Which of the following is NOT a source of radiation exposure?

Cell phone signals are indeed not a source of ionizing radiation exposure. Unlike other choices listed, which involve forms of radiation that can potentially cause ionization in biological tissues, cell phone signals operate using non-ionizing radiation. This type of radiation, which includes radio waves emitted by cell phones, does not carry enough energy to ionize atoms or molecules, meaning it cannot directly cause damage to DNA or lead to radiation-related health effects typically associated with ionizing radiation.

On the other hand, medical X-rays, air travel, and cosmic radiation all represent sources of exposure to ionizing radiation. Medical X-rays are designed for diagnostic purposes and can expose patients to radiation, which, although controlled and monitored, contributes to overall exposure. Air travel exposes individuals to increased levels of cosmic radiation due to the altitude, where there is less atmospheric protection. Cosmic radiation, originating from outer space and interacting with the Earth’s atmosphere, also contributes to everyday radiation exposure. As a result, these options are significant sources of radiation exposure compared to the non-ionizing nature of cell phone signals.