Interview with a Health Physicist

In the fallout of the TSA’s increased use of new imaging technology in many U.S. airports, there’s been a lot of controversy, here and elsewhere, over the machines’ safety.  There also is a lot of interest in radiation after the tragic Fukushima Daiichi nuclear plant problems after the earthquake and tsunami.

To get some straight answers on the science, we contacted PhysicistLisa*, a Health Physicist working in the US radiation safety industry.

Skepchick: Radiation is an easy boogeyman.  It seems that most people don’t understand what it is, how it works, where it exists, and how it affects us biologically.  They just know it’s scary. Give us a basic “radiation for dummies” rundown:  What is radiation?  Where does it come from?  When is it dangerous, and at what levels?

Lisa: The important thing to keep in mind here is that generally when we say “radiation” we are referring to ionizing radiation, which is comprised of particles or electromagnetic waves that are energetic enough to directly cause an atom to lose electrons (become ionized).

Radioactive materials can be human-made, but they also occur naturally.  In terms of the physics of how radiation interacts with the human body, there is no difference between human-made and natural radiation, though different types (photons vs. protons for example) or energies of particles/waves have different effects in terms of damage to human tissue.

We use human-created ionizing radiation fields in many aspects of our lives, notably in medical imaging such as CT scans and x-rays, but also for particle accelerators that are used to destroy tumors in cancer patients, x-ray machines that are used in quality checks in large aircraft parts, industrial production lines, and many other things.

Non-ionizing radiation is another type of radiation that is not energetic enough to directly ionize an atom.  Non-ionizing radiation has an electromagnetic wave form such as RF, visible light, UV, infrared, and microwave.

This is an important distinction! Just as a CT uses ionizing radiation to create an image and an MRI uses non-ionizing radiation to create an image, current airport security measures have also deployed two different types of technology: back-scatter x-ray which uses ionizing radiation and millimeter wave which uses EHF RF signals.  A discussion of the potential health effects of either of these machines cannot be productive if they are considered to be the same technology.

I think at this point as skeptics you’ve seen a lot of the good charts and graphs, some from unexpected sources like the love of my life (he just doesn’t know it yet) Randall Munroe, of XKCD, so there’s the beginning of an understanding of these doses in relation to each other.  Cancer rates associated with exposure to ionizing radiation are estimated to be about 0.17% per 1 rem (0.01 Sv) of radiation.

However, I think it is really important to note that these cancer risks are *extrapolations* from the results of high doses to radiation. This is based on the linear no-threshold theory of dose: the basis for the idea that there is “no safe dose” of radiation, that even a very small dose holds a small risk of radiogenic effects.

This model is used because it is very difficult to study very low level effects of radiation.  Conclusions by the National Research Council (based on a wealth of data including but not limited to early uranium miners, radium dial painters, and Japanese atomic bomb survivors) state that radiogenic health effects have only been consistently demonstrated in human populations at doses over 5-10 rem delivered at high dose rates.

Skepchick: So why do you think people are so freaked out about radiation? How does radiation exposure stack up to, say, smoking or drinking, in terms of long term health risk?

Lisa: I think a big source of fear around radiation is that we can measure it to such tiny levels.  We can’t measure any chemical to the levels that we can measure radioactive materials to.  One Curie is the activity in a gram of radium (historically named) and a Becquerel (the more logical SI unit, imagine that) is a decay per second, such that 1 Ci = 3.7E10 Bq.  So when you hear measurements in the picoCurie range (very common) remember that by saying that we were able to detect an elevation of 33 pCi we’re talking about being able to see the decay of about 73.3 ATOMS every minute.  Being able to measure something very tiny shouldn’t be inherently scary and yet with radiation, it seems to be.

I think this document from the Health Physics Society is a great explanation of Radiation Benefit and Risk Assessment.

Skepchick: There’s been a lot of speculation over the amount and possible effects of radiation dose from the TSA imaging machines.  Although they’ve been FDA approved, some claim that the studies to determine effective dosage were flawed.  I’ve even heard that with these machines in use, pilots could be exposed to more than the NRC’s allowable annual dose.  Can you elucidate this topic for us?

Lisa: Assuming that a pilot or a flight attendant went through a backscatter x-ray unit every time they flew (which is unlikely, frankly!) the dose would still continue to be a fraction of the exposure they get just from flying the plane.  The atmosphere actually significantly protects us from cosmic radiation and the higher you go, the higher your background dose rate will be as a result, so pilots and flight attendants have a higher risk just by the nature of their job.

Dose rate as a result of flying is around 0.3 mrem/hr.  Radiation generating equipment has to be regularly calibrated, and the American National Standards Institute requires that these machines deliver less than 0.025 mrem (2.5E-04 mSv) per scan.

It is in no one’s best interest to run the scanners above ANSI standards, so I find talk of the big bad companies not caring about people’s health and overdosing people illogical and akin to conspiracy thinking.  If you do not trust companies and the government to properly regulate and maintain these machines, that is an issue that is not a scientific debate.  Because these scans are so much lower than the dose from simply flying, it is not credible that this dose alone could be responsible for putting pilots over an annual dose limit.

Skepchick: Another concern being raised is that instead of delivering a whole body dose, the radiation from backscatter xray is concentrated on the skin. This quote is from an article linked in in an earlier post about TSA:

A group of scientists at the University of California, San Francisco raised concerns about the “potential serious health risks” from the scanners in a letter sent to the White House Office of Science and Technology in April.  Biochemist John Sedat and his colleagues said in the letter that most of the energy from the scanners is delivered to the skin and underlying tissue.
“While the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high,” they wrote…..The Office of Science and Technology has issued a response to the scientists’ letter, saying the scanners have been “tested extensively” by US government agencies and were found to meet safety standards.  But Mr. Sedat told AFP last Friday: “We still don’t know the beam intensity or other details of their classified system.”

Can you discuss the claim that a dose concentrated to the skin is more harmful than a dose distributed to the whole body?

Lisa: The argument here is based on the claim that this dose is more dangerous because it is reported as a whole body dose,  but they are ignoring that the dose is only on the skin.  This is an epic misunderstanding of how health physicists calculate dose.

It’s correct that it is difficult to compare the dose from one organ to the another, and that is why we report an “Effective Dose.” This is an equivalent dose to the whole body as a result of the incident radiation’s a) tissue weighting factor and b) radiation weighting factor.  Simply put, the Effective Dose gives us a way to compare doses from different situations as a result of dose to different organs and/or in different radiation fields.

A tissue weighting factor of 0.01 is applied for doses to the skin to produce an effective dose to the whole body. The dose the skin can receive is much higher than a whole body dose and still be considered safe. That is then combined with a radiation weighting factor that incorporates the type of radiation and energy (incident radiation with more momentum is more damaging; in this case, 1.0 for x-rays).

That gives us an Effective Dose that we report so we can fairly compare the dose as a result of different exposures.  This dose calculation methodology is internationally accepted and is derived from risk based data.  The whole body Effective Dose reported, thus is inclusive of the fact that all of the dose is deposited in the skin.

The majority of dose is concentrated at the skin, leaving your gonads (and/or fetuses) protected by the body.  The quality of the image is based on the ability of the radiation to scatter off the skin and not deposit deeply in the body.

Skepchick: We’ve also heard people saying that these machines use radiation with a different wavelength than the regular background stuff we’re exposed to every day.  Is there anything to this at all?

Lisa: In terms of ionizing radiation, we have a great wealth of knowledge of the biological effects of most radiation fields.   The physics of biological interactions with radiation are well understood, so this claim really doesn’t hold any water.

I would not consider non-ionizing radiation to be my area of expertise.  However, we have been using EMF for a long time in many applications so it is not “new. ”  This sort of arguing is used basically any time a new RF technology is brought up (cell phones, WiFi networks, Bluetooth, etc).

Further information on the break down of increased risk as a result of these scanners is well presented in this recent article in the Archives of Internal Medicine.

Skepchick: Thanks Lisa!

*Disclaimer: Lisa is a health physicist working in the radiation safety industry. However, all information she presents here represents her professional opinion and not necessarily that of her employer or any of their clients*



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  1. Hmm, well I guess I don’t know as much physics as I should, because I had a hard time following that, although it was very interesting :p

    This article seems to suggest that radiation levels from airport scanners are unlikely to be health concerns. Did I interpret that right?

  2. @Luthien: Yes, as best I can understand it, the dose from going through the airport x-ray scanner once is about the same as the dose from a 5 minute flight. (.3 mrem/hr/60 min/hr= .005mrem/min. 5 minutes * .005 mrem/min = .025 mrem, the same as the maximum allowed dose from the scanner.) I think the flight crew usually stays inside the security area between flights or legs of a flight, so they only get zapped once a day. If they fly for an average of 6 hours per day, this would increase their exposure by 1.4%. If they used the mm wave scanners some of the time, or the x-ray machine was producing less than the maximum allowed dose, the increase would be that much smaller.

    Apparently, the standard conversion for effective exposure already accounts for lower energy x-rays that are absorbed in the skin versus higher energy ones that go right through you or get absorbed inside you, so the concerns of the UCSF scientists are already addressed in the dosage calculations. I didn’t understand this part as well, but that’s what I think Lisa was saying.

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