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.

A water intensive process