Sievert is the amount of biological damage in the human body caused by radiation. It was named after the Swede Rolf Sievert, a radiologist who pioneered the biological impact of radiation. He invented a device to measure radiation dosage, the so-called Sievert chamber. In the 1940s and 1950s, the Sievert unit related to a specific dosage of radiation. It measures the equivalent and effective dose by applying a weighting factor specific to each type of radiation and organ. The dosage relates to the strength of the source, the distance from the source and the duration of the exposure. This is because alpha particles, for example, are much more dangerous to human tissue than gamma rays at the same dosage level.
Sieverts are often measured in thousands or even millions. According to the US Environmental Protection Agency, normal radiation exposure equals 6.2 millisieverts per year. Other research suggests that an adult receives around 2.7 millisieverts per year of radiation exposure from background sources as well as medical diagnostics and treatments. The exposure resulting from a transatlantic flight, for instance, is approximately 0.07 millisieverts, and 0.02 for a chest X-ray.
Nuclear accidents and exposure to nuclear waste of course generates much higher levels of radioactivity. During the Chernobyl catastrophe, radiation levels in the reactor control room reached some 300 sieverts per hour, a dosage that would lead to death in a matter of minutes. Even now, almost thirty years after the incident, radiation levels in the reactor hall are still as high as 35 sieverts an hour, a dose that could kill an adult in less than 20 minutes.
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