Answer to Question #1626 Submitted to "Ask the Experts"

Q

Can you explain the Annual Limit on Intake (ALI) that is listed by the NRC? Does this mean that a worker can ingest the ALI quantity each year and after 50 years the worker will have a total dose of 5 rem? What do they mean (on the NRC Web site) regarding the ALI derived for either 5 rem (stochastic) or 50 rem (nonstochastic)?

A

You are exactly right about the general interpretation. If a worker takes in one ALI, (s)he will receive an effective whole body dose of 5 rem, integrated over the next 50 years. In the vast majority of cases, this dose is all received in the year of intake, but in some cases involving materials that clear slowly from the body and have long physical half-lives, the dose may be spread out over the 50-year period. The terms "stochastic" and "nonstochastic" refer to whether the ALI was determined based on the effective whole-body dose (derived by multiplying actual organ doses by risk weighting factors), which has a dose limit of 5 rem, or from a dose to a single organ, whose limit is 50 rem. So, an ALI may commit you to 5 rem effective whole body or 50 rem to a single organ (whichever is smaller) over the 50 years following intake. Michael G. Stabin, PhD, CHP