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RADIATION SAFETY TRAINING MANUAL

CHAPTER 3
MAXIMUM PERMISSIBLE EXPOSURES

CHAPTER 3 Table Of Contents

A. GUIDELINES FOR RADIATION EXPOSURE

B. MAXIMUM PERMISSIBLE DOSE

TABLE 3.1

C. HOW DOES THE MAXIMUM PERMISSIBLE DOSE COMPARE WITH OTHER SOURCES OF RADIATION EXPOSURE?

TABLE 3.2

FIGURE 3.1

D. WHAT IS THE RISK AT THE MAXIMUM PERMISSIBLE DOSE?

TABLE 3.3

E. SPECIAL SAFEGUARDS FOR PREGNANT WOMEN

FIGURE 3.2

TABLE 3.4

A. GUIDELINES FOR RADIATION EXPOSURE

For investigators working with radioactive materials in University of California, San Francisco (UCSF) laboratories, the risk, if any, to low levels of radiation exposure is small. Nevertheless, the risk is real and can only be kept small if the policies and procedures of UCSF, along with the regulations of the State and Federal governments, are carefully followed. UCSF policies and government regulations are based, in part, on three radiation protection principles:

1. Occupational exposure should only take place when the benefit to society warrants the risk. There is little doubt that medically-related research falls into this category.

 

2. Exposure to workers should be As Low As is Reasonably Achievable (ALARA). This has been characterized as the "optimization" of radiation protection by the International Commission on Radiological Protection.

 

3. A "maximum allowable individual dose" must be established to set an upper limit on the risk to individual workers.

UCSF is fully committed to the principle of ALARA. The Radiation Safety Manual spells out this commitment and the ALARA program for the campus. Each authorized user should familiarize themselves with this material. This chapter is devoted to a description of permissible doses.

B. MAXIMUM PERMISSIBLE DOSE

The maximum permissible doses allowed by state and federal regulations have been set based on current knowledge. Scientific committees composed of the world's leading authorities in radiation science and biology are established to periodically appraise the literature and recommend changes in dose limits, if indicated.

The dose limits consider that damage caused by radiation exposure is dependent upon several factors:

1. The age of the person exposed.

 

2. The absorbed dose.

 

3. The body part exposed.

The occupational radiation dose limits first divide people into two groups: those 18 years and over, and those under 18 years of age. The latter group is limited to the same doses as the general population (i.e. non-radiation workers). Table 3.1 presents a synopsis of the dose limits contained in the Code of Federal Regulations (10CFR20). (The limits are the same throughout the country.)

Review Table 3.1. Note that the hands have a limit 10 times higher than the whole body radiation dose. Radiosensitive tissues, such as the blood forming cells, and the gonads, have the lowest maximum permissible dose.

The regulations limit radiation exposure to members of the public (i.e. those who are not occupational radiation workers) to limits that are one-fiftieth of the occupational values. These lower limits apply to visitors, custodial help, delivery persons, or administrative personnel.

UCSF's commitment to ALARA has resulted in the administrative imposition of even lower limits than those required by regulation. In essence, UCSF is committed to keeping the radiation doses to occupationally exposed workers at levels 25% or more below the State limits. For example, the limit for whole body exposure is 5 rems/year. This is roughly 400 millirems per month. UCSF is committed to keeping whole body exposures below 100 millirems per month. Based on years of monitoring the exposures of UCSF laboratory workers, radiation exposures rarely exceed the detectable limits of the film badge (approximately 10 millirem/month). In fact over 90% of personnel receive less than 100 millirem in one year.

TABLE 3.1

Summary of Recommendations^a (After Report No. 91, NCRP, 1987a)

A. Occupational exposures (annual)^b

B. Public exposures (annual)

C. HOW DOES THE MAXIMUM PERMISSIBLE DOSE COMPARE WITH OTHER SOURCES OF RADIATION EXPOSURE?

We are continuously irradiated by external ionizing radiation from cosmic and terrestrial sources, and from naturally occurring radioisotopes within our body (i.e. potassium-40 and carbon-14). For example, a person 70 years old will have received, on average, a 9 rem whole body dose from these sources alone. The internal radiation exposure accounts for approximately 20 millirem per year. The cosmic exposure varies by elevation but ranges from about 30 to 120 millirem per year. The terrestrial exposure also varies with mineral deposits and other geological considerations but generally varies form 20 to 120 millirem per year. The external background radiation in San Francisco is approximately 80 millirem/year, or about one-fiftieth of the allowable limit for radiation workers - 80% of the limit for the general public. On the open ocean, the annual dose is approximately 55 mrem/yr and in Denver about 150 mrem/yr (almost twice the level in San Francisco).The average individual in the United States accumulates a dose of 1 rem from natural sources every 12 years. The dose from natural radiation is higher in some states, such as Colorado, Wyoming and South Dakota, primarily because of increased cosmic and terrestrial irradiation. The average individual may receive 1 rem every 8 years or less. However, there are other areas in the world where natural background radiation levels are very much higher. For example, a dose of 1 rem may be received in some areas on the beach at Guarapari, Brazil, in only about 9 days, and some people in Kerala, India get a dose of 1 rem every 5 months.

In addition to natural background radiation, many people receive additional radiation exposure for medical reasons. Medical exposures are intentional and clearly have defined benefits for the individual. For purposes of comparison, the average surface skin dose from one radiographic (P/A view) chest x-ray is 0.027 rem. The estimated average surface skin dose per abdominal x- ray is 0.62 rem. Table 3.2 and Figure 3.1 list annual dose contributions from some of these sources.

TABLE 3.2

Annual GSD in the U. S. population circa 1980-82

FIGURE 3.1: Graphic Under Construction

The percentage contribution of various radiation sources to the total average effective dose equivalent in the U. S. population.

Radiation can also be received from natural sources such as rock or brick structures, from consumer products (such as smoke detectors containing radioactive materials), and from air travel. The possible annual dose from working 8 hours a day near a granite wall at the red cap stand in Grand Central Station, New York City, is 0.2 rem, and the average annual dose in the United States from consumer products and air travel is 0.0026 rem.

D. WHAT IS THE RISK AT THE MAXIMUM PERMISSIBLE DOSE?

Death due to radiation exposure requires high exposures. In measuring radiation effect, the concept of the lethal dose 50 (LD₅₀) has been borrowed from pharmacology. The LD₅₀ is defined as the dose of any agent or material that causes a mortality of 50% in the experimental group. The LD₁₀₀ produces a mortality of 100%. For acute whole body human radiation exposure, the LD_50/60 is in the range of 300 to 350 rads. This means 50% mortality within 60 days.

There are variations in the population due to age, sex, degree of health, and sensitivity to radiation exposure. Briefly stated, the young and the old appear to be more radiosensitive than the middle-aged individual. The female appears to have a greater degree of tolerance to radiation than does the male.

The effects from chronic or protracted exposure are less than from acute exposure. Exposure to the sun offers some parallels to radiation exposure. Whole body exposure to the direct sun for several hours can result in a severe sun burn. However, as more of the body is protected (using sun screens, clothing, shade, etc.) the length of exposure can be increased without the effect of sunburn. For example, one can stay out for a few minutes each day (eventually accumulating a total exposure of several hours) and have a very different effect than by receiving an acute dose within several hours. Radiation exposure may also work this way, although experts do not fully agree.

The chief risk to radionuclide users comes from intermittent exposures to very low doses not from an acute exposure to a very high dose. The risks to low doses of radiation are not fully known and so the best principle is to follow is ALARA - the minimum exposure that can be reasonably achieved.

One risk is cancer. The figures for cancer mortality are given in Table 3.3 If the average figure of 300 excess cancers per million people per rad is used, and a scenario of 20 years of exposure at the State limit is assumed, the result would be a total of 6,000 extra cancers per million workers, or a 0.8% increase in extra cases over a thirty year period. If the American Cancer Society's figures that 25% of Americans will contract cancer are used, the maximally exposed worker would increase his/her chances of getting cancer from 25% to 25.8%. Of course, there are only a few thousand UCSF radiation workers, and the average UCSF worker receives an occupational dose of less than 100 mrem/year as opposed to the 5,000 mrem/yr used in this example. The cancer risk from a radiation dose received at this rate may well be zero.

TABLE 3.3

Excess Mortality Estimates - Lifetime Risks per 100,000 Exposed Persons
(extracted from Table 4-2 of BEIR V)

However, these statistical arguments are not very comforting if we, or one of our friends or relatives, develop cancer. The way to avoid even this small risk of a radiation induced-cancer is to stay well below the maximum allowable level by following established policies and procedures.

In 1980, approximately 1.3 million workers were employed in occupations in which they were potentially exposed to radiation. About half of these workers received no measurable occupational dose. In that year, the average worker exposed to a measurable amount of external radiation received an occupational dose equivalent of 0.2 rem to the whole body, based on the readings of individual dosimeters worn on the surface of the body. We estimate (assuming a linear non-threshold model) the increased risk of premature death due to radiation-induced cancer for such a dose is ~2-5 in 100,000 and that the increased risk of serious hereditary effects is about one-third smaller. To put these estimated risks in perspective with other occupational hazards, they are comparable to the observed risk of job-related accidental death in the safest industries, wholesale and retail trades, for which the annual accidental death rate averaged about 5 per 100,000 from 1980 to 1984. The U.S. average for all industries was 11 per 100,000 in 1984 and 1985.

E. SPECIAL SAFEGUARDS FOR PREGNANT WOMEN

A number of studies have indicated that the embryo/fetus is more sensitive to radiation exposure than the adult, particularly during the first three months after conception. This is also a period when a woman may not be aware she is pregnant. Women who are pregnant or who are considering pregnancy should to be aware of the special needs of their situation. Supervisors and co-workers of fertile women should be aware of the risks to the fetus to avoid creating a situation that might put the embryo/fetus at risk. The National Council on Radiation Protection and Measurements has made two recommendations: a) the maximum dose to the fetus from occupational exposure should not exceed 0.5 rem, and, b) radionuclide workers must know about prenatal exposure risks arising from ionizing radiation. In particular they must know why pregnant women have a lower maximum permissible dose.

The Appendix of the UCSF Radiation Safety Manual contains a reprint of the U.S. Nuclear Regulatory Commission (NRC), Regulatory Guide 8.13, Instruction Concerning Prenatal Radiation Exposure. In addition, this section contains the UCSF pregnant personnel policy. Each authorized user should read and become familiar with this material.

The prediction that an unborn child would be more sensitive to radiation than an adult is supported by observations for relatively large doses. The National Academy of Sciences noted that doses of 25-50 rems to a pregnant human may cause growth disturbances in offspring. Such doses substantially exceed, of course, the maximum permissible occupational exposure limits.

Concern about prenatal exposure (i.e., exposure of a child while in its mother's uterus) at the permissible occupational levels is primarily based on the possibility that cancer (especially leukemia) may develop during the first 10 years of the child's life. According to a report by the National Academy of Sciences, the incidence of leukemia among children from birth to 10 years of age in the United States could rise from 3.7 to 5.6 cases per 10,000 children exposed to 1 rem in utero, an increase of 50%.

FIGURE 3.2: Graphic Under Construction

The Academy also estimated that an equal number of other types of cancers could result from this level of radiation. Although other scientific studies have shown a much smaller effect from radiation, women employees should be aware of any possible risk so that they can take steps they think appropriate to protect their offspring. Efforts should be made to keep the radiation exposure of an embryo/fetus the lowest practicable level during the entire period of pregnancy.

The employer should take practicable steps to minimize the radiation exposure of a potential mother. The advice of the Radiation Safety Office can be obtained to determine if radiation levels in working areas are high enough that a baby could receive 0.5 rem or more before birth.

The following facts should be noted in making a decision about continuing to work with ionizing radiation:

1. If you are planning on becoming pregnant or think you may be pregnant, discuss the matter with your supervisor or Principal Investigator so that appropriate appraisal of the potential radiation exposure may be made.

 

2. In most cases of occupational exposure, the actual dose received by the unborn baby is less than the dose received by the mother because some of the dose is absorbed by the mother's body.

 

3. At the present occupational exposure limit, the actual risk to the unborn baby is quite small, even though experts disagree about the exact level of risk.

 

4. There is no need to be concerned about a loss of your ability to bear children. The radiation dose required to produce such effects is many times larger than the State dose limits for adults.

 

5. Even if you work in an area where you receive only 0.5 rem per three-month period, in nine months you could receive 1.5 rem and the unborn baby could receive more than 0.5 rem, the full-term limit suggested by the NCRP. Therefore, if you decide to restrict your unborn baby's exposure as recommended by the NCRP, be aware that the 0.5 rem limit to the unborn baby applies to the full nine-month pregnancy.

To put the risk due to radiation in perspective, a table of the effects of various risk factors on the outcome of pregnancy is included (Table 3.4).

TABLE 3.4

Effect and Frequency of Certain Maternal Factors on Pregnancy Outcome

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