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6.1.8Radiation

Annual Effective Dose Limits

5 min read 5 parts Lesson 8 of 14

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Section 1 of 5Why Limits Exist

6.1.8 Annual Effective Dose Limits

Limits are set by law and regulations, and employers must show they are not exceeded.

Purpose
  • Dose limits set the maximum equivalent dose that a person can receive in a year from artificial sources, to keep the risk of harm low.
Annual basis
  • Limits are expressed per year, so doses received at different times in the year are added together.
Exclusions
  • Natural background radiation and medical exposures received as treatment are generally not counted against the limit.
Typical values
  • Commonly applied limits are 1 mSv per year for members of the public and 20 mSv per year for classified radiation workers.
Reason for difference
  • Workers accept a higher limit because they are trained, monitored and chosen for work involving radiation.

Section 2 of 5Keeping Doses Low

Keeping Doses Low
ALARA principle
  • Doses should be kept as low as reasonably achievable, even when they are below the legal limit.
Time
  • Shorter exposure times reduce the dose in direct proportion to the time.
Distance
  • Greater distance from a source lowers the dose rate because radiation spreads out.
Shielding
  • Lead, concrete or water placed between a source and a person absorbs radiation and cuts the dose.
Monitoring
  • Film badges and electronic dosimeters record the dose that each worker has received.
GroupAnnual limit (typical)
Member of the public1 mSv
Radiation worker20 mSv

Section 3 of 5Checking a Dose Against the Limit

1
List every artificial source of exposure and the equivalent dose received from each over the year.
2
Convert all doses into the same unit, such as mSv, before adding.
3
Find the total annual effective dose by adding the individual doses.
4
Compare the total with the correct limit for the person, public or worker.
5
If the total is close to or above the limit, change working practices by reducing time, increasing distance or adding shielding.

Section 4 of 5Worked Example

Question

  • A radiation worker receives an average equivalent dose of 1.5 mSv per month.
  • Will the worker stay within a limit of 20 mSv per year?
  • A member of the public receives 0.4 mSv from a hospital X-ray and 0.5 mSv from another artificial source in the year;
    • do they stay within a 1 mSv limit?

  1. Workerannual dose = 1.5 × 12 = 18 mSv.
  2. 18 mSv is less than 20 mSv, so the worker is within the limit, but only by 2 mSv.
  3. Publictotal = 0.4 + 0.5 = 0.9 mSv.
  4. 0.9 mSv is less than 1 mSv, so this person is also within the limit.

Answer Both are within their limits: 18 mSv against 20 mSv, and 0.9 mSv against 1 mSv.

Section 5 of 5Exam Checklist & Tips

6.1.8 – Key Points for the Exam

  • Dose limit The maximum equivalent dose allowed in a year from artificial sources.
  • ALARA The principle that doses should be as low as reasonably achievable.
  • Dosimeter A device that records the radiation dose received by a person.
  • Radiation worker A person who works with ionising radiation and is monitored for it.
  • Annual dose The total equivalent dose received in one year.

Common Mistakes

  • Believing that doses below the limit carry no risk at all.
  • Counting natural background radiation as part of a worker's limit.
  • Comparing a monthly dose directly with an annual limit.
  • Adding doses given in different units such as μSv and mSv without converting.
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