mSv for a mean lifetime of 70 years (7).