How Bananas Make Radiation Ap-peel-ing

Physics

By: Hannah Pell

On March 7, 1995, Gary Mansfield, a health physicist at the Lawrence Livermore National Laboratory, sent out an email to members of the RadSafe nuclear safety mailing list. The subject line read: “Banana Equivalent Dose.”

“Some time ago (when I almost had time to do such things), I calculated the [radiation] dose one receives from the average banana,” the email begins. Bananas contain the radionuclide K-40, a naturally occurring isotope of the element potassium (K). So, bananas are indeed radioactive, and Mansfield wanted to know how much radiation is in one “reference banana.” His rough calculation amounted to approximately .01 millirem. (Put another way, that’s 1/100 of the average dose from a 3-hour airplane trip).

Mansfield continued: “Would love to go into more detail, but have to get back to our DEADLY Human Radiation Experiments (i.e., eating bananas).” Mansfield could have just as well replaced ‘bananas’ with potatoes, red kidney beans, broccoli, tomatoes, or nuts, as these foods all contain K-40, too. “Considering the fact that the [Department of Energy] has officially stated that there is no safe dose of radiation,’ my advice to you all is to stop eating immediately.”

“Thank you, Gary, for the references and the deep [insight] into the true nature of how intense all the microgram counting has become,” a fellow health physicist joked.

What followed was an extensive email chain to further refine this estimate, which has come to be known as the “Banana Equivalent Dose” or BED, because we in physics don’t already have more than enough acronyms. Additional considerations included how our bodies naturally regulate potassium levels, how salt concentrations affect absorption, a note that the European Union had indeed defined a “Reference Banana”, and a proposition that the unit for the amount of radiation in one standard banana is aptly named the “bananacurie.”

Despite the collaborative fine-tuning, the Banana Equivalent Dose remains an informal calculation and is not an accepted standard for quantifying radiation dose. But it may help us understand it a bit better.

Although the amount of radiation in one single banana is minuscule, according to the Nuclear Threat Initiative, many bundles of bananas together can be enough to trigger a false alarm when trying to detect potentially dangerous and illicit nuclear or radiological materials. According to this estimate, up to 100 bananas is the equivalent radiation exposure to living near a nuclear power station, eating 400 bananas is the same exposure as a flight from London to New York, and it would take consuming 20,000,000 for the same radiation exposure as a typical dose in one session of radiotherapy.

The International Commission on Radiological Protection explains that there are three slightly different yet related ways to calculate radiation dose: absorbed dose, equivalent dose, and effective dose. Absorbed dose is the amount of energy deposited by radiation in some mass, and it’s measured in milligrays (Joule/kilogram). The equivalent dose is derived from the absorbed dose and is calculated for individual organs.

The idea of the effective dose was first introduced by physicist Dr. Wolfgang Jacobi in his 1975 paper, “The concept of the effective dose: a proposal for the combination of organ doses,” published in the journal Radiation and Environmental Biophysics. The U.S. Nuclear Regulatory Commission describes effective dose as a combination of “the amount of radiation absorbed and the medical effects of that type of radiation.” It is a measure of potential biological impacts that could occur in the future (i.e., percent chance of developing cancer) of one’s exposure to a particular amount of radiation. The SI unit of effective and equivalent dose is the Sievert (1 sievert is equivalent to 100 rem).

Though some have argued that the Banana Equivalent Dose can be misleading, it does demonstrate that explaining the very normal presence of radiation in our daily lives is challenging. How can we grasp something we cannot touch? How can we visualize something we cannot see? Radiation, whether found in medical treatments, nuclear power plants, or the fruit basket on the table, necessitates that we navigate complex risks and benefits. Communicating them effectively is important for protecting our personal and public health.

Is that really so bananas?

 
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