Have a look here it explains things pretty well

http://hps.org/publicinformation/ate/q8900.html

There are several principles in dose limits.

Quite often we go to the numbers (1mSv/y for the general public, 20mSv/y for a registered radiation worker) and skip over two important principles that come first.

The first of these is justification. Is there a benefit to exposing someone (public or worker) to radiation? Some exposures have clear benefit to the individual exposed - medical x-rays, for example. If the exposure has no justification then it should be avoided, regardless of how small the dose might be.

The second is optimisation. If the exposure is justified, what can be done to limit the exposure to the smallest possible dose? This is the well known ALARA principle (see the link Lindsay posted).

Following those two principles, it is of course conceivable that a dose in excess of the dose limits may be justified and unavoidable. This is not an ideal situation, but it does occur. At present residents of parts of Fukushima receive annual doses in excess of 1mSv. The Japanese government policy is to eventually remediate until annual doses due to fallout are below 1mSv, but at present the remediation criteria are to only reduce annual doses to below 5mSv. It is recognised that the 1mSv limit is currently unachievable in many places, and that the benefit of allowing people to remain in their homes rather than forced relocation is suficient to justify this.

One of the reasons for the difference for radiation workers is that the dose they may receive (and, of course, many radiation workers receive doses that are still well within the normal range for members of the public) has a greater justification. The larger doses received by health care professionals handling radioactive tracers or radiotherapy equipment, for example, is justified on the basis of the benefits such treatments have for patients. Another consideration is that radiation workers will be monitored so the dose they receive is known, and will also usually receive enhanced medical diagnosis so that any medical effect that the doses they receive may produce will be detected earlier and treatment will therefore be more effective and less costly.

As an additional info I append a pdf version of the ICRP report 103 found in the internet. You will find all wished details.

There is no technical basis for the limit of 1mSv annually to a member of the public. The limit is political, not a matter of safety. The intent is to protect at a level where no benefit is derived from the dose.

The worker dose limit was derived from the Japanese atomic bomb survivor studies. Early in the studies it was clear that no observable effects were found below 0.5 Sv. From that observation it was surmised that 10 times lower was surely safe. A worker limit of 0.05 annually was established. It was considered extra safe because the dose would be delivered over time rather than instantaneous as with the bomb dose.

A risk-benefit argument was developed for the worker dose, assuming there were indeed a risk (the 0.5 Sv safe level might be in error) that risk would be small. A worker derived benefit (income) from exposure to the risk, but the public did not. A factor of 10 was introduced for the public and the limit set at 5 mSv. This level was reduced to 1 mSv since it was obvious that most public exposures were less than 1 mSv.

The linear, no-threshold model was introduced for a conservative and easily calculated risk model. The LNT assumption lead to no-safe-dose of radiation. No-safe-dose lead to As Low As Reasonably Achievable as a radiation protection policy. The limit of 1 mSv/y is no longer defensible if one can claim that a lower dose is reasonable.

In no case has the original observation of no effects below 0.5 Sv been demonstrated to be incorrect.

Thank you all for answering and providing links for more information. Special Thanks to Joseph L Alvarez for an astounding insight on the origin of 1 mSv/year as a limit.

Along all these explanations I would like to mention the fact that the general public include children and old age persons who are more susceptible to radiation than the middle aged persons. Moreover the radiation workers are of age 20-60yrs are healthier and have dose monitoring as well as provided with regular medical check. As mentioned earlier its the Risk to Benefit analysis which decides the dose limits.

@Prasanta,

you are right with your remark to the age dependence of radiation sensitivity. For stochastic effects like cancer induction your statement is wrong for old people. They show a significantly lower sensitivity for cancer mortality, because of the needed long lag time.

@Hanno & @Prasanta Thanks for the answers. Indeed, the general public includes crucially susceptible age groups (fetal, neonatal to 20) who have a lot of dividing cells in their body. Radiation impact on dividing cell is well known. But as Hanno has pointed out, people of old ages are more more resistant to radiation dependent cancer due to same reason.

Dear Manish,

it´s right, that old people have less mitosis in their cells, but the main reason is, they don´t live long enough to experience their disease. Please have a look for further infos at the attached pdf, it´s an excerpt from one of my textbooks.

All limits are based on the LNT assumption, which has yet to be proven correct. In the case of the general public the limit is simply the expression of the average natural background (1 mSv/year) because the LNT assumes that any dose above the background is detrimental - not true but easy to apply and, as Joseph observed, it is a political decision. It is worth mentioning that there are areas with high natural background where the dose limit of 1 mSv / year simply cannot be met.

For the worker's limits, the dose limitations are a bit more elaborate: based on the 1977 ICRP risk estimation, the fatal cancer risk coefficient: 1 x 10E-2 per Sv and the hereditary risk coefficient is 0.4 x 10E-2 per Sv. In the meatime, the fatal accident rate in "safe" industries was 5 x 10E-4/year and thus the average dose to workers must be

@Radu,

correct explanation with one small disagreement. The limit of 1 mSv/a is thought additional to the natural background. One possible view for the public limit of 1 mSv/a can be, that 1 mSv/a additional dose for the population by not natural radiation exposure correlates with the standard deviation of natural background.

Yes, this is correct, the 1 mSv/h is seen as above the background.

@Radu

The worker limit was set long before 1977. It appears in several documents in the late 50s and early 60s. It became official in the US as FRG 2 in 1960. The comparison to safe industries was not used to select the limit but to justify the limit. The 1990 restrictions were an ALARA decision. Most industries world-wide were already well below the new guidance. It was ALARA to ensure standard good practices.

I agree with the details given by Prof. Joseph L Alvarez. Readers may further refer to Page Nos. 31 & 32 regarding "Evolution of ICRP recommendations 1977, 1990 & 2007" by Prof. R. H. Clarke, Ex-Chairman ICRP.

Sorry, I was misunderstood: I never said that in 1977 there was the first worker limit. The first limits appeared already at the begining of the 50's. It's just that ICRP 1977 set the first logical rationale for the worker dose limit. Until then the limits were set a bit arbitrary. It is because of the logical justification of the 50 mSv limit given by ICRP 1977 that this limit holds to this day (I believe USA still has a 50 mSv/year limit, or did they move to 20 mSv as well?).

The base is the principle o LNT and Because workers are monitored annually by physicians, while the public can not get sick and not knowing for years. This is the razon for diference of the numbers.

Natural background level multiplied by the time permanent residence about 1 mSv.

Not identified disease risks for staff below 20mSv/year and 50mSv

The value of 1 mSv/y is the result of a lot of biology experiments and epidemiological surveys. This is the reason why the ICRP in the years change this value.

IMO this limit definitely needs to be re-examined in the future, in light of the population study in Kerala, India supporting radiation hormesis.

The value of 1 mSv/y is based on the fact that general public include children whose radiation sensivity is higher. The distinction of radiation workers and non-radiation workers (general public) was evolved so that radiation effects can be followed in the workers and their progeny. Radiation effects are the same whether they are radiation workers or general public.

@Radu A. Vasilache

Earlier I used to believe what you have explained...but I have one doubt...as the risk coefficient is defined per Sv. but in calculations we are taking only 50 mSv/year. or in five years we say the limit is 100mSv. then if we calculate risk by using 100 mSv, it will be doubled and so on. So in my view, in risk coefficient the dose is committed effective dose. Then this explanation doesn't hold true.

@Munish Kumar

Sir please upload that document you are talking about.

@Manish K. Mishra

Did you get the final answer?

Risk models are based on the linear non-threshold principle. So I am not sure I understand why 50 mSv/year or 100 mSv/5 year is of importance here?

Currently it uses the dose limit for deterministic effects of 1 Sv (1000 mSv) and estimated a working period of 50 years. 1000/50 = 20 mSv / year.

There is no scientific reason for this. It is an assessment that can be technically and scientifically defensible.

This assessment is the result of long operational assessments

 Dear Tommy Berlund,

It is not 50 mSv / year but 100 mSv in five years. Average of 20 mSv per year. Operationally can release up to 50 mSv in a year, but the average over five years remains 20 mSv / year. P. Ex; first year 50 mSv in the second year.15 mSv, 15 mSv in the third year, fourth year 15 mSv. In the four years we have 90 mSv. In the fifth year the employee can only receive up to 10 mSv, to add the 100 mSv in 5 years

Public limit is a fraction of the worker limit. the worker's limit was set to 20 mSv / year for philosophical issues, practical and operaciomais. The reason for this is the worker's training, knowledge of the risks, the protection provided by the employer and periodic tests performed by the worker.

well. Are operational limits and are made to operationalize and standardize what is done in the world.

The dose limit for workers of 20 mSv/year limit is based on the annual fatal probability of 10^-3 as a reference risk, and that for the public is 10^-4. This is due to assumption that in “safe” non-radiation occupations, the average annual fatality rate was about 100 per million workers (I.e., 10^-4) and that subgroups with high risks might run a risk ten times the average (10^-3). For more details, please see Annex C of ICRP Publication 60 (i.e., ICRP 1990 recommendations), and I think that Fig. C-9 on its page 185 will be especially useful.

Best wishes,

Nobby 

Nobuyuki Hamada, Ph.D

http://researchmap.jp/read0140945/?lang=english

https://www.researchgate.net/profile/Nobuyuki_Hamada

http://scholar.google.com/citations?user=lrHbuw4AAAAJ&hl=en

Article Classification of radiation effects for dose limitation purp...

hi, how could I found or calculate the permissible dose of radiations for radiograph staff per day/week/month/season and not only for a year!

I think this is due to long research on biological effect and it includes children,woman and sensitive parts

I have just been aware of a question from Ms Shadi Tashakori . The same dose limits apply to all radiation workers, including radiological technologists and all other medical workers. Daily, weekly/monthly/quarterly permissible doses were recommended in early days, but only annual dose limits have been recommended since the late 1960s. For more details, please see Supplementary Tables 6, 7 and 8 in the following paper

Article Classification of radiation effects for dose limitation purp...

How could we maintain the public dose for the family that one of the family members had therapy with radioiodine -131? Probably more concern for children in the family, or pregnant mother in the family? I know there are some restrictions, but how we do make sure that the exposure would not give more than one mSv?

In the ICRP system of radiological protection, dose constraints of 5 mSv/episode, but not public dose limits of 1 mSv/year, apply to comforters and carers of the patients. Likewise, in the NCRP system of radiation protection, numeric protection criteria of 5 mSv/episode apply to comforters and caregivers of the patients. A reminder that dose limits do not apply for medical exposure of the patients themselves either). This is my quick feedback to a question raised by Dr Nur Hidayati

Absolutely, agree with Drs Joseph L Alvarez and Nobuyuki Hamada . The dose limits have been revised time to time as per the research findings of UNSCEAR as a part of our understanding the depth of the subject. Also please take care of Dose Constraint and Dose limit as specified in IAEA GSR Part-3.

It is based on previous accident cases (Hiroshima and Nagasaki) and effects on human health (probability based). However, based on UNSCEAR report, world populated weight average value is 0.87 mSv/y.

Second case for professions, 20 mSv; may be based on maximum effects (sure health risk at those dose).

Thank you Dr Nobuyuki Hamada for the quick answer

For big numbers of population could be a big number of complications such as cancer or autoimmune diseases. For professionals higher risks could be acceptable.

Dear respected colleagues, I find this discussion interesting and educating. I particularly align my thoughts with that of @Alan Cresswell on the basis of justification and optimisation.

My pleasure, Dr Nur Hidayati

In my opinion, the global average value of 2 mSv / a of natural radiation background is the most important basis.

Any profession has its risks, so the people of other professions from the public have dangers of their professions. As for workers in the field of radiation first, they do not have risks from any other professions. Secondly, they have much more knowledge of radiation protection methods than others.

📷

Thanks.

Regards,

Francisco José Fonseca Ferreira

For the public, they are exposed to radiation every day.

And the health of professionals is better.