Topic 3 of 3
Effects of exposure
Absorbed electromagnetic radiation transfers energy to matter. Over-exposure can damage living cells and tissue through heating or changes to atoms and molecules.
Thermal radiation can transfer energy without a material medium. The effect on a body depends on what it absorbs, not just on whether radiation is present.
Heating: absorbed energy raises temperature
Absorbed microwave or infrared radiation can increase a material's internal energy and temperature. If tissue gains energy faster than it can transfer that energy away, its temperature can rise enough to cause damage, including burns.
The microwave oven uses absorption for useful heating. That application does not imply that every microwave signal has the same heating effect: the power reaching an object, the amount absorbed, the exposure duration and the conditions differ.
Heating explanation
Radiation is absorbed → internal energy increases → temperature can rise → excessive heating damages tissue.
This is an energy-transfer explanation. Microwaves do not need to make matter radioactive in order to heat it, and non-ionising radiation is not the same as radiation that cannot cause harm.
Ionisation: electrons are removed
Ionisation is the removal of electrons from atoms or molecules, producing ions. X-rays and gamma rays provide clear examples of ionising radiation. Their interactions can damage molecules in cells, including DNA.
Ionising explanation
Radiation removes electrons → ions and molecular changes are produced → cells can be damaged.
Ionisation is a different microscopic mechanism from simply making tissue hotter. Radiation can cause molecular damage without a large noticeable temperature rise. The fact that a beam can be used to destroy cancer cells also explains why exposure of healthy tissue matters.
Ultraviolet needs a careful distinction
Some sufficiently energetic ultraviolet radiation can ionise, but not all UV is ionising. UV can also damage cells through other changes in molecules. Its hazards should not be explained by claiming that every UV wave removes electrons.
UV can damage skin and eyes, and UV exposure from sunbeds increases skin-cancer risk. A useful application, such as disinfecting water, does not make unintended exposure harmless.
Apply the same mechanism
Disinfecting water in a screened unit
The unit exposes microorganisms in the water to UV that can damage their genetic material and prevent reproduction. The intended effect is useful because it reduces viable microorganisms under the treatment conditions.
If that radiation reaches a person's tissue unintentionally, the ability to affect living cells has not disappeared. The same application therefore needs the radiation to reach its intended target while exposure elsewhere is controlled.
The explanation does not require every UV interaction to be ionisation. Identify the cellular damage and the exposure conditions instead of using "non-ionising" as a synonym for harmless.
Use exposure information, not a single spectrum label
To compare effects, consider the radiation type, energy reaching and being absorbed by the tissue, duration and the tissue exposed. Wavelength or frequency alone does not determine the outcome of every exposure.
The spectrum is therefore not divided into a universally safe side and a universally harmful side. Heating and ionisation explain important hazards, while the amount absorbed and the conditions determine how those mechanisms affect a particular case.
Name the mechanism. For a heating effect, explain absorption and temperature rise. For an ionising effect, explain electron removal and possible cellular damage. For UV, retain the qualification that harm is not always caused by ionisation.