Answer to Question #11305 Submitted to "Ask the Experts"

Category: Radiation Effects — Effects on Tissues and Organs

The following question was answered by an expert in the appropriate field:

Q

Are radiation effects dependent on the dose rate? For example, would an acute dose of 2 mSv have the same radiation risk if it was spread over a longer time period, like a year? Or would a 1 mSv dose received over a year be the same as receiving it all at one time?

A

This is a good question. An acute radiation dose, a dose over a very short period of time (an acute dose), may or may not be considered more harmful than a radiation dose spread out over a longer period of time, like a year. To explain this two-sided answer, see the following.

There are effects that are seen from very large radiation doses (around 500 mSv and more). These effects are damage to the body that can be observed as physical symptoms in a person. Symptoms can include nausea, vomiting, blood count drops, hair loss, or even skin burns. These symptoms occur within days or weeks of a high-level acute exposure. However, the body continually repairs itself, like getting over a stomach virus or healing a burn. So getting a radiation dose that is spread out over time allows the body to heal itself, and therefore an acute radiation dose is more severe.

It is different for low doses of radiation, where no physical symptoms are seen shortly after exposure. The long-term effects of low doses of radiation are called "stochastic." These are effects that may raise the chance or probability of some condition later in life. The prime condition of this type is a very slight increase in the probability of developing cancer. We all have a chance of getting cancer, even if never exposed to radiation. For radiation exposure, if a very large group of people were exposed to a low-level radiation dose, a very few of them might get cancer above what is normally expected. "For radiation protection purposes, it is assumed that the risk(s) of stochastic effects are strictly proportional to the dose without threshold, throughout the range of dose and dose rates of importance in radiation protection" (Hall 1994). In other words, it is conservatively assumed that the radiation dose risk increases with each exposure and is not affected by the dose rate. So, if you double the radiation dose you double the slightly increased risk of getting cancer.

John P. Hageman, MS, CHP

Reference
Hall E. Radiobiology for the radiologist. 4th ed. Philadelphia, PA: Lippincott-Raven; 1994.

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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