Radiation Doses: Sieverts, X-rays & Perspective

1. Types of Ionising Radiation

• Alpha (α): Helium-4 nuclei (2 protons + 2 neutrons). High ionising density; stopped by paper or skin (range ~5 cm in air). Extremely dangerous if inhaled or ingested (Polonium-210, Radium-226).
• Beta (β): High-speed electrons or positrons. Range ~1 m in air; stopped by few mm aluminium. Can penetrate skin to the germinal layer. Both external and internal hazard.
• Gamma (γ) / X-ray: High-energy photons. Most penetrating — thick lead or concrete required for shielding. Primary medical diagnostic tool; also emitted by nuclear fallout.
• Neutron (n): Emitted in fission and fusion. Very penetrating; capture by hydrogen produces protons and secondary gamma — water or polyethylene effective shielding.

The biological damage depends on how densely ionisation occurs along the particle track, measured by Linear Energy Transfer (LET).

2. Dose Units

• Gray (Gy): Absorbed dose = 1 joule deposited per kg of tissue. Physical quantity, no weighting.
• Sievert (Sv): Effective dose = absorbed dose × radiation weighting factor (w_R) × tissue weighting factor (w_T). Accounts for biological effectiveness: photons/electrons w_R = 1, protons = 2, alpha = 20, neutrons 2–20.
• Becquerel (Bq): Activity = 1 radioactive decay per second. Describes source strength, not dose received.
• Rem / mrem: Older US unit; 1 Sv = 100 rem. Still used by US NRC.

For low-LET radiation (gamma, X-ray): 1 Gy ≈ 1 Sv. For alpha: 1 Gy absorbed = 20 Sv effective dose equivalent.

3. Comparative Dose Chart

Doses on a log scale from µSv to Sv:

4. Natural Background Radiation

Global average background dose: ~3.1 mSv/year. It consists of:

• Radon gas (40%): Radioactive decay of U-238 in soil produces Rn-222, which seeps into buildings. Highly variable — Denver CO, Cornwall UK, Kerala India have 10–100× average doses. Domestic radon detectors are cheap (<$30) and advisable in high-risk areas.
• Cosmic radiation (16%): Galactic cosmic rays and solar energetic particles. Dose doubles with every 2,000 m altitude gain. Airline crews receive ~3–6 mSv/year from cosmic radiation.
• Internal (17%): K-40 (natural potassium isotope), C-14, and Ra-226 in food and body tissues. Eating 1 banana ≈ 0.1 µSv (banana equivalent dose — informal illustration).
• Terrestrial gamma (18%): Gamma from soil minerals (thorium, uranium, radium daughters).

5. Health Effects by Dose

• <100 mSv (lifetime): No confirmed direct health effect in epidemiological studies. Statistical upper bound on cancer risk increase: ~0.5% per 100 mSv.
• 100–1000 mSv: Small but measurable increase in cancer incidence in cohort studies (Hiroshima/Nagasaki survivor data, CT cohort studies).
• 1–2 Sv (acute): Acute Radiation Syndrome (ARS): nausea, fatigue, reduced white blood cell count within hours to days.
• 2–6 Sv (acute): Severe ARS; lethal to 50% without medical treatment (LD₅₀/30). Bone marrow transplant can save lives.
• >10 Sv (acute): Cerebrovascular syndrome; lethal within days regardless of treatment. Chernobyl's first responders who died received 8–16 Sv acutely.

6. LNT Model — Controversy

The Linear No-Threshold (LNT) model assumes cancer risk is proportional to dose with no safe threshold — even 1 µSv carries some tiny risk. Adopted by ICRP (International Commission on Radiological Protection) as a conservative regulatory assumption.

Critics argue:

• LNT is a statistical extrapolation below directly measurable doses (<100 mSv). Direct evidence of harm at these levels is absent.
• DNA repair systems handle ~10,000 spontaneous DNA damage events per cell per day — low-dose radiation may not overwhelm them differently than background damage.
• Radiation hormesis: Some epidemiological studies find slightly lower cancer rates in high background radiation areas (Ramsar, Iran; Kerala, India), suggesting possible adaptive response at low doses.

Counter-argument: Given millions of people exposed to slightly elevated doses (nuclear workers, frequent flyers), even a linear risk means many eventual cancers. Conservative regulation is appropriate.

7. Radiation Protection Principles (ALARA)

ICRP's three principles of radiological protection:

• Justification: No practice should be adopted unless it provides sufficient benefit to outweigh the detriment. Every medical scan must have clinical justification.
• Optimisation (ALARA): Doses should be As Low As Reasonably Achievable, economic and social factors considered. Minimise through time, distance, shielding.
• Dose limits: Occupational: 20 mSv/year averaged over 5 years. Public: 1 mSv/year above natural background.

Practical shielding guidelines: halving distance reduces dose 4× (inverse square law); 1 HVL (half-value layer) of material halves gamma dose (HVL of lead for 1 MeV: 8 mm; for concrete: 100 mm).