Radioactive tissues?

In the first radiological news story of 2012 Bed, Bath, & Beyond announced on January 12 that it had received a shipment of steel tissue holders (manufactured in India) that were contaminated with radioactive cobalt-60. As of this writing it is not quite certain exactly where the Co-60 came from, although there are some guesses (more on this in a moment). First, a little on the health effects of this contamination.

The tissue boxes are radioactive – no doubt about it. The question is whether or not they are sufficiently radioactive to cause health problems. At present I have been having problems finding the results of direct radiation dose measurements and without some numbers all that we have to go by are the assurances that have been printed in press releases. The Nuclear Regulatory Commission and the New York State Department of Health have noted that a person who purchased one of these tissue holders and who had a fairly typical exposure to the holder would receive low doses of radiation – New York State officials were quoted as saying that exposure to one of the boxes for an hour would be the equivalent of a chest x-ray.

I don’t doubt that this statement is factually correct but, as a scientist, I find it a little unsatisfying – I want numbers! In particular, I’d like to know what dose rate was measured from the boxes and at what distance – with that I can figure out the amount of Co-60 in each of the boxes and I can calculate the radiation dose at any distance and for any length of time. One number that I’ve heard – although without attribution – was that the tissue holders were reading less than 10 mrad/hr at a distance of 2-3 feet. This is consistent with the statements made by NRC and NYS officials, but it’s really just a guess.

When we have a situation like this there are any number of ways to go about determining the radiation dose and the risk, but there are two or three that are fairly common approaches. One is to assume the very worst combination of circumstances and to calculate the maximum radiation dose that anyone could possibly receive. If the dose calculated this was is acceptable then we can assume that the reality would certainly be OK. In this case, the very worst set of circumstances would be to assume that somebody holds one of these tissue holders in their lap continuously. So – if we have a tissue holder that’s reading 10 mrad/hr 3 feet away it will read about 100 mrad/hr at a foot and about 1 rad/hr at a distance of 4 inches. The radiation dose to the whole body would be a bit lower than this since most of us are more than 8 inches tall – there might be a fairly high radiation dose to the lap but radiation dose to the whole body would be lower. Even this scenario, however, would expose a person to many rads of radiation each year, and even a single rad annually is ten times the radiation exposure limit for members of the public.

But we have to ask ourselves if this dose – and the assumptions that went into calculating it – are reasonable. How long can we expect that somebody will actually hold a tissue holder and what distance will they keep it from them? I can go by my own apartment, I suppose – I have two boxes of tissues in my apartment, one next to my desk and the other next to my bed. I spend a lot of time in each of thes locations – about 6 hours nightly asleep (and about 2 feet from that tissue box at the head of the bed) and about 4 hours every evening (plus weekends) at the desk at home, with the box of tissues at my feet. If we stick with a dose rate of 10 mrad/hr at a distance of 3 feet as the standard then we can calculate that I’d receive about 100 mrad every day from tissue boxes (if I had two of this particular box in my apartment).

But what I don’t know – what I have no way of knowing – is whether or not my own tissue box placement is the same as everyone else’s, if I get the same amount of sleep as others, if I spend the same amount of time at my desk as others, and so forth. For example, if I had only one box in my bathroom my dose would be a lot lower, and if I lived in a house and not in a small (650 square feet) apartment then my dose would likely be lower yet. This is the problem with any sort of radiation dose calculation – and the risk from exposure to radioactive objects – since we don’t know how every single person will use the objects we often have to choose between a clearly ludicrous scenario that will produce the highest calculated dose or we have to develop a scenario that seems more reasonable, but that might overestimate the dose to some while underestimating the dose to others. Any set of assumptions – from the most pessimistic to the most optimistic – can be questioned. It makes the whole radiological assessment business challenging.

In the case of the Indian tissue box holders – and in the absence of any more solid information – it seems reasonable to take the statements by regulators at face value (I have a lot of professional respect for both New York State and Federal radiation regulators, who are typically quite talented – and no, I don’t work for either of these organizations and I’m not regulated by them!). This would come out to several hundred mrem of radiation exposure annually – less than what is known to cause harm, but still more exposure than we really want the public to receive. Thus, the regulatory comments make sense – while nobody is at immediate risk from these holders it is reasonable to take them off the market, keeping radiation exposure to the public As Low As Reasonably Achievable.

The issue of protecting the public from radiation exposure is important, but it’s not the only issue in this case – there is also the question of how Co-60 found its way into a bunch of tissue box holders in the first place. Co-60 is used in industry and (less frequently) in medicine – sources can range from virtually risk-free to dangerously radioactive. Interestingly, in 2010 an “orphaned” Co-60 source from Delhi University was responsible for 11 hospitalizations and a death when a radioactive source accidentally ended up at a scrap metal yard. Whether Co-60 from this source is what ended up in the metal of the tissue holders is not yet known – but at the least the 2010 incident demonstrates that India suffered one breakdown in their control of radioactive materials in the recent past and there might have been more.

Globally, “orphaned” radioactive sources have caused problems in the past and they are likely to continue doing so for years to come. The International Atomic Energy Agency has devoted considerable resources to trying to identify and control dangerous radioactive sources for years, yet there are still accidents (for copies of the IAEA reports on these accidents you can browse through the documents in the IAEA Non-Serial Publications, which are available for download as PDF files). Part of the key to keeping these sources under control is for governments to maintain up-to-date information of the whereabouts of the most dangerous sources and the other part of the problem is for governments to make sure that these sources are kept safe and secure against loss or theft. In the United States this is accomplished through something called Increased Controls that aims to increase accountability for the sources as well as for those working with them. Extending such controls to other nations is something the IAEA has been working on for years, but there is still much work to be done – as evidenced by the 2010 accident in India.

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Dr Y is a certified health physicist, trained in nuclear power plant design and operations, with experience in nuclear power, environmental science, and planning for radiological and nuclear emergencies. He has 30 years of experience in the areas of nuclear and radiation safety

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3 Responses to “Radioactive tissues?”

  1. Melissa Pledger January 17, 2012 at 12:55 PM #

    Dr. Y.,
    Thank you for the above article. It brought science and physics back to the problem at hand.

  2. Dr. Y January 17, 2012 at 8:19 PM #

    You’re quite welcome – and glad I could help!

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  1. Radioactive tissue boxes redux | ScienceWonk - February 2, 2012

    [...] weeks have brought a little more information about the radioactive tissue boxes I wrote about in an earlier posting. Since this topic is important and because it raises interesting issues it’s worth an update, so [...]

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