'Porno' Scanners


In the discussion that surrounds the airport backscatter systems (more emotionally known as 'porno scanners') the risk of radiation is frequently combined into the list of why these devices are so awful. Most of those commenting on this are ignorant of what they speak and likely do not seek true education in it. One location I read from that had been linked off of LRC (I believe the link was to dontscanme or something similar) a report was linked detailing the risk from cataracts that ionizing radiation can pose. Linking cataracts to backscatter machines either reveals profound ignorance or willful disingenuousness.

Cataracts are a deterministic effect from radiation. Even the linked report was dealing with cataract formation from exposure to individuals working in an occupational capacity in a cardiac cath lab. Cardiac cath lab workers are among the highest receivers of occupational exposure and even there cataracts are not common. The regulations (awful that they exist I know), industry standards and general ALARA (as low as reasonably achievable) principles minimize stochastic effects from radiation (cancer) and have pretty much eliminated deterministic effects. Further, the article is referring to a study from Malaysia. It is hard to speak about what their practices are. Practices matter a great deal in this area. To pass on the information on cataracts… it takes a minimum of 500,000 uSv to cause cataracts if the exposure is given in a short time… if the time frame is longer than the required value is much greater.

http://hps.org/publicinformation/ate/q5208.html

So, what is the radiation risk from these scanners. There is published data on this in NCRP Commentary No. 16 “Screening of Humans for Security Purposes Using Ionizing Radiation Scanning Systems”. The effective doses from the common field system is 0.03 uSv per scan for the anterior view and 0.02 uSv per scan from the posterior view. I think we can agree that 0.03 + 0.02 is < 0.1 uSv which is what I will use as the value from here on. First let us compare the value of 0.1 uSv to the exposure you would receive from the actual flight.

http://www.hps.org/publicinformation/ate/q444.html

This value ranges at our latitudes from 0.3 uSv/hr to 0.4 uSv/hr depending on the solar cycle. So, if you are on a two to three hour flight from Atlanta to San Antonio you would expect an exposure of between .6 – .8 uSv. So, the exposure from the backscatter system is a small component of the overall trips exposure.

Let us now compare it to principles discussed in NCRP Commentary 16. We have the concept of annual Negligible Individual Dose (NID). The NID is 0.01 mSv (which is 10 uSv). It would take 100 scans from the backscatter device to reach the NID. It would take 2500 scans to reach a particular administrative control level.
The average American receives 6.2 mSv (6,200 uSv) of radiation effective dose per year from all sources (natural, medicine etc). 3 mSv of that comes from inescapable natural radiation (no medical exposure). This data can be found in NCRP Report No. 160.

But again… what about risk? There is no really good data on such minute amounts of radiation. For regulatory purposes (oh those things again) the linear-non threshold theory applies. From that we assume no radiation exposure is good. But the data for that is extrapolation from atomic bomb survivors who received well in excess of 2,000,000 uSv in a single blast. There are papers out there studying hormesis (the concept that low levels of exposure are beneficial to humans). So while there is a regulatory concept it is not related that much to scientific evidence.

The best formula for lifetime risk of cancer to radiation exposure is for the non-occupational population 5% per Sv (1,000,000 uSv) increase in the risk of cancer. At 0.1 uSv per scan you would have a lot of frequent flyer miles to get there.

Growing radiation exposure is an interesting subject. But the bang for the buck (yeah… I went with Pareto) is not in backscatter machines but is in medicine (particularly CT). Given the number of unnecessary medical images that people demand or allow themselves to be subjected to chasing after backscatter machines on the issue of radiation demonstrates a lack of knowledge in the field and the underlying science.

There are many good reasons not to implement and deploy these devices. Radiation exposure is probably the weakest of these and certainly the most poorly understood by the people making the case.