Lesson 3 of 3
Effects of exposure
I can explain hazardous effects of over-exposure on living cells and tissue, including heating and ionisation.Syllabus
Syllabus K326 / K327, 10(d). Explain hazardous effects of over-exposure on living cells and tissue, including heating and ionisation.
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Why can ultraviolet harm skin cells in ways that infrared of the same intensity cannot?
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Ultraviolet has a higher frequency and can damage the DNA in skin cells.
Higher-frequency radiation carries energy in larger amounts, so ultraviolet can damage DNA in skin cells and lead to skin cancer. Infrared mainly heats the skin.
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.
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.
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.
Check your understanding
A screened ultraviolet water-disinfection unit uses radiation that can damage microorganisms. Why does that useful effect not establish that unintended exposure of a person is harmless?
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Radiation can damage tissue; the effect depends on the type, intensity and exposure time.
The ability to damage microorganisms explains the useful application, but it does not give immunity to nearby human tissue. UV can harm skin and eyes, and not every UV hazard is explained by ionisation. The amount absorbed and exposure conditions matter.
Practise an exam question
Write your answer first, then tick the mark points it makes.
Exam-style question
Explain why too much exposure to ultraviolet radiation from the Sun is harmful, and why sunscreen helps. [3]
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