Applications and Safety
3. Biological Effects of Radiation
Learning outcomes
- I can explain how ionizing radiation affects living cells.
- I can distinguish between short-term and long-term radiation exposure.
- I can describe how DNA can be damaged by radiation.
- I can explain why rapidly dividing cells are more sensitive.
- I can identify factors that influence radiation risk.
Biological Effects of Radiation
Ionizing radiation carries enough energy to remove electrons from atoms and molecules, producing ions.
When ionizing radiation passes through living tissue, these ionizations can alter important biological molecules. In particular, radiation can damage:
- DNA
- proteins
- cell membranes
- other structures within cells
Most radiation-induced molecular damage is repaired successfully, but sufficiently severe or incorrectly repaired damage can affect cell function, cause cell death, or contribute to mutations and cancer.
Ionizing and Non-Ionizing Radiation
Not all radiation has enough energy per quantum to ionize atoms.
Ionizing radiation
Examples include:
- alpha radiation
- beta radiation
- gamma radiation
- X-rays
- neutrons
These can cause ionization in biological tissue.
Non-ionizing radiation
Examples include:
- radio waves
- microwaves
- infrared
- visible light
These generally do not have enough photon energy to directly ionize atoms.
Ultraviolet spans a range; higher-energy UV can cause significant molecular and DNA damage, but in school-level nuclear physics the term ionizing radiation usually refers mainly to radiation such as alpha, beta, gamma and X-rays.
How Radiation Interacts with Cells
When ionizing radiation enters tissue, it transfers energy to atoms and molecules.
A simplified sequence is:
radiation enters tissue → ionization occurs → molecules are altered → cellular structures may be damaged
The biological effect depends on:
- radiation type
- radiation energy
- absorbed dose
- tissue exposed
- exposure time
Direct Damage
Radiation can interact directly with an important biological molecule such as:
DNA
Energy transferred by the radiation may:
- break chemical bonds
- alter DNA bases
- break one DNA strand
- break both DNA strands
- produce more complex clustered damage
This is called:
direct radiation damage
Indirect Damage
Much of a cell is made of:
water
Radiation can ionize water molecules.
This can produce highly reactive chemical species, including:
free radicals
These reactive molecules can then attack:
- DNA
- proteins
- membranes
- other cellular structures
This is called:
indirect radiation damage
Free Radicals
A free radical is a highly reactive chemical species containing an unpaired electron.
Radiation interactions with water can produce reactive species such as hydroxyl radicals.
These may react with nearby biological molecules.
Therefore, radiation does not necessarily need to strike DNA directly to damage it.
A simplified pathway is:
radiation → water ionization → reactive species → DNA damage
DNA Damage
DNA contains the genetic instructions used by cells.
Radiation can cause several types of DNA damage.
Examples include:
- damaged bases
- altered chemical bonds
- single-strand breaks
- double-strand breaks
- chromosome abnormalities
Cells have sophisticated repair mechanisms, so DNA damage does not automatically result in permanent harm.
The outcome depends partly on how severe the damage is and whether it is repaired correctly.
Single-Strand Breaks
A single-strand break occurs when one strand of the DNA molecule is broken.
Cells can often repair this type of damage because information from the opposite strand may help guide repair.
However, multiple nearby injuries can make repair more difficult.
Double-Strand Breaks
A double-strand break occurs when both strands of DNA are broken.
This can be more difficult for a cell to repair accurately.
Incorrect repair can potentially produce:
- mutations
- chromosome rearrangements
- loss of genetic information
Severe damage may cause the cell to die or stop dividing.
Possible Outcomes of Radiation Damage
After radiation damages a cell, several outcomes are possible.
Successful repair
The damage is repaired correctly.
The cell continues functioning normally.
Cell death or loss of reproductive ability
Damage is too severe.
The cell dies or can no longer divide successfully.
Incorrect repair
The cell survives, but DNA has been altered.
This may produce:
mutation
In some circumstances, mutations can contribute to cancer development.
Radiation and Cancer
Cancer develops when cells acquire changes that disrupt normal control of:
- cell division
- DNA repair
- programmed cell death
- growth regulation
Ionizing radiation can increase cancer risk because it can damage DNA.
However:
radiation exposure does not mean a person will definitely develop cancer.
Radiation exposure changes risk or probability.
Cancer development depends on many biological and environmental factors.
Why Radiation Can Both Cause and Treat Cancer
This may initially seem contradictory.
Radiation can increase cancer risk because it can:
damage DNA in healthy cells
But radiation can also treat cancer because sufficiently large, carefully targeted doses can:
damage cancer cells so severely that they can no longer reproduce successfully
Radiotherapy therefore attempts to maximize damage to tumor cells while limiting damage to surrounding healthy tissue.
Rapidly Dividing Cells
Cells that divide rapidly are often particularly sensitive to ionizing radiation.
This is partly because cell division requires DNA to be:
- accurately copied
- checked
- separated into daughter cells
Radiation damage can interfere with these processes.
Examples of Radiation-Sensitive Tissues
Tissues containing many rapidly dividing or developing cells can be particularly sensitive.
Examples include:
- bone marrow
- intestinal lining
- reproductive tissues
- developing embryonic and fetal tissues
- some skin tissues
Cancer cells also frequently divide rapidly, which is one reason radiation can be useful in cancer treatment.
Sensitivity varies considerably among cell and tissue types.
Bone Marrow
Bone marrow continually produces:
- red blood cells
- white blood cells
- platelets
The precursor cells involved divide frequently.
Significant radiation exposure can damage these cells.
This may interfere with blood-cell production and can contribute to effects such as:
- reduced immune function
- anemia
- increased bleeding risk
at sufficiently high doses.
Digestive System
Cells lining the digestive tract are replaced frequently.
Because these cells divide rapidly, substantial radiation exposure can damage the intestinal lining.
At high doses, effects can include:
- nausea
- vomiting
- diarrhea
- impaired absorption
- damage to the intestinal lining
The severity depends strongly on dose and exposure conditions.
Developing Tissues
Developing embryos and fetuses contain many rapidly dividing and differentiating cells.
Radiation effects depend strongly on:
- developmental stage
- dose
- dose rate
- tissues exposed
For this reason, medical radiation procedures involving pregnancy require careful professional assessment.
Short-Term Effects
Short-term effects may occur relatively soon after a sufficiently large radiation exposure.
These are sometimes called:
acute effects
Possible effects at sufficiently high doses can include:
- skin reactions
- nausea
- vomiting
- fatigue
- temporary hair loss in exposed regions
- reduced blood-cell production
- tissue damage
Very large whole-body exposures can cause:
acute radiation syndrome
Acute Radiation Syndrome
Acute radiation syndrome (ARS) can occur after a large dose of penetrating radiation is delivered to much of the body over a relatively short period.
Depending on dose, important systems affected can include:
- bone marrow
- gastrointestinal system
- cardiovascular and nervous systems at extremely high doses
ARS is associated with major radiation accidents or similarly severe exposure situations, not ordinary everyday background radiation.
Long-Term Effects
Some effects may not appear until:
- years
- decades
after exposure.
These are called:
late effects
Important long-term risks can include:
- increased probability of some cancers
- cataracts under relevant exposure conditions
- long-term tissue damage following sufficiently high exposures
Cancer is particularly important because DNA mutations can persist long after the original radiation exposure.
Short-Term vs Long-Term Exposure
The terms can refer both to how quickly radiation is received and when effects appear.
Short, intense exposure
A large dose received over a short period can produce significant immediate tissue damage.
Prolonged or repeated exposure
Smaller doses accumulated over longer periods may not produce obvious immediate symptoms but can contribute to cumulative risk.
Dose, dose rate and tissue sensitivity all matter.
Dose Rate
Dose rate describes how quickly a radiation dose is delivered.
For example, the same total dose could theoretically be received:
- within seconds
- over several hours
- gradually over months
Biological effects can differ because cells may have more opportunity to repair some types of damage when exposure is spread over time.
Therefore:
total dose and dose rate both matter.
Absorbed Dose
The amount of radiation energy absorbed by tissue is measured using:
absorbed dose
The SI unit is the:
gray (Gy)
1 Gy = 1 J/kg
Therefore:
1 Gy means that:
1 joule of radiation energy has been absorbed per kilogram of material.
Activity and Dose Are Different
Do not confuse:
activity
with:
absorbed dose
Activity measures:
nuclear decays per second
Unit:
Bq
Absorbed dose measures:
radiation energy deposited per kilogram
Unit:
Gy
A high-activity source does not automatically produce a particular absorbed dose because exposure depends on many other factors.
Equivalent and Effective Dose
Different types of radiation can cause different amounts of biological damage even when the same amount of energy is absorbed.
Radiation protection therefore uses quantities that account for biological effects.
The unit commonly used is:
sievert (Sv)
In simplified terms:
equivalent dose accounts for radiation type.
effective dose additionally accounts for the different sensitivities of tissues and organs.
Alpha, Beta and Gamma Biological Effects
The different radiation types interact with tissue differently.
Alpha
- highly ionizing
- very short range
- low penetration
Beta
- moderately ionizing
- moderate penetration
Gamma
- less densely ionizing along its path than alpha
- highly penetrating
- can reach internal organs from outside the body
These differences affect their biological risks.
Alpha Radiation: Outside the Body
Alpha particles have very low penetrating ability.
They are normally stopped by:
- a few centimetres of air
- clothing
- the outer dead layer of skin
Therefore, an external alpha source may present relatively little risk to deeper tissues when the radioactive material remains outside the body and intact.
Alpha Radiation: Inside the Body
The situation changes dramatically if an alpha-emitting substance is:
- inhaled
- swallowed
- absorbed through a wound
Alpha particles are strongly ionizing.
If emitted inside tissue, they deposit energy over a very short distance.
Therefore:
internal alpha emitters can be particularly hazardous.
Gamma Radiation and External Exposure
Gamma radiation is highly penetrating.
It can pass through the body and reach internal tissues from an external source.
Therefore, gamma radiation can present a significant:
external exposure hazard
Suitable shielding may require dense or thick materials such as:
- lead
- concrete
depending on the gamma energy and application.
Beta Radiation
Beta radiation has intermediate penetration.
It can damage:
- skin
- eyes
- internal tissues if radioactive material enters the body
Appropriate shielding may use materials such as plastic or acrylic for many beta sources, partly to reduce the production of secondary bremsstrahlung compared with very high-Z shielding placed immediately around energetic beta sources.
Internal vs External Exposure
Radiation risk depends strongly on where the radioactive material is located.
External exposure
The radioactive source remains outside the body.
Penetrating radiation such as gamma rays is particularly important.
Internal exposure
Radioactive material enters the body through:
- inhalation
- ingestion
- wounds
- absorption
Internal exposure can be particularly important when radioactive material remains in or concentrates in particular tissues.
Biological Distribution
Different elements behave differently inside the body.
For example:
- iodine tends to accumulate in the thyroid
- some forms of radioactive strontium can behave chemically somewhat like calcium and become associated with bone
- radioactive gases may be inhaled into the lungs
Therefore, risk depends not only on activity but also on:
where the radioactive material goes in the body.
Radon
Radon is a naturally occurring radioactive gas produced in uranium decay chains in rocks and soil.
Radon can enter buildings from the ground.
Its radioactive decay products can be inhaled.
Exposure to elevated radon concentrations over long periods increases the risk of:
lung cancer
This is an important example of environmental radiation exposure.
Factors Affecting Radiation Risk
Radiation risk depends on several factors.
These include:
- absorbed dose
- radiation type
- radiation energy
- dose rate
- duration of exposure
- distance from the source
- shielding
- tissue exposed
- age and biological sensitivity
- whether exposure is internal or external
- how long radioactive material remains in the body
No single factor completely determines risk.
Total Dose
Generally:
greater dose → greater potential biological effect
A larger absorbed dose means more energy has been transferred to tissue.
This usually means more:
- ionization
- molecular damage
- DNA damage
However, biological effects also depend on how and where the dose is delivered.
Radiation Type
For the same absorbed dose, different types of radiation can produce different patterns of ionization.
Alpha particles produce dense ionization over short distances.
Gamma rays tend to produce more widely distributed interactions.
This difference is one reason radiation protection considers radiation type rather than using absorbed dose alone.
Exposure Time
Reducing unnecessary exposure time can reduce radiation dose.
If other factors remain constant:
less exposure time → lower dose
This leads to one of the basic principles of radiation protection:
minimize unnecessary time near a source
Distance
For a small radiation source, intensity often decreases approximately according to the inverse-square relationship:
I ∝ 1/r²
If distance doubles:
r → 2r
then:
I → I/4
If distance triples:
I → I/9
Therefore, increasing distance can greatly reduce exposure.
Example: Distance
Suppose the radiation intensity at:
1 m
is:
360 units.
At:
2 m
the intensity is approximately:
360/4
= 90 units
At:
3 m:
360/9
= 40 units
This is why distance can be an effective radiation protection measure.
Shielding
Shielding places absorbing material between a radiation source and people.
Different radiation types require different approaches.
Alpha
Stopped by very thin barriers or the outer layer of skin.
Beta
Can be reduced by materials such as plastic or acrylic.
Gamma
Requires thicker shielding such as lead or concrete.
Neutrons
Often require materials rich in hydrogen, such as water or polyethylene, with shielding design depending on neutron energy.
Time, Distance and Shielding
Three fundamental radiation protection principles are:
time — reduce unnecessary exposure time
distance — increase distance from the source
shielding — place appropriate material between the source and people
These principles are widely used in:
- hospitals
- laboratories
- nuclear facilities
- industry
Stochastic Effects
Some radiation effects are described as:
stochastic
For stochastic effects, the main concern is that radiation exposure may increase the probability of an effect such as cancer.
In radiation protection models, greater dose is generally treated as increasing the probability of the effect.
The severity of the cancer, if it occurs, is not determined simply by the original radiation dose.
Tissue Reactions
Other effects occur primarily when sufficiently high doses are delivered to particular tissues.
These are sometimes called:
tissue reactions
Examples can include:
- skin injury
- cataracts
- damage to bone marrow
- damage to other organs
For these effects, severity generally increases once sufficient dose has been received.
Radiation and Mutations
Radiation can alter DNA sequences or chromosome structures.
Such changes are called:
mutations
A mutation may:
- have no noticeable effect
- kill the cell
- alter cell function
- contribute to cancer
The biological outcome depends on where the damage occurs and how the cell responds.
Somatic Cells
Most cells in the body are:
somatic cells
Radiation-induced mutations in somatic cells affect the exposed individual.
If such mutations disrupt genes controlling cell division, they may contribute to cancer development.
Germ Cells
Cells involved in reproduction are called:
germ cells
Damage to these cells is considered in radiation protection because genetic material could potentially be affected.
The biological consequences depend on the nature and extent of the damage.
Radiation Sensitivity and Cell Division
A useful general rule is:
rapidly dividing, less differentiated cells tend to be more radiosensitive
This helps explain why tissues such as:
- bone marrow
- intestinal lining
- reproductive tissues
can be particularly sensitive.
It also contributes to the effectiveness of radiation therapy against many rapidly proliferating tumors.
However, radiosensitivity varies among different cancers and healthy tissues.
Radiotherapy and Healthy Tissue
Radiotherapy cannot always expose only cancer cells.
Some healthy tissue may also receive radiation.
Treatment planning therefore attempts to:
- concentrate dose in the tumor
- avoid sensitive organs where possible
- divide treatment into appropriate fractions
- use multiple beam directions
- monitor the patient carefully
The goal is to maximize:
therapeutic benefit
while minimizing:
harm to healthy tissue
Fractionation
In many radiotherapy treatments, the total dose is divided into smaller doses called:
fractions
Instead of delivering the entire dose at once, treatment may occur over several sessions.
This can allow healthy tissue to repair some radiation damage between treatments while still producing substantial damage to cancer cells.
Background Radiation
Everyone is exposed to some natural background radiation.
Sources include:
- cosmic radiation
- radioactive materials in rocks
- radon
- naturally occurring isotopes in food and the body
Background radiation varies between locations.
Living in a world containing some radiation does not mean every radiation exposure produces immediate harm.
Risk depends strongly on dose and exposure circumstances.
Medical Radiation
Medical procedures can also expose patients to ionizing radiation.
Examples include:
- X-rays
- CT scans
- nuclear medicine
- radiotherapy
Medical professionals consider whether the expected benefit justifies the radiation exposure.
Diagnostic procedures aim to obtain the necessary information while keeping exposure appropriately controlled.
ALARA
An important radiation protection principle is:
ALARA
This means:
As Low As Reasonably Achievable
The goal is to avoid unnecessary exposure while still allowing beneficial uses of radiation.
This principle is important in:
- medicine
- scientific research
- industry
- nuclear technology
Comparing Two Exposures
Consider two situations.
Person A
Receives a small controlled dose during a medically necessary diagnostic procedure.
Person B
Receives a much larger whole-body dose during a serious radiation accident.
Both involve ionizing radiation, but the biological risks are very different.
Risk assessment must consider:
- dose
- dose rate
- body region exposed
- radiation type
- medical benefit or lack of benefit
Simply saying "both were exposed to radiation" provides too little information.
Radiation Risk Is About Probability
Radiation risk is often misunderstood as:
radiation exposure = certain harm
That is incorrect.
At lower doses, an important concern is generally a possible increase in the probability of long-term effects.
This means that two people receiving similar exposures do not necessarily experience the same biological outcome.
Worked Example: Absorbed Dose
Suppose:
0.020 J
of radiation energy is absorbed by:
2.0 kg
of tissue.
Use:
absorbed dose = energy absorbed / mass
Dose:
= 0.020/2.0
= 0.010 J/kg
Since:
1 Gy = 1 J/kg
absorbed dose:
= 0.010 Gy
Worked Example: Exposure Time
Suppose a worker receives radiation at an approximately constant dose rate of:
0.02 mSv/h
for:
3 hours.
Dose:
= dose rate × time
= 0.02 × 3
= 0.06 mSv
Reducing the time to:
1 hour
would reduce the dose under the same conditions to:
0.02 mSv
Worked Example: Distance
Radiation intensity is:
800 units
at:
1 m
from an approximately point-like source.
At 4 m:
I₂ = 800 × (1/4)²
I₂ = 800/16
I₂ = 50 units
Increasing distance from 1 m to 4 m reduces the intensity to:
1/16 of its original value
under the inverse-square approximation.
Common Mistakes
Mistake: Saying radiation exposure always causes cancer.
Radiation can increase cancer risk, but exposure does not guarantee cancer.
Mistake: Saying all radiation is ionizing.
Radio waves, microwaves, infrared and visible light are non-ionizing.
Mistake: Saying radiation only damages DNA by hitting it directly.
Radiation can also create reactive species that damage DNA indirectly.
Mistake: Assuming every DNA injury becomes a mutation.
Cells can successfully repair many forms of DNA damage.
Mistake: Saying alpha radiation is always the most dangerous.
Risk depends on whether the source is internal or external and on many other factors.
Mistake: Confusing activity with dose.
Activity is measured in Bq; absorbed dose is measured in Gy.
Mistake: Assuming all cells have equal radiation sensitivity.
Rapidly dividing and developing cells are often more sensitive.
Mistake: Ignoring dose rate.
The rate at which a dose is delivered can influence biological effects.
Mistake: Assuming more penetrating always means more biologically damaging.
Penetration and ionization density are different properties.
Key Terms
Ionizing radiation — radiation capable of producing ions by removing electrons from atoms or molecules.
Ionization — formation of ions through gain or loss of electrons.
DNA damage — chemical or structural alteration of DNA.
Mutation — permanent alteration in genetic material.
Free radical — highly reactive chemical species containing an unpaired electron.
Single-strand break — break in one strand of a DNA molecule.
Double-strand break — damage involving breaks in both strands of DNA.
Radiosensitivity — susceptibility of cells or tissues to damage from ionizing radiation.
Acute exposure — radiation exposure received over a relatively short period.
Chronic/prolonged exposure — exposure occurring repeatedly or continuously over a longer period.
Absorbed dose — radiation energy absorbed per kilogram of material.
Gray (Gy) — SI unit of absorbed dose; 1 Gy = 1 J/kg.
Sievert (Sv) — unit used for radiation protection quantities that account for biological effects.
Dose rate — radiation dose delivered per unit time.
Internal exposure — exposure caused by radioactive material located inside the body.
External exposure — exposure from a radiation source outside the body.
Stochastic effect — effect for which radiation exposure affects the probability of occurrence, such as cancer.
Tissue reaction — biological effect associated mainly with sufficiently high doses to particular tissues.
ALARA — principle of keeping radiation exposure As Low As Reasonably Achievable.
Key Takeaways
- Ionizing radiation can remove electrons from atoms and molecules.
- Ionization can damage biological molecules and cells.
- DNA is an important target of radiation damage.
- Radiation can damage DNA directly or indirectly through reactive chemical species.
- DNA damage may be repaired correctly, repaired incorrectly, or cause cell death.
- Incorrectly repaired DNA damage can produce mutations.
- Some mutations can contribute to cancer development.
- Radiation exposure increases risk rather than guaranteeing cancer.
- Rapidly dividing cells are often more sensitive to radiation.
- Bone marrow and intestinal tissues contain particularly radiosensitive cell populations.
- High radiation doses received quickly can produce acute effects.
- Long-term effects may appear years after exposure.
- Radiation risk depends on both dose and dose rate.
- Absorbed dose is measured in gray (Gy).
- Activity is measured in becquerels (Bq) and is not the same as dose.
- Radiation protection quantities involving biological effects are commonly expressed in sieverts (Sv).
- Alpha radiation is particularly hazardous when alpha-emitting material enters the body.
- Gamma radiation can present a significant external hazard because of its penetrating ability.
- Radiation risk depends on radiation type, dose, tissue exposed, exposure time, distance, shielding and whether the source is internal or external.
- Increasing distance can greatly reduce radiation intensity.
- Time, distance and shielding are fundamental radiation protection principles.
- Radiation can both increase cancer risk and be deliberately used to treat cancer.
- Medical and industrial uses of radiation balance expected benefits against potential risks.
Check Your Understanding
- What is ionizing radiation?
- Give four examples of ionizing radiation.
- Explain how ionizing radiation can damage a cell directly.
- Explain how radiation can damage DNA indirectly through water molecules.
- What is a free radical?
- Compare single-strand and double-strand DNA breaks.
- Give three possible outcomes after radiation damages a cell.
- Explain how radiation exposure can increase cancer risk.
- Why does radiation exposure not mean that cancer will definitely develop?
- Why are rapidly dividing cells often more sensitive to radiation?
- Name three tissues that contain relatively radiation-sensitive cell populations.
- Distinguish between short-term acute effects and long-term radiation effects.
- Explain why dose rate can influence biological effects.
- What is the SI unit of absorbed radiation dose?
- Explain the difference between Bq and Gy.
- Why can an alpha-emitting material be particularly dangerous if inhaled or swallowed?
- Explain why gamma radiation can present an external exposure hazard.
- List five factors that can influence radiation risk.
- A worker receives 0.03 mSv/h for 4 hours. Calculate the total dose.
- Radiation intensity is 900 units at 1 m from an approximately point-like source. Assuming the inverse-square relationship applies, calculate the approximate intensity at 3 m.