Photobiology: Historical Background, Sources, and Complications

Abstract

The modern understanding of ionizing radiation started in 1895 with Wilhelm Röntgen. In the process of conducting various experiments on applying currents to different vacuum tubes, he discovered that despite covering one in a screen to block light, there seemed to be rays penetrating through to react with a barium solution on a screen he’d placed nearby. After several experiments, including taking the first photo (of his wife’s hand and skeletal structure) with the new rays, he temporarily named them X-rays as a designation of something unknown, and the name stuck. Living organisms are continuously exposed to ionizing radiations from natural sources. In addition, exposures occur as a result of human activities and medical practices. Radiations are broadly categorized into natural and humanmade sources. More than 90% of radiation exposure to humans occurs from natural sources—e.g., cosmic rays; terrestrial sources that come from radionuclides in the Earth’s crust, the air, food, and water; and the human body itself. Humanmade radiation exposure to populations occurs mainly from medical uses of radiation and radioisotopes in healthcare, occupational sources in the generation of electricity from nuclear power reactors, industrial uses of nuclear techniques, and (mostly in the past) testing nuclear weapons. The use of ionizing radiation in medical diagnosis and therapy is widespread and continues to increase with new, useful applications in healthcare. Diagnostic radiation exposures can be significantly reduced by following adequate safety measures and optimizing nuclear-based procedures and practices. To better understand the biological effects of ionizing radiation, it can be divided into directly ionizing and indirectly ionizing. Electromagnetic radiation includes radio waves, microwaves, visible light, ultraviolet radiation, X-rays, and γ-rays. These waves are essentially characterized by their energy, which inversely varies with wavelength. They can be thought of as moving packets of energy (quanta), and in this form, they are called photons.