Why some waves are hazardous
Definition
Ionising radiation
Radiation that carries enough energy to remove electrons from atoms, turning them into ions.
- When body tissue absorbs electromagnetic waves, the energy they carry is transferred to the atoms and molecules in its cells.
- The higher the frequency of a wave, the more energy it transfers to each atom or molecule that absorbs it.
- Ultraviolet, X-rays and gamma rays have the highest frequencies in the spectrum, so they are the most hazardous to living tissue.
- X-rays, gamma rays and the higher-frequency part of ultraviolet are ionising radiation, because they carry enough energy to knock electrons out of atoms and turn them into ions.
- Ionisation inside a cell can break apart molecules, including the DNA that carries the cell's instructions.
- Radio waves, microwaves, infrared and visible light have frequencies too low to ionise atoms.
Damage to cells
Definition
Mutation
A change in the DNA of a cell.
- Severe damage to its DNA can kill a cell outright.
- If a cell survives with damaged DNA, the change to its DNA is a mutation.
- A mutated cell may start to divide uncontrollably, forming a lump of cells called a tumour, which can develop into cancer.
- The risk of harm increases with the amount of radiation absorbed and with the length of time spent exposed.
Ultraviolet
- Ultraviolet comes from the Sun and from artificial sources such as sunbeds.
- Ultraviolet does not travel far into the body, so it is absorbed by the skin and the eyes.
- Too much ultraviolet causes sunburn, in which skin cells are damaged, and over many years it makes the skin age early and wrinkle.
- Ultraviolet can damage the DNA in skin cells, which increases the risk of skin cancer.
- Ultraviolet can also damage the eyes, causing painful inflammation of their surface and increasing the risk of cataracts, where the lens of the eye becomes cloudy.
- You can reduce the risk by using sunscreen, wearing sunglasses that block ultraviolet, covering your skin and avoiding sunbeds.
X-rays and gamma rays
Definition
Radiation dose
A measure of the risk of harm from the radiation that a person's body absorbs.
- X-rays and gamma rays are highly penetrating, so they pass deep into the body and can damage internal organs as well as the skin.
- They are ionising, so they can damage DNA in cells throughout the body, causing mutations and increasing the risk of cancer.
- A very large dose kills so many cells that it causes radiation sickness and burns.
- Medical X-rays use the smallest dose that gives a clear image, so that the benefit of a correct diagnosis outweighs the small extra risk.
- Radiographers stand behind a lead screen or leave the room while an X-ray is taken, because they would otherwise receive a dose many times a day.
- Lead aprons protect the parts of a patient's body that are not being imaged, because lead absorbs X-rays and gamma rays strongly.
- People who work with gamma rays keep their distance from the source, limit their time near it and wear a badge that records the dose they receive.
Example
From sunburn to skin cancer
- Ultraviolet from the Sun is absorbed by cells in the outer layer of the skin.
- The energy transferred to these cells is enough to damage their DNA.
- Badly damaged cells die, which causes the redness and peeling of sunburn.
- A cell that survives with altered DNA carries a mutation.
- If the mutation affects how the cell controls its division, the cell can divide uncontrollably and grow into skin cancer years later.
Common Mistake
- Do not say that all electromagnetic waves are harmful in the same way, because only the high-frequency groups are ionising and can damage DNA.
- Do not stop at "radiation is harmful", because the marks come from the chain of energy transfer, cell or DNA damage, mutation and cancer.
- Do not say that ultraviolet damages organs deep inside the body, because it is absorbed by the skin and eyes.
Exam technique
Describing hazards
- Name the tissue that is harmed: the skin and eyes for ultraviolet, and cells throughout the body for X-rays and gamma rays.
- Use the words ionising, DNA, mutation and cancer in a logical order, since each step leads to the next.
- If a question asks how to reduce the risk, give a precaution that matches the radiation, such as sunscreen for ultraviolet or a lead screen for X-rays.
Self review
- Why are high-frequency electromagnetic waves more hazardous than low-frequency ones?
- What does ionising radiation do to atoms?
- Name two harmful effects of ultraviolet on the body.
- Why can X-rays and gamma rays damage internal organs when ultraviolet cannot?
- How does a radiographer reduce their own exposure to X-rays?
