Community medicine chapters
Radiation hazards
Ionising and non-ionising radiation, the units of radiation dose, which tissues are most sensitive, deterministic and stochastic effects, dose limits for workers and the public, and the principles of radiation protection (time, distance, shielding).
- Ionising radiation (X-rays, gamma rays, alpha and beta particles) damages DNA; the absorbed dose is measured in gray (Gy) and the effective dose in sievert (Sv).
- Deterministic effects have a threshold and get worse with dose (burns, cataract, radiation sickness); stochastic effects have no threshold and their probability rises with dose (cancer, genetic effects).
- Protection rests on time, distance and shielding and keeping doses ALARA; dose falls with the square of the distance; ICRP limits: workers 20 mSv/year (averaged), public 1 mSv/year.
On this page
Overview
Radiation is energy travelling as waves or particles. It is used widely in medicine (X-rays, CT, nuclear medicine, radiotherapy) and industry, and we are all exposed to natural background radiation.
Ionising radiation carries enough energy to remove electrons from atoms and damage DNA. It is the main concern for health.
Types of radiation
| Ionising | Non-ionising |
|---|---|
| X-rays and gamma rays (penetrate deeply) | Ultraviolet (sunburn, skin cancer, cataract) |
| Alpha particles (stopped by paper or skin; dangerous if inhaled or swallowed, for example radon) | Visible light, infrared (heat; cataract in glass and furnace workers) |
| Beta particles (stopped by a few mm of aluminium or plastic) | Microwaves, radiofrequency (heating of tissues) |
| Neutrons | Lasers (eye and skin burns) |
Sources of exposure
- Natural background (the largest share for most people): radon gas from soil and rocks, cosmic rays, radioactive elements in the earth and in food.
- Medical (the largest man-made source): X-rays, CT scans, nuclear medicine, radiotherapy.
- Occupational and others: nuclear industry, industrial radiography, fallout from nuclear accidents and tests.
Units
| Quantity | SI unit | Old unit | Meaning |
|---|---|---|---|
| Radioactivity | Becquerel (Bq) | Curie (Ci) | Disintegrations per second of a source |
| Absorbed dose | Gray (Gy) | Rad (1 Gy = 100 rad) | Energy absorbed per kg of tissue |
| Equivalent / effective dose | Sievert (Sv) | Rem (1 Sv = 100 rem) | Absorbed dose adjusted for the type of radiation and the sensitivity of tissues; used for risk and dose limits |
Gray = absorbed dose. Sievert = effective dose (biological risk), used in dose limits.
Radiosensitivity of tissues
Rapidly dividing, less specialised cells are most sensitive (law of Bergonié and Tribondeau).
| Most sensitive | Least sensitive |
|---|---|
| Bone marrow, lymphoid tissue (lymphocytes) | Nerve cells |
| Gonads (testis and ovary) | Muscle |
| Lining of the gut (crypts), skin basal layer | Bone (mature), connective tissue |
| Lens of the eye, the fetus (especially organogenesis) |
Biological effects
| Feature | Deterministic (tissue reactions) | Stochastic |
|---|---|---|
| Threshold | Yes: no effect below a threshold dose | No threshold: any dose carries some risk |
| Dose relation | Severity increases with dose | Probability increases with dose; severity unrelated to dose |
| Timing | Usually early (days to weeks), some late (cataract) | Late (years) |
| Examples | Skin erythema and burns, hair loss, cataract, sterility, bone marrow failure, acute radiation syndrome, fetal malformations | Cancer (especially leukaemia, thyroid, breast, lung), genetic (hereditary) effects |
Cancer and genetic effects are stochastic (no safe threshold). Cataract and radiation burns are deterministic.
Acute radiation syndrome
After a large whole-body dose in a short time (above about 1 Gy):
- Haematopoietic syndrome (about 1–10 Gy): bone marrow failure, infections, bleeding, after a few weeks.
- Gastrointestinal syndrome (about 10 Gy or more): vomiting, diarrhoea, dehydration within days.
- Neurovascular (CNS) syndrome (very high doses, tens of Gy): confusion, collapse and death within hours to days.
The LD50/60 (dose that kills half of exposed people within 60 days without treatment) is about 3–5 Gy.
Radiation in pregnancy
The fetus is most at risk during organogenesis (weeks 3–8) and early brain development (8–15 weeks): small head size, learning disability, malformations and childhood cancer. Most diagnostic X-rays give fetal doses far below the levels linked to malformations, but unnecessary exposure is avoided, and the "10-day rule" or pregnancy checks are used before abdominal imaging in women of reproductive age (follow local policy).
Dose limits (ICRP recommendations)
| Group | Effective dose limit |
|---|---|
| Occupational (radiation workers) | 20 mSv per year, averaged over 5 years (no more than 50 mSv in any single year) |
| General public | 1 mSv per year |
| Pregnant worker (to the fetus) | About 1 mSv for the rest of the pregnancy once declared |
| Lens of the eye (workers) | 20 mSv per year, averaged |
Medical exposures of patients have no fixed limit, but must be justified and optimised.
Principles of radiation protection
- Justification: the benefit must outweigh the harm (no unnecessary X-rays).
- Optimisation: ALARA: keep doses As Low As Reasonably Achievable.
- Dose limitation: stay within limits for workers and the public.
Time, distance, shielding
- Time: minimise time near the source.
- Distance: dose falls with the square of the distance (inverse square law).
- Shielding: lead aprons, thyroid collars, lead glass, concrete walls.
Dose at 1 metre from a source = 40 µSv per hour
At 2 metres: 40 ÷ 2² = 40 ÷ 4 = 10 µSv per hour
At 4 metres: 40 ÷ 4² = 40 ÷ 16 = 2.5 µSv per hour
Stepping back from 1 m to 2 m cuts the dose to one quarter
Monitoring
Radiation workers wear personal dosimeters: film badges or thermoluminescent dosimeters (TLDs), read regularly, and have medical surveillance. In Pakistan, the Pakistan Nuclear Regulatory Authority (PNRA) regulates radiation safety.
Clinical relevance
- Repeated CT scans in children increase lifetime cancer risk; use ultrasound or MRI where possible.
- Radiologists and interventional cardiologists need eye protection to prevent radiation cataract.
- After nuclear accidents, potassium iodide blocks uptake of radioactive iodine by the thyroid.
Expect: ionising vs non-ionising examples, the units (Gy for absorbed dose, Sv for effective dose, Bq for activity), the most and least radiosensitive tissues, deterministic vs stochastic effects with examples, dose limits for workers and the public, ALARA, and the inverse square law.
Quick revision
- Ionising: X-rays, gamma, alpha, beta, neutrons. Non-ionising: UV, light, infrared, microwaves.
- Gy = absorbed dose; Sv = effective dose; Bq = activity.
- Most sensitive: bone marrow, lymphocytes, gonads, gut, fetus. Least: nerve, muscle.
- Deterministic: threshold (cataract, burns). Stochastic: no threshold (cancer, genetic).
- ICRP: workers 20 mSv/yr (5-year average), public 1 mSv/yr.
- Time, distance (inverse square), shielding; ALARA.
Written by The House Job from Excel Community Medicine and Park's Textbook of Preventive and Social Medicine (named in the UHS PGET syllabus), with dose limits from the International Commission on Radiological Protection (ICRP).
FAQ
What is the unit of effective radiation dose?
The sievert (Sv).
Is radiation-induced cancer deterministic or stochastic?
Stochastic: it has no threshold, and its probability increases with dose.
Which tissues are most sensitive to radiation?
Rapidly dividing tissues such as bone marrow, lymphoid tissue, gonads, gut lining and the developing fetus.
What is the annual dose limit for the general public?
1 millisievert per year, according to ICRP recommendations.