Radiation protection is the discipline of protecting people, society and the environment from the harmful effects of ionising radiation while allowing its beneficial uses. The international system rests on recommendations from the International Commission on Radiological Protection (ICRP), basic safety standards from the IAEA (GSR Part 3, the "BSS"), and implementing regulations at the national level.

The three principles

ICRP's recommendations rest on three principles:

  • Justification — any decision that alters the radiation exposure situation should do more good than harm.
  • Optimisation — the magnitude of individual doses, the number of people exposed, and the likelihood of incurring exposure should all be kept as low as reasonably achievable, taking economic and societal factors into account. This is the ALARA principle (or ALARP in the UK).
  • Dose limitation — total dose to any individual from regulated sources in planned exposure situations, except medical exposure of patients, should not exceed defined limits.

Dose quantities

Three quantities are easy to confuse:

  • Absorbed dose (gray, Gy) — energy deposited per unit mass in matter (J/kg). Physical quantity, no biological weighting.
  • Equivalent dose (sievert, Sv) — absorbed dose weighted by radiation type (wR: 1 for photons and electrons, 20 for alpha particles, 2-20 for neutrons depending on energy). Quantifies the stochastic biological hazard at the tissue or organ level.
  • Effective dose (sievert, Sv) — sum of equivalent dose to each organ weighted by a tissue weighting factor wT reflecting the organ's contribution to whole-body radiation detriment. Designed for risk comparison across exposure situations.

Effective dose is widely misused as a personal-risk metric for an individual patient or worker. The ICRP designs it as a population-averaged quantity, useful for radiation protection decisions but not as a clinical prognosis.

Dose limits (planned exposure)

Public limits as recommended by ICRP and adopted in most national regulations (with national variations on averaging period):

  • Worker: 20 mSv/year effective dose averaged over 5 years (or 50 mSv in a single year). Equivalent dose to the lens of the eye: 20 mSv/year averaged. Skin and extremities: 500 mSv/year.
  • Member of the public: 1 mSv/year effective dose. Equivalent dose to the lens: 15 mSv/year. Skin: 50 mSv/year.
  • Pregnant worker (after declaration): protection equivalent to that of a member of the public for the rest of the pregnancy.

These limits apply to planned exposure situations. Different decision metrics apply in emergency and existing exposure situations: reference levels (rather than limits) define the upper boundary of acceptable optimised dose.

Three exposure situations

The ICRP system distinguishes:

  • Planned — deliberate introduction or operation of sources (medical, occupational, industrial).
  • Emergency — an unexpected situation requiring urgent protective action.
  • Existing — a situation that already exists when a decision on control is taken (radon in homes, post-accident residual contamination, natural radiation, radioactive contamination of consumer products).

Operational quantities

Practical dosimetry uses operational quantities defined by the International Commission on Radiation Units and Measurements (ICRU). Personal dose equivalent Hp(d) is measured by personal dosemeters (TLD, OSL, electronic) calibrated for the chest at d = 10 mm. Ambient dose equivalent H*(10) is measured by area monitors. These are conservative estimators of effective dose under most workplace conditions.

Regulator role

Regulators turn the ICRP system into enforceable rules via:

  • Licensing and notification of sources and activities (the BSS "graded approach").
  • Dose record-keeping and worker classification (Cat A / Cat B in EU BSSD).
  • Workplace classification (controlled, supervised) and zoning.
  • Medical exposure justification, optimisation, and diagnostic reference levels (DRLs).
  • Public dose constraints around licensed sites.
  • Environmental monitoring and assessment of doses to representative persons.