Here's why:

* Sunlight: The Sun emits a wide range of electromagnetic radiation, including visible light, infrared radiation (heat), and ultraviolet (UV) radiation.

* Cosmic Rays: High-energy particles from outer space also contribute to Earth's radiation exposure. These cosmic rays can penetrate the Earth's atmosphere and interact with atoms in the air, creating secondary radiation.

While other sources of radiation exist, such as radon gas in homes or medical procedures, the Sun's radiation is the most significant and widespread source we encounter daily.