Radiation is a form of energy that exists naturally in the environment and is widely used in medicine, industry, research, and other technological fields. Radiation can generally be classified into ionizing and non-ionizing radiation. Their biological effects differ according to the type and energy of radiation, absorbed dose, exposure duration, dose rate, and the sensitivity of the exposed tissues. Ionizing radiation is of particular importance in medical physics because of its extensive use in diagnostic imaging, nuclear medicine, and radiotherapy. Exposure to high doses of ionizing radiation can cause acute tissue damage, including skin erythema, hair loss, radiation burns, and acute radiation syndrome. Lower doses, particularly when exposure occurs repeatedly or over long periods, may increase the probability of long-term health effects, including certain types of cancer. The severity and probability of radiation-induced effects depend on several factors, including dose, radiation type, dose rate, exposed tissue, age, and exposure conditions. Despite these potential risks, radiation provides major benefits in modern medicine. Diagnostic radiology allows physicians to identify diseases and injuries, nuclear medicine provides functional information and therapeutic applications, and radiotherapy plays a fundamental role in the treatment of cancer. Therefore, radiation protection aims not to eliminate the use of radiation but to ensure that it is justified, optimized, and performed safely. This article reviews the basic concepts of radiation, its major sources and types, mechanisms of radiation-induced cellular damage, acute and long-term health effects, medical applications, radiation protection principles, and future developments in radiation science and medical physics. Keywords: Radiation, Ionizing Radiation, Human Body, Biological Effects, Radiation Dose, DNA Damage, Cancer, Radiation Protection, Medical Physics. 1. Introduction Radiation is energy transmitted in the form of electromagnetic waves or particles. It is a natural component of the human environment, and people are continuously exposed to radiation from natural sources such as soil, water, food, air, and cosmic radiation. Human-made sources also contribute to exposure, with medical applications such as X-ray imaging, computed tomography, nuclear medicine, and radiotherapy representing important sources of ionizing radiation exposure. Radiation itself is not necessarily harmful. Its biological effects depend on factors such as radiation type, absorbed dose, dose rate, exposure duration, and the sensitivity of the exposed tissue. Understanding these factors is therefore fundamental to medical physics and radiation protection. 2. Types of Radiation 2.1 Ionizing Radiation Ionizing radiation has sufficient energy to remove electrons from atoms or molecules, producing ionization. It includes X-rays, gamma rays, alpha particles, beta particles, neutrons, and other high-energy forms of radiation. Ionizing radiation is particularly important from a biological perspective because it can alter molecules within living cells, including DNA. 2.2 Non-Ionizing Radiation Non-ionizing radiation does not have sufficient energy to cause direct ionization of atoms. It includes radio waves, microwaves, infrared radiation, visible light, and some forms of ultraviolet radiation. The mechanisms through which non-ionizing radiation interacts with biological tissues differ from those associated with ionizing radiation. 3. Sources of Human Radiation Exposure Human exposure to radiation can originate from natural or artificial sources. 3.1 Natural Sources Natural sources include: Cosmic radiation. Radioactive materials in soil and rocks. Radon gas. Naturally occurring radionuclides in food and water. Naturally occurring radioactive elements within the human body. Radon is considered an important source of natural radiation exposure, while background radiation levels vary considerably between geographical regions. 3.2 Medical Sources Medical sources include: Conventional X-ray imaging. Computed tomography (CT). Fluoroscopy. Nuclear medicine. Radiation therapy. Image-guided interventional procedures. The development of medical imaging and radiation therapy has greatly improved disease diagnosis and treatment. However, radiation exposure must always be appropriately justified and optimized. 3.3 Occupational Exposure Radiation workers in diagnostic radiology, nuclear medicine, radiotherapy, research facilities, and some industrial environments may experience occupational exposure. Radiation monitoring, training, shielding, and safety procedures are therefore essential components of occupational radiation protection. 4. How Radiation Affects Cells When ionizing radiation interacts with biological tissues, it can transfer energy to atoms and molecules within cells. This interaction may result in ionization and the production of highly reactive chemical species. Radiation-induced cellular damage can occur through two major mechanisms. 4.1 Direct Effects Direct effects occur when radiation interacts directly with critical biological molecules such as DNA. Such interactions may cause structural changes or breaks in DNA strands. 4.2 Indirect Effects Because biological tissues contain large amounts of water, radiation may interact with water molecules and produce highly reactive free radicals and reactive oxygen species. These species can subsequently damage DNA, proteins, cell membranes, and other cellular structures. Cells possess repair mechanisms capable of correcting some forms of radiation-induced damage. However, severe or incorrectly repaired damage may result in cell death, mutations, or permanent genetic alterations. 5. Radiation-Induced DNA Damage DNA is one of the most important biological targets of ionizing radiation. Radiation can produce several forms of DNA damage, including: Base damage. Single-strand breaks. Double-strand breaks. Chromosomal abnormalities. Gene mutations. Double-strand DNA breaks are particularly important because incorrect repair can result in chromosomal abnormalities and genetic mutations. Depending on the severity of the damage, a cell may successfully repair itself, undergo programmed cell death, or survive with permanent genetic alterations. The accumulation of certain genetic alterations may contribute to carcinogenesis. 6. Factors Affecting Radiation Damage The biological effects of radiation depend on several factors. 6.1 Radiation Dose In general, the probability and severity of radiation-induced effects increase with increasing dose. Absorbed dose is measured in gray (Gy), while equivalent and effective doses are expressed in sievert (Sv) to account for radiation type and tissue sensitivity. 6.2 Dose Rate The rate at which radiation is delivered is biologically important. When exposure is distributed over a longer period, cells may have more opportunity to repair some forms of radiation-induced damage. 6.3 Radiation Type Different types of radiation have different abilities to produce biological damage. For this reason, radiation protection calculations consider the characteristics of the radiation as well as the absorbed energy. 6.4 Tissue Sensitivity Tissues containing rapidly dividing cells tend to be more sensitive to radiation. The biological response also depends on the specific characteristics and function of each tissue. 6.5 Age Children are generally more sensitive to some radiation effects than adults. Their bodies are still developing, they contain more rapidly dividing cells, and they have a longer period during which late effects may develop. 7. Acute Effects of Radiation Acute radiation effects generally occur following exposure to high doses over a relatively short period. Possible effects include: Nausea and vomiting. Skin redness. Hair loss. Tissue injury. Radiation burns. Damage to blood-forming tissues. Acute radiation syndrome. At sufficiently high doses, radiation can impair tissue and organ function. The World Health Organization notes that acute radiation syndrome may occur at very high exposures, with an approximate threshold around 1 Sv under relevant exposure conditions. 8. Long-Term Health Effects Some radiation-induced effects may appear years or even decades after exposure. 8.1 Cancer Cancer is one of the most important long-term health effects associated with ionizing radiation. Epidemiological studies have demonstrated increased risks for certain cancers among populations exposed to radiation. According to the WHO, epidemiological evidence has demonstrated increased cancer risk at doses above approximately 100 mSv, while more recent studies have suggested possible increases in risk at lower doses in certain circumstances, including some childhood medical exposures. 8.2 Cataracts The lens of the eye is sensitive to radiation, and radiation-induced cataracts are an important consideration in radiation protection, particularly for workers who may experience repeated occupational exposure. 8.3 Reproductive Effects High radiation doses can damage reproductive tissues and affect reproductive function. The severity depends on dose, radiation type, exposed organs, and exposure conditions. 8.4 Effects During Pregnancy The developing embryo and fetus can be particularly sensitive to radiation during certain stages of development. Therefore, radiation exposure during pregnancy should be carefully evaluated according to the type of examination, radiation dose, gestational stage, and anatomical region exposed. 9. Radiation and Cancer Development The relationship between radiation exposure and cancer is one of the major areas of research in medical physics and radiation biology. When radiation damages DNA, the affected cell may repair the damage, die, or survive with genetic alterations. If specific mutations accumulate over time, they may contribute to malignant transformation. However, exposure to radiation does not mean that cancer will inevitably develop. Cancer risk depends on several factors, including radiation dose, radiation type, age at exposure, tissue sensitivity, and individual characteristics. UNSCEAR continues to evaluate radiation-related cancer risks using epidemiological evidence and scientific studies, including assessments of medical exposures and secondary primary cancers following radiotherapy. 10. Medical Applications of Radiation Despite its potential risks, radiation is essential to modern medicine. Diagnostic Imaging X-ray and CT imaging provide valuable anatomical information and assist in the diagnosis of many diseases and injuries. Nuclear Medicine Radiopharmaceuticals can be used to evaluate physiological and metabolic functions and, in some cases, to deliver targeted therapeutic radiation. Radiotherapy Radiotherapy uses ionizing radiation to destroy cancer cells or prevent them from multiplying. Modern treatment techniques combine advanced imaging, computer-based treatment planning, and precise dose delivery to maximize tumor control while minimizing exposure to surrounding healthy tissues. Thus, radiation protection does not seek to eliminate radiation use but rather to ensure that radiation is used when medically justified and in an optimized manner. 11. Radiation Protection Radiation protection aims to minimize unnecessary exposure while maintaining the required medical or scientific benefit. Important principles include: 11.1 Time Reducing the time spent near a radiation source generally reduces radiation exposure. 11.2 Distance Increasing the distance from an external radiation source can substantially reduce exposure in many situations. 11.3 Shielding Appropriate shielding materials, including lead, concrete, or specialized protective materials, may be used depending on the radiation type and energy. 11.4 Justification and Optimization Medical radiation procedures should be clinically justified, and their technical parameters should be optimized to obtain the required diagnostic or therapeutic benefit with the lowest reasonable radiation exposure. WHO emphasizes that radiation should be used when its benefits outweigh its risks and that appropriate protective barriers, safety procedures, and training should be implemented. 12. The Role of Medical Physics in Radiation Protection Medical physics plays a central role in ensuring the safe and effective use of radiation in healthcare. Medical physicists contribute to: Radiation dose assessment. Equipment performance testing. Quality assurance programs. Radiation protection. Treatment planning. Optimization of imaging protocols. Patient and occupational dose monitoring. Implementation of new radiation technologies. The role of medical physics has become increasingly important as imaging and radiotherapy technologies continue to develop. 13. Future Perspectives Future research is expected to improve our understanding of radiation biology and individual radiation sensitivity. Important areas of development include: Artificial intelligence for radiotherapy treatment planning. Low-dose medical imaging. Personalized radiation dosimetry. Individualized radiation risk assessment. Advanced radiation shielding materials. Image-guided and adaptive radiotherapy. Improved understanding of radiation effects on different organs and biological systems. UNSCEAR's current scientific program includes ongoing evaluations of radiation effects on the nervous system and eye, as well as planned assessments of medical exposure, immune-system effects, and non-cancer health effects of ionizing radiation. Almustaqbal University – The First University in Iraq