Scanner radiation caused 1% of flight-related cancers.

After extended public anxiety about cancer risks associated with back-scatter scanners, EU and U.S. banned them in 2012 and 2013 respectively. But how many people actually developed cancer from these scans before they were banned?

I have yet to find articles that estimate the world-wide mortality using consensus numbers. Most just state that the risk is “negligible” or “truly trivial”. That vague language is not comforting to a pedantic like me, so let’s look at the actual numbers. See the end of this post for a full list of sources and informative infographics.

Risk per scan: 0.3 millionth of a percent

The risk that an individual develops cancer when exposed to 1 microsievert of radiation is around 0.0000041 % for adults. The risk increases approximately linearly with radiation, so four scans quadruples the chance – not more and not less. The risk is only slightly higher (0.0000057 %) when including children, elderly, and heritable effects.

A typical back-scatter scan exposes you to 0.07 microsieverts of radiation (Multiple sources: see end of post.)

Multiply these two numbers and you end up with an elevated risk-per-scan of 0.07 * 0.0000041 % = 0.00000029 %.

Risk due to other radiation sources on your flight

The effect of a single scan exposed passengers to about as much radiation as being outside on the ground for 10 minutes or inside a flying airplane for 1.5 minutes (around 0.07 microsievert). This is due background radiation (most importantly cosmic radiation, i.e., the bombardment of particles from outer space, which decays into X-rays and other stuff when colliding with our atmosphere).

I leave it as an exercise to the reader to figure out whether scanners or background radiation constitute the largest risk factor for cancer on typical flights.

Scenario: camping in a scanner

If you want to increase the chance of developing cancer at some point in your life by 1% (one in a hundred), you would have to find one of the old back-scatter scanners and camp in it for four months straight, 24 hours a day. Every time you leave for the loo or to stretch yourself, you’d have to go back in and stay longer to compensate. And you’d have to have done it before 2013, because they are really hard to get now. Good luck on your adventure.

Image result for MR scanner sleep
Your home for the next four months.

What if we multiply by a few billion?

Even minuscule effects on individuals add up when they apply to a large number of people. In 2012, there were roughly three billion passengers who spend on average two hours per flight, totaling six billion hours in-flight.

Image result for gif passengers plane
Three billion passengers.

If all of them went through one back-scatter scan per flight, the result is that:

In 2012, 8.3 passengers developed cancer because of airport scanner radiation. 664 passengers developed cancer because of the background radiation in-flight but 95 of these would have developed cancer for the same reason anyway, had they stayed on the ground for the same duration. An additional 601 passengers developed cancer while commuting due to “normal” non-flight-related ageing and risk factors.

Sources: see end of post.

That is, scanners, background radiation, and ageing caused 767 cancers of which scanners make up one percent. The numbers above contains some estimates and could be off by a factor of two, i.e., between 0.5 and 2% of flight-related cancers were due to the scanners.

Risk of cancer after back-scatters were banned

In 2017 there were roughly 4 billion passengers, so while modern scanners should cause virtually zero cancers, 918 passengers developed cancer because of background radiation in-air and 832 simply due to regular ageing.

In comparison, 7.28 million of these 2017-passengers would develop cancer in the same year anyway, regardless of whether they flew or not. Flying only added one in 8,000 to that number.

Morale: statistical illiteracy and a note on war and terror

If you care about cancer, please do not waste your time thinking about airport scanners. Neither the old nor the new. If you felt scared, it is not your fault. We humans are notoriously poor at dealing with risk for rare events. We are statistically illiterate. I am too. There are just so many ways we fail that it is hard to count them. But take a look at the availability heuristic, loss aversion, and base-rate fallacy, as a way to get started.

Image result for i am stupid gif

You know about another rare event? Death by terrorism. In the sea of all sources of human suffering, terrorism makes up but a minuscule fraction of almost any other cause (guns, flu, traffic, etc.). The amount of money and time spent on terror prevention is truly staggering in comparison.

It costs around a million dollars per year to have a US or European soldier in war. It currently cost around 700 dollars on average to save a children’s life through the Deworm the World initiative. 1.400 children’s lives each year or one soldier at war?

Appendix 1: Where I got the numbers from

The risk of cancer onset (not necessarily death!) is around 0.0000041% per microsievert (μSv) for adults. It is around 5.7 * 10-6 % per uSv when including children, elderly, and heritable effects. This according to the a 2007-report (see table 1) by the International Commision on Radiological Protection which almost everyone cites in academia. Read more about radiation-induced cancer on Wikipedia.

Backscatter scanner dose is in the order of 0.07 μSv per scan:

This is about the same exposure as when sleeping next to someone for a night or eating a banana.

Background radiation on earth’s surface is in the order of 0.4 μSv per hour:

Background radiation while in flight is in the order of 2.8 μSv per hour, i.e. seven times that on earth’s surface:

  • 2.40 μSv per hour (0.04 μSv per minute) according to multiple sources cited in Mehta (2011).
  • 2.94  μSv per hour according to Enyinna (2016).
  • Many other sources whith full-text behind paywalls put this in the order of 3 μSv

There were approximately 3 billion passengers in 2013 and 4 billion passengers in 2017:

Each flight is around 2.0 hours in recent years (since 2005 at least) when reading off the chart on page 13 of this Boing-report.

There are 0.182 % chance of being diagnosed with cancer each year according to World Cancer Research Fund National. This translates into 0.0000208% chance every hour.

Appendix 2: Quick ways to learn more