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The word “radiation” often evokes fear in people.  What is “radiation” and how does it affect us?  Let’s learn a bit more about this term.  Radiation is a form of energy to which we are constantly exposed and which may or may not have health consequences. It is given off by matter as either rays (or waves) of pure energy, or high-speed particles. The former, rays or waves of energy, is known as electromagnetic energy. Some types of electromagnetic energy are sunlight, x-rays, radar, and radio waves. The latter, particle radiation, includes, for example alpha and beta particles, and neutrons.

Radiation can also be categorized as either non-ionizing or ionizing. Non-ionizing radiation such as visible light, heat, radar, microwaves, and radio waves deposit energy in the biological or inert material through which they pass but do not have adequate energy to break molecular bonds. Ionizing radiation is more energetic. Thus, when passing through matter, it can break molecular bonds and displace electrons from atoms. This displacement of electrons may cause changes in the living cells of plants, animals, and people. Alpha and beta particles, gamma rays, and x-rays are all considered ionizing radiation, which is generally much more dangerous than non-ionizing radiation. Alpha and beta particles are emitted from naturally occurring substances such as uranium, thorium, and radium, and some man-made elements such plutonium. Beta particles have a greater ability to penetrate materials than the larger alpha particle. Gamma rays and x-rays are high energy waves that have an even greater ability to penetrate materials.

Despite the potential adverse effects of ionizing radiation on biological organisms, it also plays a very positive role in nuclear power and in medical applications in both the diagnosis and the treatment of disease. It is also important to understand that the known adverse health effects from radiation exposure, as with chemicals, are dependent not just on the type of radiation but also on the dose or amount of radiation exposure.

Expressions of radiation doses absorbed into the body, also referred to as dosimetry, are unique to radiation exposures. Radiation dosimetry is a fairly complex subject. Briefly, however, 1 millisievert (mSv) is defined as the average accumulated background radiation dose to an individual for 1 year, exclusive of radon, in the United States. In other words, individuals are exposed to a whole body equivalent of about 1 mSv of ionizing radiation per year from terrestrial and cosmic sources, primarily gamma radiation. The greatest environmental exposure is, however, from naturally-occurring radon gas in some homes.

To give readers a better sense of exposure potential and dose received from various common sources, the accompanying bulleted list at the end of this article shows sources of radiation and in some instances the organs most affected by that type of exposure.

The follow-up of the atomic bomb survivors has been the primary basis for our understanding of the risks of radiation exposure. In these studies of about 100,000 individuals, the most sensitive health endpoint has been cancer and increases in cancer rates were not seen until the exposure was greater than 200 mSv. For public health purposes it is assumed that there may still be a risk at low doses (low defined as less than 10 mSv) and risk is proportional to the dose. It is, however, debated among scientists whether or not there is any true risk at low doses and some even argue that there is a benefit from low level radiation exposures, which prepares biological systems to cope with higher level exposures. For comparison, it has been estimated that the people in Fukushima City, Japan, in the wake of the nuclear reactor leakage, were only exposed on average to 4 mSv and those in the evacuation zones averaged less than 10 mSv.

Radiation that is a potential problem for the general public is radon gas that in some areas of the U.S. is quite high in the home. It has been estimated that about 10% of lung cancers in the U.S. can be attributed to radon exposure. Another radiation exposure that clearly poses a risk for many people, yet which is not ionizing radiation, is ultra violet (UV) light from the sun. As we know, it can cause sunburn and is also a suspected major contributor to skin cancer.

Cancer is not the only health effect of concern for some radiation exposures.  Based upon studies of atomic bomb survivors and some types of medical treatments using high doses of radiation there clearly is an increased risk for cardiovascular diseases. It is undecided whether there is a threshold for these effects. For the workplace, exposure limits of UV light to the eye are currently being lowered due to the risk of cataracts. The increases of cataracts, mainly posterior subcapsular cataracts, have also been observed in atomic bomb survivors, and, in the Ukraine, in Chernobyl nuclear accident cleanup workers, and in radiological technicians. These risks of cataracts can also be a problem for some cardiologists who work with relatively high doses of x-rays.

Mobile phones emit very low levels of non-ionizing radiofrequency radiation. The World Health Organization plans to place more emphasis on assessing health risks of radiofrequency sources of radiation since the available health effects studies thus far are neither entirely consistent nor conclusive. The health concerns are of a possible increased risk of acoustic neuromas (an uncommon, benign tumor that is usually slow growing and can affect balance and hearing) and also gliomas (a type of malignant brain tumor).  Additional research to address this possible concern is ongoing.

Doses of radiation absorbed after exposures from some different sources are summarized below. Keep in mind that different forms of radiation have differing potentials for penetrating through tissue.

    • Routine chest x-ray to lungs: 0.15 mSv
    • Whole body average annual dose from terrestrial and cosmic sources: 1 mSv
    • Routine mammogram dose to breasts: 3 mSv
    • Public health community’s definition of low dose (whole body annual dose):Stomach dose from abdominal CT scan (visualizes multiple sections of organs using x-ray): 10 mSv
    • Average annual dose to lungs from radon and its decay products found some homes: 24 mSv
      (Dose data sources: Health Effects of Exposure to Low Levels of Ionizing Radiation: BEIR V,
      National Research Council, and Brenner et al., N Engl J Med 2007; 357:2277-84).

Additional resource links

  • Radiation Protection ( – Basics about radiation, its health effects, the work of the Nuclear Regulatory Commission (NRC), and more. Includes an Ask an Expert about Radiation e-mail page to query the NRC.
  • Radiation Protection and Health Effects ( – A brief fact sheet from the EPA’s Office of Radiation Protection outlining radiation and health, effects of radiation type and exposure pathway, estimating Health Effects, and protecting against exposure.
  • Public Health Statement for Ionizing Radiation ( – A summary chapter about ionizing radiation and the effects of exposure. From the Agency for Toxic Substances and Disease Registry.
  • The Effects of Radiation on our Health ( – From IRSN (Institut de Sûreté et de Radioprotection Nucléaire), France’s Radioprotection and Nuclear Safety Institute, this video focuses on the impact of radiation on our DNA. V
  • Ionizing Radiation, health effects and protective measures ( – A fact sheet from the World Health Organization (WHO) offering information on radiation sources, types of exposure, health effects, nuclear emergencies, and protective health actions.
  • Electromagnetic fields and public health: mobile phones ( – A fact sheet from the World Health Organization (WHO) discussing the possibility of health effects.
  • Is Radiation from Your Cell Phone a Health Hazard? ( – Presented by the National Capital Area Skeptics, but not necessarily representing their views, this talk presented by a Professor of Electrical and Computer Engineering at the University of Maryland addresses the concern that cell phone use may pose a health hazard. V
  • Revisiting Chernobyl: A Nuclear Disaster Site of Epic Proportions ( – A PBS News Hour report about the 1986 nuclear meltdown at Chernobyl. Viewers should note that although this is a well produced and generally accurate summary of the accident and its consequences, some of the effects attributed to radiation exposure are individual opinions and not necessarily based on scientific or medical evidence. V
  • Sackler Colloquia, 2013 (Session V: Radiation Hazards) ( – These colloquia, convened by the National Academy of Sciences, address scientific topics of broad and current interest, and are presented as videos. V

Related topic: Hazard vs. Risk


(revised 8/3/2016 from original posting on 3/30/2015)

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