Answer to Question #2514 Submitted to "Ask the Experts"

Q

As emergency response personnel (Hazardous Materials Response Team for Salt Lake City Fire Department) we often wonder at what reading of our detection meters should we consider further approach to an emergency scene involving radioactive materials unsafe In other words, when do we decide to continue investigation or to run away? (We have no other protection from radiation but time, distance, and common materials shielding.) How do we determine a perimeter of the "Hot Zone" at such an incident?

A

Under state and federal regulations, any area in which you can receive a radiation dose in excess of 5 mrem in an hour must be controlled as a radiation area and any area in which you can receive a dose in excess of 100 mrem in an hour must be controlled as a high radiation area. Both of these dose limits are considered safe in that the risk of radiation injury or sickness from this level of exposure is no more risky than many other activities we undertake routinely (such as driving or going to work). You should wear some sort of radiation dosimetry to enter any radiation area.

If you are a radiation worker, you are permitted to receive an annual dose of 5 rem. Nonradiation workers are limited to 100 mrem per year. However, in extraordinary circumstances (such as a radiological emergency) personnel may receive up to 25 rem as a once-in-a-lifetime event. This is called a Planned Special Exposure (PSE). Even this level of exposure is not considered harmful or overly risky—certainly not as risky as entering an area in which an explosion has just taken place.

When considering what your instrument readings mean, you have to consider not only dose rate, but the overall dose that will be accumulated while you're in a radiation area. For example, spending six minutes (0.1 hours) in a 100 rem/hr area will give you a lower dose than spending an hour in a 20 rem/hr area (10 rem versus 20 rem). You also need to consider whether or not you can trust your detector reading—a GM "pancake" probe is meant to be used to measure contamination levels and will probably give inaccurate radiation level readings. And, since many Geiger counters are calibrated with a cesium-137 source, they may give incorrect radiation level readings if they are used to measure cobalt-60, iridium-192, or any isotope other than cesium-137. It is better to use an ion chamber, which directly measures radiation exposure and can be used without having to interpret the results.

Finally, as an emergency responder, you need to know that contaminated victims do not pose a health risk to you or your colleagues. The contamination can make a mess, and your equipment or vehicles might need to be decontaminated, but you are not going to get sick from saving the life or treating even a heavily contaminated person.

The bottom line is that a dose even as high as the 25 rem PSE limit is not going to cause harm to you. So your planning should center around how to save and treat the largest number of people without exceeding this dose, and accepting the fact that you will probably have to decontaminate yourself, your equipment, and your vehicles after you're done. A small price to pay, I think.

Andrew Karam, CHP, PhD