The Science Behind Radiation

How much radiation is in a chest x-ray?

When people talk about radiation exposure – especially from flying – they often reach for a familiar comparison: the chest X-ray. Government agencies, pharmaceutical companies, and doctors might state that the dose from a flight is equal to some fraction of a "single chest X-ray," a shorthand meant to make an invisible risk easier to understand.

It sounds simple, right? It turns out, unlike in elementary school math class, one plus one does not always equal two, at least not when it comes to how some U.S. government officials count chest X-rays. In fact, the Centers for Disease Control and Prevention acknowledges it sometimes counts procedures that take two different X-rays as just one X-ray, for the purposes of this comparative calculation.

In an emailed response to questions, a CDC spokesperson acknowledged the CDC’s calculation method and offered an explanation.

"'Chest x-ray' refers to one or two x-rays which allows for values to be more conservative in nature," she wrote.

The practice differs from how radiation officials in the United Kingdom and other countries define a chest X-ray, and it can have a subtle but important effect: when the bundled, two-image definition is used as the yardstick, the amount of radiation people receive on a flight can appear to translate into fewer chest X-rays than it actually would if each image were counted on its own. In other words, the comparison can leave people feeling more reassured than the underlying math really supports.

How 1 + 1 still equals one when it comes to X-rays (at least, in the United States)

Here is the math usually presented to the public: The CDC estimates a cross-country flight’s radiation dose to flight crews and passengers is roughly 0.035 millisievert (mSv). They say this is less than a standard chest x-ray, which they list at 0.1 mSv.

However, when we began fact-checking this math, we found that there is more than one way to define a chest x-ray, and the amount of radiation you get from one.

To run down this trail, we first needed to learn there are two main kinds of outpatient chest x-rays:

  1. The standard view from the back (called the "PA view"): This is the common x-ray you would get to diagnose pneumonia or tuberculosis. This comes with an effective dose of about 0.02 mSv. That number, in plain English, adds up all the radiation exposure each of your organs take in.
  2. The side view ("lateral view"): A slightly less common type taken from the side, that is increasingly only advised in particular circumstances. This delivers a higher dose of about 0.04-0.06 mSv.

There’s also a front view (“AP view”), but this x-ray is considered sub-optimal. It is typically ordered when patients are in a hospital bed and have a hard time standing up.

When the CDC says "a typical dose of radiation from a chest x-ray" is 0.1 mSv, that figure is based on combining two exposures from the back and side x-rays.

When we asked the CDC why they use the combined figure, they stated: "Most patients receive the PA/lateral examinations, so in estimating the highest risk, we selected the upper range of radiation doses to the lung, 0.1 mSv."

While it is true that many doctors still order two views (back and side) to diagnose pneumonia, the side view is increasingly falling out of favor.

A review in the journal Radiography concludes the side view "radiograph should not be performed routinely unless clinically indicated." A primary concern the study authors cited was "increased radiation dose associated with the additional chest view." In other words, a second X-ray really is an additional X-ray, and truly does increase your radiation dose.

Notably, even the CDC’s own guidelines for tuberculosis screening recommend only the back view chest x-ray for anyone over 10 years old.

The Society for Radiological Protection in the UK told us that exams there "often consist of a single view."

This changes the math significantly. The UK society notes that for a single view, the effective dose is "typically around 0.02 mSv." The CDC’s method of counting effectively translates five typical chest x-rays, taken from the back, to just one exposure.

For this report, we chose to count one standard back view chest X-ray (with an estimated dose of 0.02 mSv), as one image, when referring to a single chest X-ray.

Is there such a thing as a “safe” dose of radiation?

When assessing the dangers of ionizing radiation, many scientists believe that even the smallest dose carries some level of risk. 1  In this model, the probability of developing cancer rises in direct proportion to the amount of radiation received, meaning there is no "floor" or safe minimum. This approach is formally called the linear no-threshold theory. 2

This model is supported by many leading scientists 3 and influential bodies, including a committee of the National Academies of Sciences’ Board of Radiation Effects 4. As a long-standing 5, independent organization 6, the National Academies of Sciences provides evidence-based advice to the federal government 7, lending significant scientific credibility to the linear no-threshold theory as the standard for considering ionizing radiation danger.

"Newer epidemiological evidence recently has shown that LNT – the linear no-threshold model for dose versus risk – is the most accurate approach to ionizing radiation risk assessment,” 8 said John Boone, a Distinguished Professor of Radiology and Biomedical Engineering at the University of California Davis, in an email to the Howard Center 9. "What this means is that no amount of ionizing radiation is risk-free, which is the consequence of the 'no threshold' model."

Under the linear no-threshold model, ionizing radiation exposure is cumulative. This means every exposure you receive throughout your life adds up.

David Brenner, the director of the Center for Radiological Research at Columbia University, has compared radiation exposure to buying tickets for a lottery that you do not want to win, where every ticket purchased increases the odds of a negative outcome. 10

In this analogy:

  • High levels of exposure are equal to buying many tickets, leading to a higher risk.
  • Low levels of exposure are equal to buying only a few tickets, resulting in much lower risk.

The critical point is that even one "ticket" or very low exposure carries some risk.

Ionizing and Non-Ionizing Radiation

The linear no-threshold theory only applies to ionizing radiation. 11 This type of radiation includes cosmic radiation and radiation from x-rays. 12 The theory does not apply to non-ionizing radiation from devices like cell phones or Wi-Fi routers. 13

Ionizing radiation is more dangerous than non-ionizing radiation because it can pass through many materials and change them as it moves. Unlike ultraviolet (UV) radiation or the type of radiation that comes from cell phones, ionizing radiation has enough energy to knock electrons out of atoms 14 and break chemical bonds. 15 This means it can change a material’s molecular structure 16, something non-ionizing radiation does not normally have enough energy to do. These changes can damage cells 17 and tissues 18 inside the body.

NASA explains this danger, saying on its website, "Ionizing radiation is like an atomic-scale cannonball that blasts through material, leaving significant damage behind." 19 This same type of ionizing radiation is what airline pilots, flight attendants, and passengers encounter when they fly at high altitudes, where Earth’s protective atmosphere is thinner. 20 Because radiation increases with altitude 21, those who fly receive more exposure to cosmic radiation than people on the ground, though they are still far more protected than astronauts, who travel much higher with even less shielding from Earth’s atmosphere and magnetic field. 22

Natural Background Radiation and the Linear No-Threshold Theory

In our reporting, we encountered individuals and organizations implicitly minimizing the risks of radiation exposure during air travel by comparing it to the level of natural background radiation, the radiation we are naturally exposed to in our environment.

The U.S. Environmental Protection Agency, for example, states that "Most people do not fly frequently enough to add a significant amount to their total radiation dose." However, they note that aircrew should "consider their flying time more carefully." 23

Natural background radiation that we experience at sea level includes radiation from the Earth’s crust and cosmic radiation from outer space 24. We’re also exposed to radiation from other people, since all humans contain small amounts of naturally occurring radioactive materials. 25

It is common to see estimates that a person in the U.S. receives about 3.1 millisieverts (mSv) of background radiation per year 26, which is roughly equivalent to the radiation dose from 155 standard chest x-rays. 27

Arjun Makhijani, president of the Institute for Energy and Environmental Research 28, has written that U.S. regulators often invoke comparisons to background radiation to wrongly suggest that radiation levels below background levels are harmless or can be ignored. 29

Indeed, according to the linear no-threshold theory, even though we are around a little bit of radiation every day, it doesn't make extra radiation any safer. Instead, that daily radiation is just the starting point, and every new dose adds more risk on top of it. Even this unavoidable background radiation may itself "cause significant adverse health outcomes," according to Makhijani. 30

The Radon Dilemma: Natural Background or Preventable Risk?

There is another reason why comparisons of voluntary radiation exposure to background exposure can lead people into a false sense of security: much of our exposure to background radiation is in a sense not actually "natural."

Radon exposure accounts for about two-thirds of most background radiation estimates 31. Almost all of that radon exposure is indoor radon. 32

While radon gas is naturally occurring 33, its concentration indoors is often an artifact of human construction 34. Building methods that restrict ventilation trap the gas, leading to levels far higher than those found in nature 35. This human influence is even recognized by the U.S. Toxic Substances Control Act. 36

Additionally, while radon levels in water sourced from private wells can be extremely high 37, they can be effectively reduced through aeration systems 38.

Makhijani argues that including indoor radon in "natural" totals, as is common practice, is misleading 40. Because radon is the second leading cause of lung cancer (after smoking), it represents a preventable public health risk rather than an inevitable environmental constant.

The Illusion of a "Safe" Baseline

In short, exposure to "natural" radiation alone, as it is commonly calculated, includes the kind of radiation that is already known to be unsafe.

If we remove radon, the baseline for what is considered a "natural" dose drops significantly.

How we measure radiation changes how safe a job looks. Currently, the government says the average American gets 3.1 mSv of background radiation a year 41. Since pilots and flight attendants get about 3 mSv at work annually on average 42, their job can seem "safe," as their exposure can fall slightly below the "natural" average.

But if we decide that trapped radon is a man-made problem caused by poor ventilation and remove it from the total, the real natural background drops to only 1.1 mSv 43.

Suddenly, the pilot’s dose of 3 mSv isn't "low" anymore, it is nearly triple natural background levels.

When facing health issues, patients should listen to the medical advice of their doctors, who can determine when it is smart to accept specific exposures. Doctors provide the expertise needed to decide when limited exposure, such as a required X-ray, is appropriate and necessary.

But Makhijani notes that, "Exposure of air crews during flights is, of course, not medical. It is workplace exposure. 44 And unlike medical imaging, where people choose exposures, flight crews in the U.S. are often completely unaware of their level of workplace exposure to ionizing radiation.

 

Footnotes

  1. This is an overview sentence. Subsequent sentences with footnotes support this claim. ↩
  2. "The LNT model provides that ionizing radiation is always considered harmful and that there is no threshold below which an amount of radiation exposure to the human body is not harmful. The LNT model further holds that biological damage caused by ionizing radiation (the cancer risk and adverse hereditary effects) is directly proportional to the amount of radiation exposure to the human body (response linearity). Thus, the higher the amount of radiation exposure, or dose,[7] the higher the likelihood that the human receptor will suffer biological damage." (source) ↩
  3. See next paragraph for footnote for the "leading scientist" claim. Also see this prominent 2015 academic article with different authors which concludes: "Results suggest a linear increase in the rate of cancer with increasing radiation exposure." ↩
  4. National Academies of Sciences, Engineering, and Medicine. 2006. Health Risks from Exposure to Low Levels of Ionizing Radiation: BEIR VII Phase 2. Washington, DC: The National Academies Press. (source). pg 15, conclusion: "The committee concludes that current scientific evidence is consistent with the hypothesis that there is a linear, no-threshold dose-response relationship between exposure to ionizing radiation and the development of cancer in humans."; For clause about the board see here, "Library of Congress Cataloging-in-Publication Data Health risks from exposure to low levels of ionizing radiation : BEIR VII, Phase 2 / Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation, Board on Radiation Effects, Research Division on Earth and Life Studies, National Research Council of the National Academies." ↩
  5. "The National Academies of Sciences, Engineering, and Medicine operate under a congressional charter signed by President Abraham Lincoln in 1863, at the height of the Civil War, to advise the nation on issues related to science and technology." (source) ↩
  6. "The National Academies provide independent, trustworthy advice and facilitate solutions to complex challenges by mobilizing expertise, practice, and knowledge in science, engineering, and medicine." (source) ↩
  7. "The National Academies of Sciences, Engineering, and Medicine operate under a congressional charter signed by President Abraham Lincoln in 1863, at the height of the Civil War, to advise the nation on issues related to science and technology." (source) ↩
  8. Email with Dr. Boone "Newer epidemiological evidence recently has shown that LNT – the linear no-threshold model for dose versus risk, is the most accurate approach to radiation risk assessment. Therefore, radiation professionals such as myself use this more validated model for risk assessment. What this means is that no amount of radiation is risk-free, which is the consequence of the 'no threshold' model." In follow up email, included in link, he says I can add "ionizing" to the quote. ↩
  9. Email with Dr. Boone "Newer epidemiological evidence recently has shown that LNT – the linear no-threshold model for dose versus risk, is the most accurate approach to radiation risk assessment. Therefore, radiation professionals such as myself use this more validated model for risk assessment. What this means is that no amount of radiation is risk-free, which is the consequence of the 'no threshold' model." In follow up email, included in link, he says I can add "ionizing" to the quote. ↩
  10. (source). "David Brenner, director of the Center for Radiological Research at Columbia University, rephrased my question. It’s not a matter of whether Hawaii is safe in terms of radiation; scientifically speaking, safe implies zero risk, and that’s not the case. In general, the risk of developing cancer in the U.S. is in the range of 40%; tacking on whatever additional exposure we’d receive in Hawaii would be akin to increasing our risk to 40.001%." ↩
  11. "The LNT model provides that ionizing radiation [...]" (source) ↩
  12. https://www.cdc.gov/radiation-health/about/ionizing-radiation.html ↩
  13. "Electronic devices that send information through the air are everywhere. Between Wi-Fi, cell phones and other networks, people are in a nearly constant cloud of wireless signals. These devices use RF energy to send and receive information. RF energy is a type of non-ionizing radiation." (source) ↩
  14. EPA "Ionizing radiation has so much energy it can knock electrons out of atoms, a process known as ionization." ↩
  15. Australia source "Because ionising radiation has enough energy to remove electrons from atoms, it has the ability to change the chemical composition of the material it interacts with." ↩
  16. Source: "Radiation ionizes cellular atoms and molecules; if immediate recombination does not occur, these can manifest as some type of molecular, cellular, or organic system alteration." ↩
  17. Australia source "Because ionising radiation has enough energy to remove electrons from atoms, it has the ability to change the chemical composition of the material it interacts with. In living tissue this effect can result in a process that damages DNA and can result in the death or mutation (cancer) of the impacted cell." ↩
  18. EPA "Ionizing radiation can affect the atoms in living things, so it poses a health risk by damaging tissue and DNA in genes." ↩
  19. NASA "Ionizing radiation is like an atomic-scale cannonball that blasts through material, leaving significant damage behind." ↩
  20. CDC "Aircrew and passengers are exposed to cosmic ionizing radiation on every flight." ↩
  21. CDC "At flight altitudes, passengers and crewmembers are exposed to higher levels of cosmic radiation." ↩
  22. IAEA "Aircrew and frequent flyers receive higher radiation doses from cosmic radiation than the public. Astronauts receive even higher radiation doses." ↩
  23. Source "In the United States, the average dose of radiation people receive is 620 mrem (6.2 mSv) per year. Most people do not fly frequently enough to add a significant amount to their total radiation dose. However, airline crew members need to consider their flying time more carefully." ↩
  24. https://www.epa.gov/radtown/background-radiation ↩
  25. https://www.ehs.iastate.edu/background-radiation ↩
  26. Chapter from this NCRP report: "effective dose per individual in the U.S. population (EUS ): 3.1 mSv (arithmetic mean)" ↩
  27. "We are using 0.02 mSv as a chest x-ray. We will link to chest x-ray explainer. Australian government: "For example a single chest X-ray exposes a patient to around 0.02 mSv" ↩
  28. Arjun Makhijani, President of IEER, holds a Ph.D. in engineering (specialization: nuclear fusion) (source) ↩
  29. P. 43-44 “The argument is often made that there is natural radiation and men, women whether they are pregnant or not, infants, children, and the embryo/fetus are exposed to it. According to this view, levels of radiation exposure that are smaller that natural background can be considered as having minimal effects or can even be ignored. This premise of often implicit, but nonetheless clear, in the many appeals that are made to levels of natural radiation by the nuclear industry and even regulators.” That sentence footnotes to something about the EPA. (source) ↩
  30. P. 76 "The radiation protection limit for the general public at 100 millirem per year is approximately equal to the natural background amount at sea level. This is not a health-based level, since natural background radiation may cause significant adverse health outcomes, just as many other natural factors do. For instance, cosmic rays or natural potassium-40 may contribute to the large proportion of pregnancies that fail in the first two weeks (generally before a woman realizes she is pregnant)." (source) ↩
  31. Source: "This figure illustrates that the dose from exposure to indoor radon (200 mrem/year EDE) represents over 50% of the total dose." (Note: total is referring to something more than background radiation, but the raw 200 mrem (2 mSv) number still holds.) ↩
  32. Source: "This figure illustrates that the dose from exposure to indoor radon (200 mrem/year EDE) represents over 50% of the total dose." (Note: total is referring to something more than background radiation, but the raw 200 mrem (2 mSv) number still holds.)" ↩
  33. https://www.arpansa.gov.au/understanding-radiation/what-is-background-radiation ↩
  34. Source. "Without ventilation, radon can build up inside the home, with the highest concentrations usually recorded in the lower and basement levels." p 11 ↩
  35. Source. "Without ventilation, radon can build up inside the home, with the highest concentrations usually recorded in the lower and basement levels." p 11 ↩
  36. "The Administrator of the Environmental Protection Agency shall develop model construction standards and techniques for controlling radon levels within new buildings." (source)
    (Arjun makes this point in this report.) ↩
  37. Source: See Table B6. The "maximum" level is 120 mSv/year, which is almost 40 times higher than conventional background estimates. This document is an appendix of this NCRP report. ↩
  38. Source: "Aeration systems are the only effective method for reducing radon levels that are at or above 10,000 pCi/L." ↩
  39. Source: "They should not be added to involuntary exposures. Including radon exposures is similarly misleading. The vast majority of radon exposure is indoor radon; it is not natural because it is an artifact of building construction." ↩
  40. Source ↩
  41. Chapter from this NCRP report: "effective dose per individual in the U.S. population (EUS ): 3.1 mSv (arithmetic mean)" ↩
  42. "SOURCES AND EFFECTS OF IONIZING RADIATION", United Nations Scientific Committee on the Effects of Atomic Radiation, United Nations, published 2000, under Table 3 “Occupational radiation exposures”, p. 14, row “Air travel (crew)” and column “Average annual effective dose (mSv)”: “3.0” Aircrews are occupationally exposed to an average of 3 mSv of radiation per year. ↩
  43. 3.1-2 = 1.1↩
  44. Source ↩