National 5PhysicsSQARevision NotesRadiationNuclear RadiationAnnual Effective Dose Limits

Annual Effective Dose Limits (SQA National 5 Physics): Revision Note

Exam code: X857 75

Written by: Leander Oates

Reviewed by: Katie M

Updated on 22 October 2025

Annual effective dose limits

The annual effective dose limit is defined as:

The maximum equivalent dose (H) that should be received per year

The equivalent dose is a measure of how harmful a dose of radiation is
- The unit is sieverts (Sv)
- However, 1 Sv is a very large amount, so millisieverts (mSv) are often used
The annual effective dose limit for a member of the public is 1 mSv
The annual effective dose limit for a radiation worker is 20 mSv
These limits exclude the average amount of background radiation that everyone is consistently exposed to
The average annual background radiation in the UK is 2.2 mSv
- Therefore, the general public is recommended to be exposed to no more than 1 mSv above the average background radiation per year

Examiner Tips and Tricks

You do need to know the annual effective doses for the public and for those who work with radiation.

You also need to know the average annual background radiation in the UK.

You will not be given these values on your data sheet, so you do need to memorise them.

The annual effective dose limits are used to assess workplace safety in different situations where workers are exposed to radiation
For example:
- Dentists who commonly perform X-rays
- Radiologists
- Nuclear power technicians
- Aircraft crew
It is also used for risk assessment to keep members of the public safe
For example:
- Travelling on an aeroplane
- Radon levels in residential areas
- Setting industry safety standards for radiation use
- Setting safety standards for locations where radiation is, or has been used
Medical procedures are not included in the annual effective dose limit for members of the public
- The risks of radiation exposure have to be weighed against the benefits of medical treatment for each patient individually

Worked Example

A technician operates an X-ray fluorescence machine.

The equivalent dose rate received by the technician during the operation is measured to be $15 μSv h^{- 1}$ .

Determine the maximum number of hours the technician may operate the machine in one year without exceeding this annual dose limit.

Answer:

Step 1: List the known quantities

Equivalent dose rate received, $\dot{H} = 15 μSv h^{- 1}$
Annual effective dose limit for a radiation worker, $H = 20 mSv$

Step 2: Convert the annual effective dose limit to μSv

milli = 10^-3
micro = 10^-6

$\frac{1 mSv}{1 μSv} = \frac{10^{- 3}}{10^{- 6}} = 10^{(- 3) - (- 6)} = 10^{3}$

$20 mSv = 20 \times 10^{- 3} μSv = 20 000 μSv$

Step 3: Write out the appropriate relationship

$\dot{H} = \frac{H}{t}$

Step 4: Rearrange to make time the subject

Multiply both sides by $t$

$\dot{H} t = \frac{H t}{t}$

$H = \dot{H} t$

Divide both sides by $\dot{H}$

$\frac{H}{\dot{H}} = \frac{\dot{H} t}{\dot{H}}$

$t = \frac{H}{\dot{H}}$

Step 5: Substitute in the values to calculate

$t = \frac{20 000 μSv}{15 μSv h^{- 1}}$

$t = 1333.33 h$

Step 6: Round to an appropriate number of significant figures

The least precise input value is 2 s.f.
Therefore, the final answer can only be given to the same precision

$t = 1300 h (2 s . f .)$

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