What is the Sum of Fractions Rule?

Dear Dr. Zoomie – I was reading about posting areas that hold radioactive materials and they said something about the “sum of fractions” that I didn’t quite understand. There was some mathematical equation, but I didn’t exactly understand that either. Can you tell me what’s up with the “sum of fractions?” Thanks!

This one is actually sort of important, but it’s not as hard as the regs make it seem. And you can use the same thinking for a couple of things, so it’s worth trying to understand. Here’s how it works.

Say you’ve got a room where you’re storing radioactive materials and you’re trying to figure out whether or not you need to put up a sign indicating it’s being used for radioactive materials storage. If you look in the regulations you’ll find a table telling you what level of activity requires licensing (anything less than this limit is exempt from licensing) and possibly another table telling you what level requires labeling (if you can’t find the second table then the labeling level is 10 times as high as the exemption limit). So it seems pretty simple – if you have more than the limit for labeling then you have to post the room and if you have less than the limit, you don’t. In reality it’s a little more complicated.

Let’s try an easy one – according to 10 CFR 30 Appendix B (which is titled Quantities of Licensed Materials Requiring Labeling) you will find that if you are storing tritium (H-3) in a room then you have to post the room if you have more than 1000 µCi (or 1 mCi). So if you have, say, 900 µCi of tritium in the room you don’t have to worry about posting it. Similarly, the level for P-32 is 10 µCi and the limit for I-125 (both of these are used in research) is 1 µCi. So if you have less than 10 µCi of P-32 or less than 1 µCi of I-125 you’re also exempt from having to post the room for radioactive materials storage. But what if you’ve got a bit of all of these nuclides – or others?

Say, for example, you have a room with 400 µCi of H-3, 4 µCi of P-32, and 0.4 µCi of I-125? None of these are high enough to require posting in and of themselves. But using the “sum of fractions” you need to put a sign on the room anyhow. The table here explains why.


Nuclide Labeling limit (µCi) Amount on-hand (µCi) Fraction of limit
H-3 1000 400 0.4
P-32 10 4 0.4
I-125 1 0.4 0.4
Sum of fractions 1.2


So – the first thing you need to do is to figure out how much activity you’ve got compared to the labeling limit (half, a third, three quarters, etc.). To do this you divide the amount of activity you’ve got by the labeling limit (for example, 400 µCi is 40% of the limit of 1000 µCi for tritium). After you’ve done this for each of the nuclides you’re storing you just add up all of the fractions – if the sum is greater than 1 you have to post the room.

So let’s take the table above. For each of these nuclides you have 40% of the allowable limit (40% is the same as 0.4). Since you have three nuclides, each of them with 40% of the allowable limit, the sum of the fractions comes out to 0.4+0.4+0.4+1.2. Since this is greater than 1.0 the room has to be posted. Easy, right?

You see the Sum of Fractions show up in a number of places. For example, 10 CFR 37 talks about when you have to take increased controls over the security of radioactive materials – and you use the sum of fractions if you have multiple nuclides in storage. If you’re discharging radionuclides into the sanitary sewer system (or letting them escape into the air) you’ll have limits for each radionuclide being discharged – the sum of fractions is used here as well. There’s more, but you get the idea. Once you understand how the idea works then you can use the same technique – the same calculations (or the same spreadsheet) across the board.

Rad Area Posting – Millirem Per Hour Versus Millirem In An Hour (mR/hr v. mR in an hour)

Dear Dr. Zoomie – I work in a laboratory where we use x-ray machines on a regular basis. When the machine is on we have measured dose rates of up to 100 mR/hr in parts of the lab and as high as 50 mR/hr in the storage room on the other side of the wall from the x-ray machine. The previous RSO never posted these rooms as radiation areas, even though the regulations say that anyplace where dose rates are higher than 5 mR/hr have to be posted. Did my predecessor screw up, or do we have to post these rooms as radiation areas?

Great question! And it also brings up one of the fine points when it comes to hanging radiation area signs. In fact, the important thing isn’t so much the maximum dose rate you can measure at any one time but, rather, the radiation dose a person can receive when the room(s) and machine(s) are used on a routine basis. In fact, according to 10 CFR 20.1003 (definitions), “Radiation area means an area, accessible to individuals, in which radiation levels could result in an individual receiving a dose equivalent in excess of 0.005 rem (0.05 mSv) in 1 hour at 30 centimeters from the radiation source or from any surface that the radiation penetrates.” So, using this definition, let’s see what the difference is between “5 mrem per hour” and “5 mrem in one hour.”

Radiation Area Sign

Example of a Radiation Area sign. According to 10CFR20.1902 the licensee shall post each radiation area with a conspicuous sign or signs bearing the radiation symbol and the words “CAUTION, RADIATION AREA.”

Say, for example, that your x-ray machine is usually used only about 5% of the time on a typical day and never more than 10% of the time – this is the time that the machine is actually turned on (time preparing for an x-ray doesn’t count). This is called the machine’s workload and is abbreviated as W (it can also be called the utilization factor, abbreviated U). If you are in this room for an hour with the machine used the way it’s normally used, in one hour you can receive a radiation dose of only 10 mR (100 mR/hr when the machine is used x 10% of the time the machine is in use). In the adjacent room a person would receive a dose of 5 mR in one hour with the machine in normal use. So – taking into account only the machine’s workload – you might have to post both of these rooms as radiation areas. But what if you also take into account how the rooms are used?

You said that the adjacent room is a storage room – it’s a pretty good guess that nobody stays in this room 100% of the time. In fact, a reasonable occupancy factor (abbreviated as O) for storage areas is about 5%. So, using your understanding of how both the machine AND the room are used you find out that, even though the highest dose you measure in the storage room is 50 mR per hour, a person using that room the way it’s normally used when the machine is used the way it’s normally used will receive a radiation dose of less than 1 mR in an hour. Or, mathematically, this is how it looks:

D (in one hour) = DR x O x U

D (in one hour) = 50 mR/hr x 5% occupancy x 10% workload

D (in one hour) = 50 x 0.05 x 0.1 = 0.25 mR

So – even though the storage room might have dose rates as high as 50 mR/hr, you don’t need to post it as a radiation area because of your knowledge of the way the room is used combined with your knowledge of the way the machine is used.

We still have your lab to worry about. With a dose rate of 100 mR/hr when the machine is on you could be justified in posting the room as a high radiation area. On the other hand, we’ve already seen that, with the machine’s maximum workload, a person in the room would receive a dose of about 10 mR in one hour – this would make the room a radiation area, but not a high radiation area. If people normally stay in this room all the time then you’d stop here. On the other hand, if people come and go then you can apply the occupancy factor to your lab as well to figure out how to post your lab.

X-ray Machine

Radiography of knee in modern x-ray-machine at Sandnessjøen Hospital, Norway. The patients knee is examined to check for possible bone fractures after an injury.

There are two important points to keep in mind here.

One is that this is a way to credit for how your normally use the rooms and the machines (or radiation sources) – it is not meant to be a way to “pencil-whip” your numbers to avoid regulatory trouble. Use legitimate numbers when you’re doing these calculations, not the numbers you need to use to get the result you want. For example, you can look up occupancy factors online or you can measure them yourself by observing a room for a few days. And you can use equipment logs to determine a machine’s workload. But you need to be able to justify every number that you use when you’re trying to figure out the amount of dose a person is likely to receive in one hour.

The other point is that you can’t go wrong by posting a room based on the actual dose rates, leaving out the occupancy factor and machine workload. But if you don’t want to post, say, a hallway or stairwell or storage room (or laboratory) you can use these calculations to find out whether or not a posting is really necessary.

Finally, this same calculation is the same whether you’re talking about an x-ray machine in a laboratory (or in a hospital), a linear accelerator used for cancer treatment, or a radioactive source. As long as you know the highest radiation dose rate, the occupancy factor, and the workload you can make this determination.

Hopefully this helps!

Radioactive Materials Security – What is IC Quantity?

Hi, Dr. Zoomie – I am putting together a radioactive materials license and they’re asking me what sort of security I have and whether or not I have an “IC quantity” of radionuclides. To be honest with you, I’m not sure what an “IC quantity” is, and I’m also not sure how much security I need. Help!

OK – so let’s try to get this sorted out for you, and it shouldn’t be too bad (hopefully)!

First let’s tackle the easy one. “IC” stands for Increased Controls – a concept that came out within the last decade or so (the new security regulations are only a year or so old). That’s the amount of radioactivity that calls for higher levels of security. If you have less than the IC quantity then you don’t have to worry; more than the IC quantity and security becomes a bigger issue. IC quantities are listed in 10 CFR 37 – for Cs-137 for example the IC quantity is 27 curies; as long as you have less than 27 Ci of Cs-137 on your license then you don’t have to worry about the added IC precautions.

Let’s say you have three nuclides (A, B, and C) and the IC limit is 10 Ci for nuclide A, 20 Ci for nuclide B, and 30 Ci for nuclide C. What you have on hand is 6 Ci of nuclide A, 10 Ci for nuclide B, and 6 Ci of nuclide C. You don’t have an IC quantity of any single nuclide – are you off the hook? Sadly no, and it’s because of the sum of fractions rule. 6 Ci of nuclide A is 60% of the IC limit for that nuclide, 10 Ci of nuclide B is 50% of the limit for it, and 6 Ci of nuclide C is 20% of that limit. If you add these up (0.6+0.5+0.2) you come out with 1.3 – if the sum of the fractions is less than 1.0 then you’re OK; since you come in above that level then IC applies to you (sorry).

OK – so that’s what IC means and when you have to use it; now let’s get into the harder question about how to secure your materials.

The bottom line is that, no matter how much (or how little) radionuclide you have on hand, you need to make sure that nobody can steal it and that nobody can accidentally be exposed to enough radiation to hurt them. The less risk a source poses the less security you need to have. But no matter how minor a source, you are required to take steps to make sure that nobody can just come in off the street and take your radioactive materials without being stopped.

Fiber Optic Active Seal Installation

Fiber Optic Active Seal

So – say all you have is a soil density (“nuclear”) gauge, a lead paint analyzer, or a tank level gauge. The portable devices (lead paint analyzer and soil density gauge) are going to have to be kept locked up at all times. In your office they will have to be kept in a locked room, preferably in a locked cabinet or safe inside that room for added security. Better yet is to limit access to the room to only those people who are permitted to use the source. But the bottom line is that you have to do what you can to keep it from being stolen. In the field, by the way, this means keeping a device locked into the trunk of a car or in a toolbox that locks and that’s fastened to your van or pickup truck.

If you have a higher-activity source or a combination of sources that call under the IC regimen then you’ve got even more work to do. In addition to seeing to the physical security of the source you’re going to have to work with your on-site security or with a security contractor because you’re going to have to submit fingerprints of all the workers who will have access to the source, conduct a background check, and make sure that they are considered to be trustworthy and reliable. If they don’t pass the background check, if they refuse to be fingerprinted, or if their fingerprints have shown up somewhere that they shouldn’t then you probably won’t be able to give that person access to your higher-activity sources.

There are a lot of other aspects to the IC program – you should really check out 10 CFR 37 for all the details – but these are what seem to trip people up the most.

A few other things to keep in mind, with any level of radioactive materials. One is that keys are a weak point of many security systems because they can be copied, lost, or taken with a departing worker. It is far better to have keycard access or, at least, a numeric keypad to gain access to a room. If a key is compromised then you have to call a locksmith and issue new keys to everyone with access to a particular room. On the other hand, if a disgruntled employee leaves your company all you have to do is to revoke authorization for his keycard, or change the key code – much easier and less expensive.

The other thing to remember is that it’s hard to defend against an insider who has legitimate access to a radioactive source. This could be a disgruntled employee, someone who’s looking for quick cash, or a criminal who managed to make it past your background check. But it could also be a loyal employee who is being threatened or who is under substantial pressure. The bottom line is that anyone who has legitimate access to your radioactive sources has the potential to become a weak point in your security program – this is one good reason to conduct periodic evaluations of a person’s reliability to try to make sure they haven’t become a threat since their last check.

Finally, you should consider going beyond the bare minimum required by the regs. Consider asking your local police precinct to visit your facility and take a look at your physical security – they might see things that you would miss. You might want to put in cameras at critical doors to keep track of who’s entering and leaving secured rooms, and you might even consider thermal or motion detectors in your most sensitive spaces. Don’t use flimsy doors (and especially not doors with windows in them) to safeguard high-activity sources, don’t store them near (or in) a loading dock area, and so forth. Let’s face it – in addition to the cost of the sources themselves, a terrorist attack can cause a huge economic impact on your community – you (and your company) don’t want that to come from one of your sources. This is another good reason to ask a professional to evaluate your facility and make recommendations – and for you to follow those recommendations.

Good luck!

Is It Safe To Wear Jewelry With Irradiated Gems?

Dear Dr. Zoomie – I heard that jewelers sometimes irradiate gemstones. Why do they do this (and how is it done)? Is it safe for me to wear jewelry with irradiated gems? I’ve got enough to worry about already!

Well, it is true that a lot of gemstones are irradiated, but they’re completely safe to wear so you can relax a little bit! And now that you’re (hopefully) calmed down a little bit, here’s what happens.

It’s no secret that gems come in a lot of colors and there’s been a tremendous amount of scientific study into why, say, rubies are red and sapphires are blue (both have similar chemical formulae) while diamonds can be pink, blue, colorless, or any of a number of other colors. The general answer is that gemstone color comes from the interaction of the stones with light – some colors are absorbed, some are refracted within the gem, and some are reflected. There’s a ton of sciences, including some fairly heavy-duty physics – that goes into understanding why this occurs and, to be honest, much of it is over my head. But part of the short and (relatively) simple explanation is that different elements absorb (or fail to absorb) different colors of light. The presence of trace amounts of manganese (for example) can give rise to a purple color, iron can turn an otherwise colorless crystal gray or black, copper tints it green or blue, uranium gives us yellow and orange, and so forth. So a crystal’s chemistry can help determine its color.

Irradiated Gemstones - Colorless and other diamonds (left) can be artificially irradiated causing a variety of colors. Some of the irradiated colors are then heated as a second step, resulting in additional colors (group right).

Colorless gemstones can be artificially irradiated causing a variety of colors. Photo by GIA.

But there’s more than this because light has to pass through a crystal and the structure of that crystal also helps to determine which wavelengths (or colors) of light pass through the crystal, are absorbed, or are reflected. Not only that, but the distribution of electrical charge within a crystal also plays a role. This is where the radiation comes in – ionizing radiation can alter the distribution of electrical charge, and slamming neutrons into a crystal can dislodge atoms from their precise alignment, disrupting the crystal structure. Both of these phenomena will change a gemstone’s color – the most common example is topaz; irradiation can turn a rather boring brown or tan topaz into a lovely blue; light blue if it sits in a beam of high-energy electrons and a deeper blue after being bombarded with neutrons in a nuclear reactor. And other gems are irradiated as well – diamonds can be made yellow (for example), but topaz is the most common.

OK – so that’s why and how it works, but you’re concerned about the health risks, and justifiably so, if only because it would be too bad to spend good money on beautiful gemstone that places you at risk every time you put it on. So what we need to find out is whether or not an irradiated gemstone somehow stores the radiation it’s exposed to, or if it becomes radioactive itself.

The first question is easy – irradiated objects do not store radiation and they don’t re-emit it later. Think of a brightly lit room that lacks windows (light is, after all, just a form of radiation). When you turn the lights off the room gets dark – the chairs, tables, walls and so forth don’t glow with the stored light because the light isn’t stored in these objects; when the lights are turned off the irradiation stops and everything gets dark. Just the same with ionizing radiation – when the irradiation stops, that’s it; end of story.

Having said that, some forms of irradiation can make objects become radioactive. I know that this sounds as though I’m contradicting what I just said, but there’s an important distinction between an irradiated object storing and releasing the radiation it was exposed to versus it becoming radioactive itself. What if, for example, you paint a table with glow-in-the-dark paint that’s activated by exposure to light? In this case, exposure to the lights causes the paint to become activated – it causes chemical changes in the paint. Now when you turn out the lights the paint will glow (but the unpainted table will not), fading slowly over time. The paint is NOT releasing the same light that it absorbed – what the light does is to induce a chemical change in the paint that causes it to glow for awhile. By the same token, hitting a non-radioactive material with neutrons (which are present in a nuclear reactor core) causes some of the atoms to become radioactive. But this only happens with objects that are exposed to neutron radiation or to the very highest energies of beta or gamma radiation, and this can only happen in the core of a nuclear reactor or in very high-energy particle accelerators. And, as with the glow-in-the-dark paint, this induced radioactivity fades relatively quickly. By the time a gemstone is sold any radioactivity that was created has long since faded to undetectable levels and they certainly pose no risk to the buyer (or to the wearer).

Another part of this is regulatory – in order for an irradiation facility to sell their gemstones and ship them to the jewelry store they have to be able to show that the stones meet regulatory criteria – they simply aren’t allowed to ship anything that’s still “hot.” This is another protection against your buying jewelry that might put you at risk.

So – the physics of irradiation are such that only gemstones irradiated in a nuclear reactor can become radioactive at all; everything else will simply have the color changed. Physics also controls the rate at which these gems will become non-radioactive, and this typically happens fairly quickly (a few days to a few weeks). And regulations require that these gems be confirmed to be safe before they can be sold to you. For all of these reasons I feel comfortable saying that, even though your beautiful deep blue topaz might have once spent some time inside of a nuclear reactor, it’s safe for you to wear.

But wasn’t there an incident awhile back where people were hurt by radioactive jewelry?

Yes there was – several people were hurt in fact. But this had nothing to do with irradiated gemstones and we think that all of the jewelry in question was rounded up and accounted for. Here’s what happened.

About a century ago people started using radioactivity to treat cancer. One of the therapies that was used involved putting radioactivity (radium, primarily) into gold capsules. These capsules would be inserted into a tumor and the radiation would destroy the tumor. After they were used (or if they weren’t used) these gold capsules were supposed to be disposed of as radioactive waste. Except that some weren’t – somehow a number of these capsules were sold to gold dealer and melted down into gold that was sold to jewelers and made into jewelry. The whole story is too involved to really get into here, but the short version is that several people got skin burns from radioactive rings before the source was tracked down – a number of scientific and medical papers were written on the subject. Luckily, some perceptive physicians figured out what was happening and contacted the regulators – notices went out and the tainted gold was rounded up and disposed of properly. There might still be minor amounts of this gold out there somewhere, but the vast majority seems to have been rounded up and there haven’t been any reported injuries (or contaminated gold) in the last few decades.

Finally – keep in mind that this involved gold that was contaminated by radioactivity; not gold (or gems) that was irradiated. Again, any irradiated gems you buy are a different kettle of fish and they won’t put you at risk.

Can You Blow Up an Asteroid With a Nuclear Weapon?

Hey Dr. Zoomie – I heard that an asteroid just missed us, and then I watched a movie about an asteroid killing everyone on Earth. Then there’s that asteroid that killed the dinosaurs. Sounds like something we should be worried about – why can’t we just blow them up with nuclear weapons?

Image of Vesta Asteroid

Image of Vesta Asteroid

Good question – especially since we only live on one planet. Let’s face it – if something whacks the Earth hard enough it can destroy our civilization and push humanity back to the Stone Age. An even bigger strike might even push us into extinction along with most of the life on Earth. We know that it’s happened in the past, and there’s no doubt it’s going to happen again sometime in the future. I don’t want to face a world without Internet, tablet computers, and cable TV – not to mention high-quality medical care, air conditioning, and refrigeration – so it’s reasonable to wonder what might be done to help keep us (and our Kindles!) safe. And since nuclear weapons are the biggest things we’ve got it seems to make sense to just blast an asteroid out of the sky. Right?

Well…not always. Here’s why.

First let’s take an easy case. When a nuclear weapon goes off it creates a fireball where temperatures are thousands to millions of degrees – enough to vaporize anything inside the fireball. For a 1 megaton nuclear weapon (technically a thermonuclear weapon or hydrogen bomb) the fireball will be 1-2 miles in radius. So setting off a bomb this size could, in theory, completely vaporize an asteroid less than about a mile or two in diameter (if the bomb is sitting on the surface). But in reality, the asteroid will absorb a lot of energy from the weapon and the fireball will start to cool down – chances are that even a smaller asteroid wouldn’t be completely vaporized. Instead it might be blown apart.

OK – blowing an asteroid apart sounds good – at least we’re taking the size down a bit, right? Well…maybe. But we also run the risk that, instead of a single big asteroid, we could instead get pummeled with dozens of smaller – but still lethal – rocks. So we trade one massive hit for a dozen (or so) big hits. This could actually be worse, so this might not be the best way to go. There’s also a small legal issue – nuclear weapons are banned in space. Granted, with the fate of humanity at stake people might not be too worried about the fine points of international law, but it does prevent us from testing any systems before we’d use them – with something as complicated as trying to nudge an asteroid off of a collision course it would be nice to be able to know what we’re doing.

Another possibility might be to try to blast an asteroid off-course so that it misses us. This is maybe better than turning a single lethal rock into a dozen lethal rocks, but only if we get it right. What if, for example, the rock is weaker than we thought and instead of being pushed off-course it just breaks up? Or what if the explosion goes off at the wrong time and it doesn’t miss us at all – maybe even hits in a worse location? The bottom line is that we have to know exactly how much the asteroid weighs and how tough it is – and we have to know its orbit and how quickly it rotates – with enough precision to make sure that we are really going to steer a single asteroid in just the right direction at exactly the right time. If we get anything wrong, as I mentioned before, then we can make things much worse.

The bottom line is that under exactly the right conditions a nuclear weapon might be able to help, but under anything less than that the effects would be anything from no change at all to making things worse.

Right now the bottom line is that we just don’t have anything we can do except to keep an eye on the skies and hope we don’t see anything with our name on it. And if we do, hopefully we’ll have enough time to figure out how to convince it to miss us. Luckily we have a pretty good handle on most of the biggest rocks in our part of the Solar System and we can predict their orbits for decades or centuries in the future. One could pop up tomorrow that would give us only a few months to respond, or even smack into us out of the blue. But every year that goes by gives us a better understanding of what’s out there and a better chance to protect the Earth. But whatever it might be that we might see, I’m pretty sure that nuclear weapons won’t be part of the solution.

Are Dental and Medical X-rays for Kids Safe?

Hey Dr. Zoomie – I’ve heard that kids are much more sensitive to radiation than adults are. Should I worry about taking my kids to the dentist or the doctor?

Image of Dental X-ray

Image of Dental X-ray

When my younger son was about two years old he broke out in big red splotches and then started having some trouble breathing. My wife and I rushed him to the hospital and they decided that an x-ray was needed. As they were prepping him the technologist started explaining the risks of x-rays to me – after a few minutes I cut him off, explained that I was a radiation safety professional and fully understood the risks, and told them to go ahead and take the x-rays. Luckily there were no serious problems and he recovered over the next week. My thinking was that the radiation dose from an x-ray is low – not much more than taking a cross-country plane flight – and the risks were somewhere between very low and nothing; in exchange for gaining valuable diagnostic information. On the other hand, even if there was a long-term risk from the radiation, that would lie many years – even a few decades – in the future. Not breathing, on the other hand, can be immediately dangerous – my feeling was that it was a good idea to make sure my son didn’t have any problems that were immediately life-threatening. Any risks that the radiation might pose wouldn’t show up (if at all) for 20 or 30 years.

The bottom line is that medical x-rays (including dental x-rays) don’t give a very high radiation dose – at most, a few tens of millirem (fluoroscopy and CT scans are higher-dose procedures; let’s get back to them in a minute). And if we dig into the radiation biology we find that the health risks from a low dose of radiation is incredibly low – about as close to zero as you can get. At the same time, we also have to consider the risk from a doctor (or dentist) not having the diagnostic information they think they need – if the risk of missing this information is higher than the risk from the radiation then there’s a good case to be made for taking the x-ray.

There are some procedures that expose us to a higher radiation dose – CT scans are one and fluoroscopy is another.

A single CT scan exposes a person to about as much radiation as 100 x-rays (give or take a bit). This sounds like a lot – and it’s not a trivial radiation dose – it’s about as much as we get from nature in a few years. But even here the risk of getting cancer is really low. How low? According to one calculation it’s about one twentieth of one percent – which means that there’s a 99.95% chance that nothing will happen. And in reality the risk might be even lower – we don’t know for sure exactly how low the risk is, but we know that it’s no higher than 0.05%, which is about twenty times safer than driving. And if you look at the risks of other options – watchful waiting or exploratory surgery – the risks from a CT scan that will provide medically necessary information seem to be pretty reasonable. At least they seemed reasonable to me when I had to make this same decision for another child several years later.

The bottom line is that if we’re going to decide the right thing to do we have to keep in mind all of the risks – and we have to think about the real risks of the diagnostic radiation as opposed to what we fear might be the case. And, while we don’t want to use medical x-rays when they’re not called for (and we still want to do what we can to reduce radiation dose whenever possible), we have to remember that they provide information that often can’t be gained in any other manner – to see inside the body we only have two choices; using some form of radiation or cutting into the body. Surely we can all agree that x-rays are better than scalpels.

What is Food Irradiation? Why is it Done?

Hi, Dr. Zoomie –

I got some ground beef at the store the other day that says it was irradiated. Can you tell me what that means and why it’s done? My brother tells me irradiation makes the food dangerous – is there any truth to that?

It’s not only the ground beef that’s irradiated – a lot of spices are irradiated, along with some fruits, meats, and more. What’s happening is that the food is exposed to very high levels of radiation; that radiation is enough to kill off any bacteria that might be inside (or on the outside)of the food. We all know that radiation can harm people – what many don’t realize that a high enough dose of radiation can kill microbes to boot. So if we slam enough radiation into food, it ought to kill all of the microbes; since those microbes are what makes the food go bad, food irradiation keeps it fresher longer.

Incidentally, it’s not only food that’s irradiated to sterilize it – there are a number of facilities in the US that use radiation to sterilize surgical supplies, some mail, and much of the blood supply.

Some types of food don’t do well under irradiation – it can cause strawberries to become mushy, for example. But in addition, some people are concerned about food irradiation because exposure to high levels of radiation can cause chemical changes in the food. And it’s true – exposing food to radiation can cause new chemicals to form, but these chemicals don’t seem to be harmful to people. And I should point out that any sort of cooking causes chemical changes to the food – that’s what cooking is. When you brown meet or caramelize onions, for example, the sugars in the food are undergoing a chemical reaction called the Maillard reaction. But what really causes chemical changes is grilling food over charcoal, wood, or even a gas flame, and some of these changes (especially the ones that include the soot or combustion products from the flame) can create chemicals that are potentially much more dangerous than anything caused by radiation. This isn’t to say that grilling out is dangerous – more to put the changes caused by irradiation in perspective. The bottom line is that cooking, grilling, and food irradiation can all cause chemical changes in your food – and irradiation causes fewer changes than the other techniques.

Something else that people worry about is that irradiation might cause food to become radioactive. Luckily there’s nothing to this at all – the gamma radiation from cobalt-60 or cesium-137 is simply unable to cause food to become radioactive (neither can the x-rays or electron beams that are sometimes used). And the food doesn’t somehow store up the radiation – no more than, say, your furniture stores up light and glows when you turn the lights off. For that matter, the only type of radiation that is effective at causing things to become radioactive is neutron radiation, and neutron radiation isn’t used for irradiating food.

Something else to keep in mind is that the whole purpose of food irradiation is to kill microbes. Think of all the food recalls and food poisoning outbreaks that have been in the news – millions of pounds of ground beef have been recalled, contaminated spinach caused illness and death, and much more. The bottom line is that food irradiation makes food safer – another of the GAO’s findings.

When you put it all together, you can reassure your brother that we get a lot more benefit than risk from food irradiation. In fact, the greatest risk is to the people working at the food irradiation facility – there have been injuries and fatalities from people who just didn’t work safely with the high levels of radioactivity that’s present. But as long as the workers are careful to follow the proper safety precautions they’ll stay safe also.

What Do First Responders Need to Know for Radiation Safety Training?

Dear Dr. Zoomie –

I’m a firefighter and we’re starting to prepare for radiological emergencies. What sort of stuff do we need to know? Is there some sort of accepted standard for training or certification? Where can we get trained?

Tough questions – and a lot of ground to cover. So let’s get started with some of the relatively easy parts and work our way from there.

FDNY EMS Conduct a Dirty Bomb Drill

FDNY EMS Conduct a Dirty Bomb Drill

First, there is no standard at the moment for training emergency responders for radiological emergencies, and there are no certification programs at present. There are a number of on-line training programs that can go over some of the basic information, but that will only take you so far – to really be effective you need to have in-person training that includes hands-on work as well as the classroom stuff. As far as where you can receive this training, Nevada Technical Associates put together a training class for the Fire Department of New York – I’ll describe this in a moment – that went over very well.

As far as what you need to know…since there is no standard at the moment I can only describe the curriculum that was put together for FDNY – since they had a huge input into the training, the curriculum not only covers what NTA felt was important, but also what the firefighters felt needed to be covered. As an aside, this training is similar to training that NTA instructors have given to other emergency responders elsewhere in the US.

FDNY Firefighter

A FDNY firefighter rushes to help victims during a simulated bus bombing, at FDNY Fire Academy on Randall’s Island, N.Y

Any radiation training, no matter who the audience is, has to include radiation fundamentals – what types of radiation are there and what are their properties, how does radiation affect our health, how can we detect it, and so forth. But emergency responders have different information needs than others – a radiation safety officer needs to be able to calculate radioactive decay, for example, but an emergency response isn’t likely to last that long so there’s no need to go through the decay calculations. Similarly, it’s helpful to understand the concepts of time-distance-shielding to reduce radiation dose, but it’s not necessary to go into all the calculations for emergency response efforts. So the “radiation fundamentals” part of the training should concentrate on the basic principles – the practical stuff – but there’s no need to get into the mathematical nitty-gritty.

Another part of the classroom part of the training should involve discussing different types of radiological and nuclear emergencies – what might happen during a dirty bomb attack, what happened at Chernobyl and Fukushima (and what might happen during a US reactor accident), as well as what a responder might see during a small-scale event such as a fire in a hospital’s nuclear pharmacy or during a vehicular accident involving radioactivity. Finally, the different places where radioactivity is used (e.g. industrial radiography) and the types of radioactive sources used in these places can help the responders to understand the risks they might face and how to deal with them. And then, on top of all this, responders should also know which regulations apply to their work and how to comply with them.

Firefighter Decontaminates a Soldier

A firefighter, left, with the Tulsa Fire Department decontaminates a U.S. Soldier assigned to the 63rd Civil Support Team

OK – so that’s the classroom part, but there’s still a practical side of the training as well. Responders have to understand how to use radiation instruments to perform surveys, how they respond to a variety of radioactive sources, how to screen people with both hand-held instruments and portal monitors, how to decontaminate themselves and others, and so forth. And ideally there should be some sort of practical exercise where the responders tackle a simulated radiological emergency – suiting up, conducting surveys, mapping radiation and contamination levels, and so forth; as well as tackling some of the “managerial” decisions such as assigning stay times, determining what sort of protective gear to wear, and so forth.

All of this came out to about 40 hours of training – the same length of time that’s normally spent in the NTA Radiation Safety Officer class.

Is Food Irradiation Safe?

Hey Dr. Zoomie – what’s the big deal with food irradiation? Is it safe, or is going to turn me into a lizard or something?

My mother used to work as an epidemiologist, investigating outbreaks of food poisoning. She told me once that if all food was irradiated her job would be mostly checking up on people who left their potato salad out too long at picnics. Food irradiation has been in use for over a half-century – it looks to be a pretty effective way to help make food safer. But first, let’s back up a little bit to talk about how it works.

Irradiated Fruits & Vegetables

Irradiated Fruits & Vegetables

High levels of radiation are dangerous, not only to humans, but to microbes as well. The idea is that blasting, say, a pound of ground beef with withering doses of radiation should kill off any microbes that might otherwise make us sick. Many microbes are fairly hardy, thought, when it comes to radiation – we have to expose the food to very high doses of radiation to make sure it’s sterilized; doses that would be fatal to humans many times over.

There are a few ways to produce such a high radiation dose. One way is to use extremely high levels of radioactivity and the other is to use very high-energy electrons to create x-rays or to directly irradiate the food. The last of these – directly bombarding food with high-energy electrons – is fairly simple; the electrons hit the microbes and kill them. The problem is that even the highest-energy electrons can only penetrate about a half inch into food – anything that’s more than about an inch thick (since it can be irradiated from both sides) will have area that can’t be reached by the electrons. The other methods – using gamma rays or x-rays – are a bit more effective since these can penetrate all the way through fairly thick foods to reach every bit of them.

Growth of E Coli

Growth of E. Coli Bacteria

So the radiation can penetrate into the food to make it safe, but a lot of people are concerned that the treatment itself might affect the food, making it unsafe. The thing is, these worries are unfounded – the benefit from killing off hostile microbes (such as the E. coli and salmonella that have caused a number of food poisoning outbreaks) far exceeds any negative effects. Here’s why.

First, irradiated food does not become radioactive – in fact, irradiating anything with gamma rays, x-rays, or even electrons is physically unable to cause things to become radioactive. Think about the lights in your home – when you turn the lights off at night your furniture doesn’t glow with the stored energy. And when you get off the table after having an x-ray taken you’re not emitting radiation either – just as food irradiation doesn’t make your food radioactive.

That’s not to say that there aren’t any chemical changes in your food when it’s irradiated. Radiation is known to cause chemical changes – it’s the whole subject of the field of radiochemistry. As one example, radiation exposure can cause chemical bonds to break, turning water into a mixture of hydrogen and oxygen. Other chemicals can form as well – especially in foods that are fatty or oily. But you have to remember that cooking also causes chemical changes in food – especially barbequing and grilling out. So the question shouldn’t be “Does irradiation cause chemical changes in the food” so much as “How do the chemical changes caused by irradiation compare to those caused by cooking?” And when it comes down to it, while food irradiation dose cause small chemical changes in the food, they pale in comparison to what happens when you put food on the grill. The bottom line is that food irradiation has been tested for over a half-century – and has been in use for about that same amount of time – and no credible studies have shown it to be any more dangerous than regular cooking.

It’s easy to say that food something is safe but somewhat more difficult to prove it. In this case, food irradiation has been studied extensively by the International Atomic Energy Agency and by the US General Accountability Office – both reputable organizations. These studies were pretty clear that food irradiation is safe and that the benefits far outweigh the risks. There have been some who have challenged these findings, but the science is not on their side.

Deadly Foodborne Pathogens

31 of the most important known agents of foodborne disease found in foods consumed in the United States each year cause approximately 9.4 million illnesses, 56,000 hospitalizations, and 1,400 deaths each year.

The bottom line is that food poisoning harms and kills people every year. How many times have we heard about illness linked to tainted chicken, ground beef, or farm vegetables, and how many people have died from these outbreaks over the years? By comparison, irradiated food hasn’t killed anybody, and there’s no telling how many people it’s saved.

What is Instrument Calibration?

Dear Dr. Zoomie –

I keep hearing about instrument calibration and don’t quite know what that means. Can you tell me what it is, when we have to do it, and whether it’s something I can do myself or should hire a contractor?

Let’s start off with why we have radiation instruments at all – it’s because we have no way of sensing radiation ourselves. No matter how high the rad levels are, you won’t feel your skin tingle, you won’t get a strange taste in your mouth, and you certainly won’t see or hear anything. That’s why we have instruments – to make up for this deficit in our senses. So if we’re using instruments to tell us whether or not we’re at risk – or even just to meet regulatory requirements – it behooves us to make sure that the instruments are working properly and to make sure we can trust their readings. That’s what calibration is for.

So calibration makes sense from a practical perspective, but it’s also a regulatory requirement – you’re required to perform a yearly calibration on all radiation instruments that are used for health and safety purposes or to perform surveys used to meet regulatory requirements. So every one of your radiation meters that’s used for routine radiation or calibration surveys has got to be calibrated every year. You should also have your instruments calibrated anytime they undergo extensive repairs – changing the batteries or cables is OK, but replacing probes or repairing the inner workings calls for calibration to make sure it’s still working properly.

If you’ve got a lot of radiation meters you might want to calibrate them yourself, but you’ll need to amend your radioactive materials license to let you do so, you’ll need to purchase the appropriate equipment, and you’ll need to get trained up so you can do it properly. Unless you have a few hundred instruments or more it probably doesn’t make sense to go through this process – but if you have a lot of instruments, calibrating them in-house can save you enough money to make it worth considering.

If you only have a few instruments you should send them out for calibration. Here you need to be careful to send them only to a facility that’s licensed to do the calibrations – most of the major instrument manufacturers can do this, but there are a lot of other facilities as well who can help you out. Doing an on-line search for “instrument calibration services” will give you a bunch of options; you can also look through the companies listed under Instrument Calibration Services in the Affiliates section of the Health Physics Society’s website (https://hps.org/aboutthesociety/affiliates/services.html).

One last thing – in addition to the annual calibrations there are quick and simple checks you should be performing every day that you use your instruments. Check to make sure the batteries are charged (many meters have a “Bat Test” button or switch position), make sure the meter’s physical condition is OK, and (for contamination detectors) check the meter for proper response against a check source to make sure your meter reads within 20% of the expected reading. For this one you can purchase a “button source” that can be mounted on the side of the meter; when your instrument is calibrated the calibration certificate will include information on the expected count rate. So, for example, if the expected count rate is 1000 cpm, you should get a reading that’s between 800-1200 cpm when you do the response check.