Eating spinach does expose you to a small amount of radiation from potassium-40, a naturally occurring radioactive isotope found in potassium, but the level of radiation is extremely low and comparable to everyday background radiation, far less than what you would receive from a chest X-ray.
Potassium is an essential mineral found in many foods, including spinach. About 0.012% of naturally occurring potassium is potassium-40, which is radioactive. This isotope undergoes radioactive decay, emitting beta particles and gamma rays, but the amount of radiation released from the potassium-40 in food is minuscule and harmless in normal dietary amounts. For example, bananas, which are also high in potassium, contain similar levels of potassium-40 and are often cited in discussions about natural radioactivity in food.
When you eat spinach, you ingest potassium-40 along with stable potassium. Your body contains potassium naturally, including potassium-40, so the radiation dose from eating spinach is just a tiny addition to the radiation your body already has internally. The radiation dose from eating a typical serving of spinach is so low that it is negligible compared to medical imaging procedures. A chest X-ray delivers a radiation dose many thousands of times higher than the dose from potassium-40 in spinach.
To put it in perspective, you would have to eat an enormous amount of potassium-rich foods—on the order of thousands or millions of servings—to approach the radiation dose of a single chest X-ray. The natural radioactivity in foods like spinach is part of the normal background radiation environment that humans have evolved with and is not considered a health risk.
In summary, while spinach does contain potassium-40 and therefore a tiny amount of radiation, eating spinach is not equivalent to receiving a chest X-ray in terms of radiation exposure. The radiation from potassium-40 in spinach is extremely low and safe as part of a normal diet.