Where does < 7.5 micro Sv/h come from (as used in UK radiation protection practice)
Published: Apr 04, 2026
Prelim
At Ionactive we occasionally take a look backwards at radiation protection and analyse where certain common practices originate. One such example is the use of the dose rate constraint of < 7.5 micro Sv/h. Before we delve into it’s derivation, and current occurrence in the Ionising Radiations Regulations 2017 (IRR17), it is worth noting where it is being used today in 2026.
One example is in the HSE (Health and Safety Executive) application documents for consents for the specified practice of operating accelerators. In their application notes the following is noted under section 4 (estimation of dose rates) : ‘These estimates must show that dose rates do not exceed 7.5 micro Sv/h outside any shielded enclosure’. As we shall review in this blog article, and also mentioned in this earlier Ionactive article ‘New UK IRR17 consent - dose rate and accelerators’, the current wording in the accelerator example does not define dose rate, or explicitly state if it relates to any existing definition in IRR17 (so the reader might reasonably assume the definition of dose rate IS that defined in IRR17). One might expect / hope that < 7.5 micro Sv/h is so steeped in historical validation that the regulator chooses to make it a key requirement in all these other specified practices too (over and above the criteria actually in the IRR17 statutory instrument (SI) and tested in the radiation risk assessment).
- Administration to persons/animals [Section 4: These estimates must show that dose rates do not exceed 7.5 μSv/h outside the controlled/immediate area].
- Industrial Radiography [Section 8: Demonstrate that on each occasion radiography is undertaken, dose rates do not exceed 7.5 μSvh/h outside any shielded enclosure or at the controlled area barrier for site radiography].
- Industrial Irradiation [Section 4: These estimates must show that dose rates will not exceed 7.5 μSv/h outside any shielded enclosure or at the boundaries of controlled areas established around irradiation activities].
- High-activity sealed source (HASS) [Section 4: These estimates must show that dose rates do not exceed 7.5 μSv/h outside any shielded enclosure].
It is interesting to note that whilst investigating the origin of < 7.5 micro Sv/h, we turned back to the 1985 regulations (IRR85), and it is apparent in our view that some definitions in the earlier regulations are more helpful than those we have to work with today in IRR17. Read on and see if you agree.
A step back into earlier UK radiation protection legislation
To trace the origin of < 7.5 micro Sv/h we must go back to 1985 with the Ionising Radiations Regulations 1985 (IRR85). In doing so we will also note some rather useful dose rate definitions (in our opinion).
[Ionactive comment: In actual fact you can go back further, such as to the The Ionising Radiations (Sealed Sources) Regulations 1969 where, for example, the following is written for "adequate shielding" : 'means shielding or a demarcating barrier outside which the radiation dose rate averaged over any one minute does not exceed 0.75 millirems per hour'. Here 0.75 millirems per hour is equivalent to 7.5 micro Sv/h. The 1969 regulations define a ""radiation area" similarly. However, we will stick with IRR85 as our starting point in this article.]
This dose rate criteria is historically about designating workplaces as Controlled Areas or Supervised Areas. We will provide references below, but if you want to check this out for yourself, one link you will need to look at is here: Ionising Radiations Regulations 1985 (IRR85).
An important point to note in Schedule 1 of IRR85 are the dose limits, some of which are higher than those specified today in IRR17 (and also in IRR99). With relevance to this blog article, the key dose limit is: 'effective dose equivalent for the whole body for persons aged over 18 being 50 mSv/year'.
A controlled area in IRR85 was defined in Regulation 8 as an area where doses of ionising radiation were likely to exceed 3/10 of any dose limit for employees over the age of 18. It further states that a supervised area would be designated where any person (note not just an employee) was likely to exceed 1/10 of any dose limit. This regulation also leaves designation open ended in that it allows designation regardless, which appears to be the forerunner of the IRR17 controlled area designation based on special procedures to restrict exposure (based on risk assessment which was not included in the IRR85 regulations).
[Ionactive comment: Ionactive RPA Mark Ramsay was still at school in 1985 !]
Unlike IRR17 (or IRR99), in IRR85 the dose rate discussed in this article was explicitly mentioned in schedule 6 (Designation of controlled areas), so directly in the SI. In this schedule, and putting aside certain qualifiers which are not important for this article, it states that the employer shall designate as a controlled area any area where the instantaneous dose rate exceeds or is likely to exceed 7.5 micro Sv/h.
Whilst not written into schedule 6 of IRR85, but appearing instead in Regulation 8, the designation of a supervised area would be where 1/10 of the dose limit otherwise applied to controlled areas. Readers familiar with this subject will recognise that based on the analysis so far, a dose rate which exceeds 2.5 micro Sv/h but is < 7.5 micro Sv/h would require a supervised area. The analysis in this article will not consider internal radiation exposure which is treated quite differently in IRR85 as compared to the later IRR99 and IRR17 (especially in how designated areas are defined).
So now we have this mapped out, let’s look at how these dose rates arise.
Controlled Area (IRR85)
- 3/10 of dose limit is 3/10 of 50 mSv/y which is 15 mSv/y.
- It is assumed that design base working hours over a year is 2000 hours.
- Therefore, 15 mSv / 2000 hours is 7.5 micro Sv/h (hence a controlled area above this dose rate).
Supervised Area (IRR85)
- 1/10 of a dose limit is 1/10 of 50 mSv/y which is 5 mSv/y.
- Therefore 5 mSv / 2000 hours is 2.5 micro Sv/h (so a supervised area > 2.5 micro Sv/h but less than 7.5 micro Sv/h).
It is important to note that in the IRR85 SI, these dose rate criteria are written explicitly in the legislation. They do NOT appear later in the SI of IRR99 or the current IRR17.
[Ionactive comment: To avoid doubt, the statutory instrument (SI) is the law, the written regulation. The ACoP to IRR17 contains the SI, the ACoP and guidance. The SI is the law and must be complied with. The ACoP is HSE derived and provides the practical methods on complying with regulations. If you fail to follow the ACoP and are prosecuted you would need to prove that an alternative method also complied with the law (i.e. the SI). The guidance is not legal and is not compulsory, however usually following the guidance will keep you on the right side of the law and following best practice.]
IRR85 interpretations (definitions)
Before we move on to the latest legislation (IRR17) it is useful to consider IRR85 Regulation 2, which provides interpretation (definitions) which we feel would be useful today in 2026. They are as follows:
- Dose rate means (etc) ‘the rate at which a person or part of a person would receive a dose of ionising radiation from external radiation if he were at that place’.
- Instantaneous dose rate means ‘a dose rate at that place averaged over one minute’.
- Time average dose rate means ‘a dose rate at that place averaged over any 8 hour working period’ .
[Ionactive comment: the above definitions are useful and clear. In particular, the definition of instantaneous dose rate does not appear in the SI IRR17, but is often quoted in the ACoP and guidance. It would be nice to see these definitions clearly defined and added to IRR17. Note our comment earlier on the HSE consent application for accelerators where it is it written ‘ These estimates must show that dose rates do not exceed 7.5 micro Sv/h outside any shielded enclosure’. It would be helpful if dose rate was so defined as per IRR85! It so happens that the IRR17 interpretation of dose rate is broadly similar to the IRR85 definition of instantaneous dose rate (IDR). However, it is useful to note the IRR17 ACoP para 297(a) which states 'the external dose rate in the area exceeds 7.5 µSv per hour when averaged over the working day' (in relation to needing special procedures and designating controlled areas). Note that dose rate is defined in IRR17 as that averaged over a minute (see next section below), so this paragraph (297a) takes dose rate (as interpreted) and applies secondary averaging. ]
A step forward to the present (IRR17)
Continuing the dose rate definition (interpretation) theme from above, we see that in IRR17, dose rate is defined as follows : ‘the rate at which a person or part of a person would receive a dose of ionising radiation from external radiation if that person were at that place, being a dose rate at that place averaged over one minute’.
This is fair enough, but no longer is instantaneous dose rate or time averaged dose rate defined, but these terms do appear in the IRR17 ACoP. In addition, given the quoted HSE consent application (noted as an example), it HAS to be assumed that the < 7.5 micro Sv/h outside an enclosure meets the IRR17 definition, i.e averaged over one minute. Or, in addition, ACoP para 297(a) noted in previous section above applies and allows this dose rate to be averaged over a working day.
The implication of this is important, especially in modern UK radiotherapy enclosures which are routinely running accelerator driven VMAT (Volumetric Modulated Arc Therapy). With VMAT the radiation beam (via rotation) is moving all the time, therefore the instantaneous dose rate (not defined in IRR17) may significantly exaggerate the exposure potential outside the enclosure at any point, unless the IRR17 definition of dose rate is correctly interpreted.
Example (this is purely to illustrate the above discussion). Assume that a VMAT radiation beam takes 1 minute for full rotation (6 degrees per second). With a 40 by 40 cm field size (highly unrealistic but gives worse case) projected onto an internal shielding wall, with a radiation monitor placed outside a primary shielding wall (at position gantry = 90 degrees), and further assuming that the beam subtends an angle of 23 degrees over the wall, then the radiation monitor will detect a dose rate (above background) for about 4 seconds of the 60 second rotation. That is, 4 seconds at maximum dose rate (let’s say 10 micro Sv/h) and 56 seconds at background (let’s assume 0.1 micro Sv/h). So using the IRR17 definition (interpretation) of dose rate, this particular situation would not breach < 7.5 micro Sv/h! Of course, if static beams were used at 90 degrees gantry for over a minute (say during medical physics calibrations) then the conclusion might be different. However, if IRR17 ACoP para 297(a) is complied with, then the above scenario can be averaged over a working day, and this would then likely account for physics work or conventional radiotherapy with static gantry angles.
ACoP para 297 also states the following which cannot be ignored: 'In addition, an area should be designated as a controlled area if the dose rate (averaged over a minute) exceeds 7.5 µSv per hour and employees untrained in radiation protection are likely to enter that area'. This paragraph has always given Ionactive some pause for thought. If it is assumed 'the area' is the area around the outside of an accelerator enclosure (in our particular example), then this provides a potential restriction on employees which does not exist for members of the public ?! This also uses a double definition (interpretation) of dose rate since dose rate is already defined as that averaged over one minute.
This above analysis will apply to all the other consent specified practices mentioned earlier in this article.
[Ionactive comment: practically every UK hospital radiotherapy enclosure (which requires an accelerator consent from HSE) is currently designed based on time average dose rates in combination with workload, gantry position, and occupancy outside the treatment room (enclosure). The sensible (and current approach) would be to evaluate the maximum possible accumulated dose over one hour at a certain position, and integrate over a whole year of treatment. Misusing the IRR17 definition of dose rate, or worse simply stating anything over 7.5 micro Sv/h is a problem, would be a huge spanner in the works for existing facilities. Furthermore, it then degrades the point of radiation risk assessments. For newly designed radiotherapy enclosures Ionactive is noting a move towards achieving < 7.5 micro Sv/h instantaneous dose rate at the design stage, specifically due to the HSE consent process already mentioned. This uses more concrete and increases costs.]
7.5 micro Sv/h and IRR17
Now putting our IRR17 dose rate interpretations aside, let’s return to the issue of 7.5 micro Sv/h.
As noted earlier, 7.5 micro Sv/h does not appear anywhere within the SI of IRR17. It only appears for the first time in the ACOP to IRR17, specifically 297(a) where it is written ‘the external dose rate in the area exceeds 7.5 µSv per hour when averaged over the working day’ (i.e where a controlled area is required). Whilst it is noted that the dose rate here does have a time average qualifier, it would be more helpful if time average dose rate was defined explicitly as in IRR85, it would remove the potential interpretation issues present in the previously mentioned consent application process for accelerators.
So – why is 7.5 micro Sv/h still being used? Let’s use a similar process to IRR85 and derive what we might expect to be the relevant dose rates (which are not relevant at all!).
Controlled Area (IRR17)
- 3/10 of dose limit is 3/10 of 20 mSv/y which is 6 mSv/y.
- It is assumed that design based working hours over a year is 2000 hours.
- Therefore, 6 mSv / 2000 hours is 3 micro Sv/h (but this does not appear in IRR17, read on shortly)
Supervised Area (IRR17)
- 1/10 of a dose limit is 1/10 of 20 mSv/y which is 2 mSv/y.
- It is assumed that design based working hours over a year is 2000 hours.
- Therefore 2 mSv / 2000 hours is 1 micro Sv/h (and this also does not appear in IRR17).
So using the above derivation, based on lower annual dose limits, does not yield dose rates which are used in IRR17 to designate controlled or supervised areas.
Why?
Part of the reason (but this is by no means the only reason) is that IRR17 is goal setting and based on risk assessment, so the onus is always going to be using ALARP (as low as reasonably practicable) to limit dose rates. That said, the IRR17 ACoP still uses controlled area criteria dose rate limits, but the criteria of ‘special procedures to restrict exposure’ is a stronger driver.
Regulation (17-1) of IRR17 SI states the following (with respect to designating a controlled area): any person working in the area is likely to receive an effective dose greater than 6 mSv a year or an equivalent dose greater than 15 mSv a year for the lens of the eye or greater than 150 mSv a year for the skin or the extremities.
Note that this returns to the 3/10 of a dose limit (with the exception of the eye) as found in IRR85, but with revised dose limits specified in IRR17. But as already shown, using these values does not lead to the 7.5 micro Sv/h specified in IRR85 and which is now still written into the ACoP of IRR17 and explicitly mentioned by HSE with regards accelerator consents (and other consents noted earlier).
So 7.5 micro Sv/h (whole body dose rate), as used in IRR17, is at best an ACoP requirement based on a long known and recognised standard, rather than based on any particular modern practice (or maths or physics).
So combining the above findings, with the departure from the IRR85 style of explicitly defining [dose rate, instantaneous dose rate and time average dose rate], we are left wondering are we in a better position now or not?! One might suggest that Regulation 8 of IRR17 (Radiation risk assessment) sorts this all out – since a risk assessment would be used to decide designation in a particular case. However, Ionactive findings of late (eg. 2021 onwards) does not always support this analysis. For example, generally HSE insist that any x-ray generator enclosure (from desktop to room size) be designated as controlled area, regardless of any risk assessment that might suggest otherwise based on engineered physical safety systems such as interlocks etc. This includes small enclosures with interlocked doors and where it is not possible to be inside when x-rays are being generated due to its physical size.
From IRR17 and onwards
So we can conclude from this analysis that the current use of 7.5 micro Sv/h (i.e. > implies a controlled area, and < is an HSE expectation for the exterior of an accelerator enclosure requiring a consent), is not based on any tangible analyses of current dose limits, maths or physics. It is a requirement of the regulator and / or IRR17. This may not overall be a bad thing, since ALARP will always play in the background (who can argue that further dose reduction is not to be welcomed in many cases)?
However, in the case of radiotherapy facilities mentioned earlier, particularly running mostly VMAT, an acceptable enclosure (to meet consent requirements) which simply meets < 7.5 micro Sv/h is a bit of a nonsense; i.e. if it were simply assumed that <6 micro Sv/h would be acceptable without further analysis. It would not be acceptable be any means without analysis of workload, gantry orientations and occupancy. Therefore, this analysis should be equally valid where a dose rate of > 7.5 micro Sv / is present, rather than simply suggesting that the facility does not meet consent application criteria (see further numeral analysis below to support this comment).
If all this followed a certain logic (!), the IRR17 external radiation controlled area criteria would be anything > 3 micro Sv/h and the supervised area criteria would be > 1 micro Sv/h but < 3 micro Sv/h. That would cause significant issues for some UK facilities using ionising radiation, and particularly in the medical sector.
IRR17 ACoP guidance says in para 306 ‘If special procedures are necessary, employers must designate the area as a controlled area whether or not the dose rate is above 7.5 micro Sv/h’. Since in our experience special procedures are an expectation of the regulator (HSE) in nearly every use of ionising radiation, the use of 7.5 micro Sv/h is apparently less important (or is it ...).
The retention of 7.5 micro Sv/h clearly helps certain users of ionising radiation, even where the derivation is qustionable in 2026. This is mitigated by the multiple caveats and requirements in IRR17. For example, IRR17 AcoP (17-1) states that ‘Employers should designate controlled areas in cases where: (a) the external dose rate in the area exceeds 7.5 micro Sv/h when averaged over the working day’. Let's analyse this specific point.
Assume the dose rate is 5 micro Sv/h averaged over the working day and for 2000 hours of work in a year. That is 10 mSv / year whole body dose and clearly in controlled area territory, despite being < 7.5 micro Sv/h. This is dealt with by 17(1)(b) of the IRR17 SI (i.e. not the ACoP) which states ‘any person working in the area is likely to receive an effective dose greater than 6 mSv a year or an equivalent dose greater than 15 mSv a year for the lens of the eye or greater than 150 mSv a year for the skin or the extremities’. And this is further dealt with by 17(1)(a) of the IRR17 SI which deals with ‘special procedures to restrict exposures’.
So in our view, the use of < 7.5 micro Sv/h to determine that an area need not be a controlled area is, at best, of some use for short terms exposures at barriers, for example during open site industrial radiography. It is not reasonably foreseeable that an accumulated whole body dose exceeding 6 mSv to any person could be exceeded (hence the derivation of 7.5 micro Sv/h is of little importance, just use it!).
Conversely, stating < 7.5 micro Sv/h as a blanket dose rate requirement around an accelerator facility (i.e. HSE regulator requirement for consents), is a bit rich when you consider that both the IRR17 definition (interpretation) of dose rate, and the use of 7.5 micro Sv/h, is not even derivable from IRR17! Clearly a more sensible approach, and one that has been used for many years, is to use a much more detailed analysis of exposure at points around an enclosure and prove that exposures comply with IRR17 limits and are also ALARP. Time and time again we have seen environmental monitoring (i.e. passive monthly / 3 monthly) results that show exposures around such facilities are negligible when compared to background radiation (and to headline dose rates of > 7.5 micro Sv/h which might be present from time to time).
Ionactive - Radiation Protection Adviser
This article is important radiation protection resource. However, it does not influence the Ionactive general Radiation Protection Adviser (RPA) service. We look after our ionising radiation user clients, and whilst we will always question IRR17 compliance where necessary , we will always work with the client and UK regulators to achieve desired compliance at all times. However, it is absolutely right to challenge and discuss current radiation protection regulation.
In addition, nothing in this article criticises HSE or any other regulator. The HSE need to apply IRR17 regulation as it is written (i.e the law as it currently stands).
Mark Ramsay
Radiation Protection Adviser (RPA)
Ionactive Consulting Limited (April 2026)