Answer to Question #10463 Submitted to "Ask the Experts"

I work in a nuclear pharmacy where ¹³¹I is dispensed in a negative pressure room. The work stations and storage cabinets in this room are exhausted by an extraction system which pulls the air from the room at approximately 28.3 m^-3 per minute through a scrubber system (charcoal filters) and then released into the environment. While the release into the environment falls well within the maximum detectable activity levels of Table 2 guidelines of 10 CFR, I do have a question. Is it possible for the charcoal filters to release the trapped ¹³¹I over time?  

The short answer to your question is “Yes, it is possible for charcoal retention systems to release trapped iodine.” The answer deserves some elaboration, however. I note that your question refers to “charcoal filters,” I assume that these are not really filters but actual beds of charcoal that contain a significant volume of activated charcoal. Such systems are common for the exhaust cleanup function that you describe.

Activated charcoal has a great tenacity for elemental iodine, and is generally very efficient for its collection and retention, especially under usual environmental conditions with temperatures less than about 80^o C and humidities generally less than about 70 percent. A notable advantage for ¹³¹I is that it has a relatively short half-life of about eight days; the pertinent implication of this is that the ¹³¹I is likely to decay in the charcoal bed before it might have a chance to be released.

As temperatures and/or humidity increase, the possibility of release and/or reduced retention of iodine increases. High humidity means that more water-vapor molecules are available to compete with iodine for available active sites in the charcoal. Elevated temperatures result in higher gaseous molecule mobility that can reduce initial retention and increase mobility of previously captured iodine in the charcoal, and this can be gradually moved downstream by the airflow. The penetration of iodine through the charcoal bed may also be increased by increasing the linear flow rate of air through the charcoal bed. The manufacturer of the particular charcoal system that you are using should have recommendations as to the recommended range of air flow rates.

Another consideration that affects the retention of radioiodine in the charcoal is the chemical form of the iodine. While elemental iodine vapor, I₂, is very strongly retained, many organic forms of iodine, such as methyl iodide, CH₃I, are less strongly retained and may be collected with less efficiency and be released more readily than the elemental form. Retention of such forms can be enhanced by using charcoal that has been impregnated with particular chemicals, such as triethylenediamine (TEDA).

Typically, facilities that have the potential for release of radioiodines use duct or stack monitoring to ensure that exhaust releases are within acceptable limits. If your pharmacy is using quantities of radioiodine typical in performing iodinations, I would expect that such monitoring would be in place. It usually involves having a sampling device, often consisting of a high-efficiency particulate filter backed up by a small charcoal cartridge held in an appropriate holder in place downstream of the charcoal cleanup system in the duct or release stack. Regular analysis of the filter and charcoal cartridge for radioiodine would show whether there was any problem with radioiodine penetrating the charcoal cleanup system.

I hope this addresses your concerns.

George Chabot, PhD