Plume forecasts

Erin, you are absolutely right. I was sloppy in what I said in that earlier post that "it is not very dangerous because your body doesn't take it up." I meant that it is not very dangerous because its biological half-life is so short and its internal dose is tiny compared to its external dose, and its external dose is also tiny compared to other isotopes.

As the studies you cited point out, Xe-133 is indeed taken up by the body since it is routinely used as a tracer in medical studies, especially of lung function. As with any gas, it will reach equilibrium with the blood if inhaled over a period of time. Xenon appears to concentrate more in fatty tissues.

Other elements, such as iodine and cesium, are more chemically active and have very long biological half-lives. Iodine-131 has a biological half-life of 138 days, which is much longer than its radiological half-life of 8 days. So its effective half-life in the body is approximately 8 days -- i.e., essentially all of its radiation is deposited inside the body. Cesium-137 has a biological half-life of 110 days, which is much shorter than its radiological half-life of 30 years, so its effective half-life in the body is 109 days.

However xenon, like all noble gases, is not very chemically active and as a result will not remain in the body very long. The reference for those other biological half-lives says Xe-133 has a biological half-life of only "a few minutes". To get a better number, there is journal article I found about this: Aitken, et al. "Estimates of Radiation Absorbed Doses from Radioxenons in Lung Imaging", Journal of Nuclear Medicine 21:459-465. In it the authors list the "biological disappearance half-time" of xenon. The majority of the xenon that has been absorbed (78%) has a 21.7 second half-time, while smaller fractions have longer half-times -- the longest is 10.5 hours (2.4% of the xenon). So the overall timescale of xenon leaving the body is on average only a few minutes, but a lot is gone within only seconds.

Another point about Xe-133 is that health physicists have quantified the amount of internal dose (from inhalation) and external dose (from the gas being around you) for Xe-133 gas, and internal dose is insignificant compared to the external dose. From this answer to a question about dose from noble gases on the Health Physics Society website:

"In the International Commission on Radiological Protection (ICRP) Publication 30 (Pergamon Press 1979), the authors show that for the noble gases the radiation dose to a person's body from external radiation... is more than 130 times higher than the dose from any gas contained in the person's lungs, and more than 200 times higher than any gas which may have crossed over into the body tissues. So they provide only dose factors that give the external radiation dose per unit time (Sv/h) per unit radioactivity in the air (Bq/m^3). Similar factors are given by the International Atomic Energy Agency in their 'Safety Series 115' report and are recommended for use in radiation protection in these circumstances."

This is reflected in the Nuclear Regulatory Commission's Table of dose limits for Xe-133. The table only lists a DAC (derived air concentration) not an ALI (annual limit on intake). The DAC is calculated from an external dose limit -- i.e., what exposure are you getting from the air around you -- while the ALI refers to internal dose through ingestion or inhalation -- this is what we use in our calculations for dose from water that contains I-131 and Cs-137, for example. The outside dose is the only important effect if Xe-133 is present in the air.

By the way, comparing the DAC for Xe-133 to that of I-131, we see that Xe-133's DAC is 5,000 times that of I-131. So compared to I-131, Xe-133 is that much less dangerous if present in the air around you.

So the bottom line is: Xe-133 does not stay in the body very long, and it's not very dangerous even if it's in the air around you.

Mark [BRAWM Team Member]