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

When assessing the dangers of ionizing radiation, many scientists believe that even minimal exposure is not considered risk-free. 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.

This model is supported by many leading scientists and influential bodies, who find that a preponderance of evidence supports it, including a committee of the National Academies of Sciences’ Board of Radiation Effects. As a long-standing, independent organization, the National Academies of Sciences provides evidence-based advice to the federal government, 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," said John Boone, a Distinguished Professor of Radiology and Biomedical Engineering at the University of California Davis, in an email to the Howard Center. "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.

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. This type of radiation includes cosmic radiation and radiation from x-rays. The theory does not apply to non-ionizing radiation from devices like cell phones or Wi-Fi routers.

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 and break chemical bonds. This means it can change a material's molecular structure, something non-ionizing radiation does not normally have enough energy to do. These changes can damage cells and tissues 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." 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. Because radiation increases with altitude, 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.

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."

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

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

Arjun Makhijani, president of the Institute for Energy and Environmental Research, 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.

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.

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. Almost all of that radon exposure is indoor radon.

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

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

Makhijani argues that including indoor radon in "natural" totals, as is common practice, is misleading. 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. Since pilots and flight attendants get about 3 mSv at work annually on average, 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.

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. 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.