Deposition of Energy

Deposition of energy refers to events where energetic subatomic particles, nuclei, or electromagnetic radiation deposit energy in the media through which they transverse. The energy may either be sufficient (e.g. ionizing radiation) or insufficient (e.g. non-ionizing radiation) to ionize atoms or molecules (Beir et al.,1999).  

Ionizing radiation can cause the ejection of electrons from atoms and molecules, thereby resulting in their ionization and the breakage of chemical bonds. The excitation of molecules can also occur without ionization. These events are stochastic and unpredictable. The energy of these subatomic particles or electromagnetic waves mostly range from 124 KeV to 5.4 MeV and is dependent on the source and type of radiation (Zyla et al., 2020). To be ionizing the incident radiation must have sufficient energy to free electrons from atomic or molecular electron orbitals. The energy deposited can induce direct and indirect ionization events and this can be via internal (injections, inhalation, or absorption of radionuclides) or external exposure from radiation fields -- this also applies to non-ionizing radiation.

Not all electromagnetic radiation is ionizing; as the incident radiation must have sufficient energy to free electrons from the electron orbitals of the atom or molecule. The energy deposited can induce direct and indirect ionization events and can result from internal (injections, inhalation, ingestion) or external exposure. Direct ionization is the principal path where charged particles interact with biological structures such as DNA, proteins or membranes to cause biological damage. Photons, which are electromagnetic waves can also create electrons that can cause direct ionization. Indirect ionization produces free radicals from other molecules, specifically water, which can then transform to cause damage to critical targets such as DNA. Ionization of water, which is a major constituent of tissues and organs, produces free radical and molecular species, which themselves can indirectly damage critical targets such as DNA (Beir et al., 1999; Balagamwala et al., 2013) or alter cellular processes. Given the fundamental nature of energy deposition by radioactive/unstable nuclei, nucleons or elementary particles in material, this process is universal to all biological contexts.

Energy deposition differs with the linear energy transfer (LET) defined as deposition of energy per unit distance (Hall and Giaccia, 2018 UNSCEAR, 2020). High LET radiation refers to energy mostly above 10 keV μm-1 which often produces more complex, dense structural damage than low LET radiation (below 10 keV μm-1). High LET radiation includes heavy ions, alpha particles and high-energy neutrons. Low-LET radiation such as photons (X- and gamma rays), electrons as well as high-energy protons produces sparse ionization events. Low LET radiation travels farther into tissue but deposits smaller amounts of energy, whereas typically high LET particles, do not travel as far but deposits larger amounts of energy into tissue at the same absorbed dose. The biological effect of the deposition of energy can be modulated by varying dose and dose rate of exposure, such as in acute, chronic, or fractionated exposures (Hall and Giaccia, 2018). 

Non-ionizing radiation is electromagnetic waves that does not have enough energy to break bonds and induce ion formation but it can cause molecules to excite and vibrate faster resulting in ensuing biological effects. Examples of non-ionizing radiation include radio waves (wavelength: 100 km-1m), microwaves (wavelength: 1m-1mm), infrared radiation (wavelength: 1mm- 1 um), visible light (wavelengths: 400-700 nm), and ultraviolet radiation of longer wavelengths such as UVB (wavelengths: 315-400nm) and UVA (wavelengths: 280-315 nm). UVC radiation (200-289 nm) is, in contrast to UVB and UVA, considered to be a type of ionizing radiation. Exposure to non-ionizing radiation occurs either from natural or anthropogenic sources, and include radio waves used for communication (broadcasting and cell phones), microwaves used in cooking food and in radar systems, infrared radiation emitted by warm objects or used in remote controls, thermal imaging and medical treatments. Visible light is the range of electromagnetic radiation and is commonly used in photosynthesis in primary producers. UV radiation has key functions in melanisation (tanning) of a number of species and exhibits key signalling roles in navigation and communication (e.g insects, aquatic invertebrates and fish), locomotory and predatory behavior (e.g. reptiles, birds and crustaceans) and growth and development (e.g. plants). UV radiation is also used in some medical treatments such as skin diseases (e.g. psoriasis, eczema, vitiligo and skin cancers).  

Energy can be deposited into any substrate, both living and non-living; it is independent of age, taxa, sex, or life-stage.

Taxonomic applicability: This MIE is not taxonomically specific.  

Life stage applicability: This MIE is not life stage specific. 

Sex applicability: This MIE is not sex specific. 

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