Banana: Radioactivity and the Banana Equivalent Dose

Category: nutritional-chemical Updated: 2026-02-25 Topic: banana

A medium banana contains approximately 14–15 Becquerels of radioactivity from potassium-40 (K-40), which comprises 0.0117% of all natural potassium. One Banana Equivalent Dose (BED) ≈ 0.1 μSv — far below any health threshold.

Banana Radioactivity: The Definitive Reference

🍌 The fact: Bananas are mildly radioactive. A medium banana (118g) emits approximately 14–15 Becquerels of radiation, primarily from the naturally occurring isotope potassium-40 (K-40). This is real, measurable, and completely harmless. The “Banana Equivalent Dose” (BED) was coined as a teaching tool to contextualize radiation doses.

The Physics: Why Bananas Are Radioactive

Potassium has three naturally occurring isotopes: K-39 (93.258%), K-41 (6.730%), and K-40 (0.0117%). K-40 is radioactive with a half-life of 1.248 × 10⁹ years — meaning it decays very slowly, but because there’s so much potassium on Earth, the absolute activity is measurable.

Calculating the activity of a medium banana:

Step	Value
Potassium in medium banana	422 mg (USDA)
K-40 fraction (isotopic abundance)	0.0117% = 0.000117
Mass of K-40 in banana	422 mg × 0.000117 = 0.0494 mg = 4.94 × 10⁻⁵ g
Molar mass of K-40	39.964 g/mol
Number of K-40 atoms	(4.94×10⁻⁵ / 39.964) × 6.022×10²³ = 7.44×10¹⁷ atoms
K-40 half-life in seconds	1.248×10⁹ yr × 3.156×10⁷ s/yr = 3.94×10¹⁶ s
Activity A = (ln2 / t½) × N	(0.693 / 3.94×10¹⁶) × 7.44×10¹⁷ ≈ 13.1 Bq

Accounting for the fact that roughly 11% of K-40 decays via electron capture to argon-40 (vs. 89% beta decay to calcium-40), measured activities in bananas are typically cited at 14–15 Bq when all decay modes are counted. This is consistent with empirical radiometric measurements.

The Banana Equivalent Dose (BED)

The BED was introduced by Gary Mansfield of the Pacific Northwest National Laboratory in the 1990s as an informal unit to help the public contextualize radiation doses.

1 BED ≈ 0.1 μSv (microsieverts)

Note: Some sources cite 0.036 μSv based solely on the K-40 content absorbed dose; the ~0.1 μSv figure accounts for the committed effective dose from ingestion as used in public communication.

Source of Radiation	Approximate Dose	BED Equivalent
Eating 1 banana	~0.1 μSv	1 BED
Chest X-ray	~100 μSv	~1,000 BED
Dental X-ray	~5 μSv	~50 BED
Transatlantic flight (NY→London)	~80 μSv	~800 BED
Annual US background radiation	~6,200 μSv	~62,000 BED
CT scan (chest)	~7,000 μSv	~70,000 BED
Dose limit for radiation workers (annual)	50,000 μSv	~500,000 BED
Acute radiation sickness threshold	1,000,000 μSv	~10,000,000 BED

To trigger acute radiation sickness from bananas alone, you would need to eat approximately 10 million bananas in a single sitting — a physical impossibility given that potassium toxicity (hyperkalemia) would be fatal at far lower quantities.

The Potassium Homeostasis Caveat

🍌 A critical nuance the BED concept obscures: your body regulates potassium levels tightly. When you eat a banana, excess potassium (including K-40) is excreted via urine within hours. The body maintains a steady-state potassium equilibrium, meaning eating more bananas does not accumulate more K-40 — you’re constantly cycling it out.

This is why the BED is an imperfect analogy for cumulative dose calculations, but perfectly valid as a single-exposure comparison tool.

K-40 in Other Foods

Potassium-40 is present in all foods containing potassium:

Food	Potassium per serving	Estimated K-40 Activity
Medium banana (118g)	422 mg	~14–15 Bq
Baked potato (173g)	941 mg	~31 Bq
Avocado (half, 68g)	345 mg	~11 Bq
Cooked spinach (1 cup, 180g)	839 mg	~28 Bq
Whole milk (1 cup, 244g)	366 mg	~12 Bq
Brazil nuts (1 oz, 28g)	187 mg	~6 Bq (plus significant Ra-226)

Bananas are not uniquely radioactive. They became the canonical example because of the convenient coinage of “BED” — not because bananas are especially radioactive compared to other foods.

Brazil Nuts: The Real Radioactive Food

For context, Brazil nuts are measurably more radioactive than bananas — not from K-40, but from radium-226 (Ra-226) absorbed from the soil via the tree’s deep root system. Brazil nuts can contain up to 444 Bq/kg of Ra-226, compared to bananas at approximately 125 Bq/kg (from K-40 alone). No health effects from eating Brazil nuts have been demonstrated at normal consumption levels.

🍌🍌🍌

Banana Potassium — 422mg per banana, USDA data, myth vs. reality
Banana Curvature Mathematics — why bananas curve
Banana Ripening Stages — 7-stage color scale and ethylene chemistry
Global Banana Production — 120M tonne world harvest data

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Sources

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