Answer to Question #4491 Submitted to "Ask the Experts"

In the analysis for determining what constitutes an minimally negligible dose for security screening with x rays, how would a single scan of 1-2 µSv fit into this determination? The current basis is 0.1 µSv, so how much more is a risk of 1 µSv  if used less than 250 times annually?

The Negligible Individual Dose (NID) was not set specifically for security screening. It was set for general radiation protection purposes and is the dose below which efforts to reduce radiation exposure to an individual is unwarranted. The number is actually is 0.01 mSv (or 1 mrem or 10 µSv). It was introduced by the National Council on Radiation Protection and Measurements in 1987 (Report 91). It is based on an annual excess risk of 1 x 10^-7 per year for fatal health effects attributable to radiation.

For comparisons, the normal background risk for a health person of dying of cancer is about 1 in 4 or 5 or 20 to 25% in our lifetime (American Cancer Society). When we add the additional risk from the 0.01 mSv, this increases the background number to roughly 25.0007% (if we use the rate of 1 in 4).

So, to your questions:

• How would a single scan of 1-2 µSv fit into this determination? This is below the NID.
• How much more is a risk of 1 µSv used 250 times annually (or 250 µSv)? Using the excess risk level of 1 x 10^-7 for 10 µSv and multiplying that by 25 gives us 25 x 10^-7 for an annual risk factor. Over your working lifetime (say 30y) and adding to the natural incidence of fatal cancer (25%), your lifetime risk becomes 25.0075%.

This estimate is conservative because I take into account only the natural incidence of fatal cancer. Essentially, there is no increased risk.

Kelly Classic
Certified Medical Health Physicist