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2001-01-15 NCRP-95(1987年）

NCRP-95（1987年）を見る機会を得ました。全文99ページの米国らしい実務的なレポートでした。およそ、生活環境には放射線を出すのもが自然、人工を含めて山のようにあるなかで、それを人体への線量当量（人・シーベルト、人・レム）としてどれだけ影響を与えるかというレポートであり、全体像の見えるものでした。

１）TVについては高電圧の使用が放射線をうむといことで、かなり昔から米国では法律を作り規制値(0.5 mR/h)が設けられてきたこと、初期のモデルには問題があったが、新しいものでは改善されているとのこと。

２）ガラス製品中のウランについては、建築物に用いられる窓ガラスやセラミックタイル等を除いて、花瓶や蝋燭台では重量の10％まで、ウラン又はトリウムの含有が認められる。これらの建築物に用いる場合は含有量は0.05%に制限される。

３）うわぐすり（グレーズ）中のウラン量についてはより多く評価されています。US-NRCは20%ウラン濃度までは使用してよいとしており、結構、高い線量率（20mRad/h）のものもあるようです。

４）陶歯についてはかなり詳細に評価しています（3,150人レム）。

５）眼鏡のレンズに不純物として含まれるウラン及びトリウムの評価も詳細でした（20,000人レム以下）。

当方のコメントとしては、陶歯とか眼鏡については、普段から人が身に着けるもので、集団線量をおおざっぱであるが、計算しているが、ウランガラスやうわぐすり中のウランは目的が鑑賞用であり、集団線量を評価するまでもないほど放射線の影響は小さいとNCRPレポートは見ているのであろうということでした。

Radiation Exposure of the US Population from Consumer Products and Miscellaneous Sources, NCRP(National Council on Radiation Protection and Measurements) Report No.95,  December 30, 1987

Contents              

l . Introduction .               1
2. Electronic Products .               3
2.1 Unwanted By-product X Rays . . . . . 3              
2.1.1 Television Receivers . . . 3
2.1.2 Video Display Terminals (VDTs) . . . . 6
2.2 Intentional X Rays .               6
2.2.1 Airport Luggage Inspection Systems . . . . . 6
2.2.2 Personnel Scanning Systems . . . .              
2.2.3 Shoe-Fitting Fluoroscopes . . . . . 8  
3. Radioactive Materials               9
3.1 Processed Radioactive Materials . . . 9              
3.1.1 Radioluminous Products . . . . . 9
3.1.1.1 Miscellaneous Radioluminous Items 14
3.1.2 Static Eliminators . . . ･ 14  
3.1.3 Spark Gap lrradiators and Electron Tubes 15
3.1.3.1 Spark Gap lrradiators . . . . . . . . . 15
3.1.3.2 Electron Tubes . . . . . 16
3.1.4 Gas and Aerosol Detectors ("Smoke Detectors " ) . 18
3.1.5 Check Sources . . 20              
3.1.6 Plutonium-Powered Cardiac Pacemakers . . . 21
3.1.7 Lightning Rods . . . . . . . . . . 22
3.2 Natural Radioactive Materials . . . . . . 23
3.2.1 Tobacco Products . . . . 23
3.2.2 Building Materials . . . . 24        
3.2.3 Domestic Water Supplies . . . . . 27        
3.2.4 Highway and Road Construction Materials . 28
3.2.5 Mining and Agricultural Products . . . . . . . 29   
3.2.5.1 Fertilizer Products . . . 29              
3.2.5.2 Phosphate Products, By-products and Wastes . 32
3.2.6 Combustible Fuels . . . . 32
3.2.6.1 Combustion of Coal . 32              
3.2.6.2 Combustion of Oil . . 37
3.2.6.3 Combustion of Natural Gas. . . 38        
3.2.7 Glass and Ceramics . . 39
3.2.7.1 Uranium in Glassware . . 39
3.2.7.2 Uranium in Glazes . . . 40
3 2 7.3 Uranium in Glass Enamel . . . . 40
3.2.7.4 Dental Products . . . 41
3.2.7.5 Uranium and Thorium Impurities in Ophthalmic Glass . . . 42              
3.2.8 Thorium Products . . . . 44
3.2.8.1 Thoriated Optical Glass . . 44
3.2.8.2 Gas Mantles . . . 45
3.2.8.3 Camera Lenses . . . . . 47
3.2.8.4 Thoriated Tungsten Welding Rods 47
3.2.8.5 Fluorescent Lamp Starters . . . . . . 49
4. Miscellaneous Exposure Sources . . . . . . 50         
4.1 High Voltage Vacuum Electronic Units . . . . 50              
4.2 Contaminated or lrradiated Materials . . . . . 53        
4.3 Disposal of Radioactive Surplus Items . . . . 57              
4.4 Aircraft Transport of Radioactive Materials . . . . . . . 58
5. Summary               60
5.1 Sources and Estimates ofAssociated Population Dose Equivalents . . . . 60
5.2 Special Considerations . . . . 65              
5.3 Discussion .               66
5 4 Recommendations for Dose Reduction and Research . 68              
References .               70
The NCRP .               81
NCRP Publications .               88
Index . 96

2.2.1 Television Receivers

(略)In the United States, Congress passed the "Radiation Control for Health and Safety Act of 1968" (PL 90-602, 1968), with enforcement principally delegated to the Food and Drug Administration. Performance standards for television receivers were adopted in 1970 (CFR, 1970) and are enforceable through the Act. The exposure rate limit was set at 1.3 x 10~7 C/(kg h) (0.5 mR/h), measured in accordance with the recommendations of the NCRP (1960). New measurement conditions, as detailed in the performance standard, are designed to ensure that the exposure rate limit will not be exceeded even under the most adverse operating conditions. If these conditions are met, the exposure rates under normal operating conditions are ordinarily a small fraction of the 1.3 x 10~7 C/(kg h) (0.5 mR/h) at 5 cm, as specified in the standard. （略）

3.2.7 Glass and Ceramics

Naturally occurring radioactive materials have been used in the glass and ceramic industry for over 150 years (Jensen, 1952). Uranium compounds have been employed to produce fluorescent glassware, a variety of colored glazes, and wall tiles. More recently uranium has been incorporated into artificial teeth both for coloring and fluorescent properties. Thorium compounds have been used in wall tiles and electrical materials.

3.2.7.1 Uranium in Glassware.  Sodium uranyl carbonate has been commonly employed in the production of fluorescent and iridescent glass. In particular, it was popular until the 1940's to use this material to produce dichroic properties in glass. As the concentration of uranium is increased, the glass becomes more opaque. In 1972 two manufacturers were identified as using uranium as a colorant, in no food glass products such as candlesticks ,and flower containers.  Federal regulations allows glassware to contain up to ten percent by weight uranium or thorium, except in commercially manufactured glass brick, pane glass, ceramic tile, or other glass or ceramic used in construction. Uranium or thorium concentrations in these latter items are limited to 0.05 percent (CFR, 1986c).

3.2.7.2 Uranium in Glazes. Uranium in the form of oxides and as sodium uranite has been used to produce glazes of black, brown, green, and the spectrum from yellow to red. The glazes were frequently used to decorate tableware and pottery at concentrations ranging from one to twenty percent by weight. As with glassware, the restrictions on the availability of uranium during the 1940's forced manufacturers to find other coloring agents. Since the substitutes were frequently more economical, uranium has not often been used in glazes in recent years.

At present no manufacturer is known to use uranium as a glaze for dinnerware. U.S. Nuclear Regulatory Commission (USNRC) exemption limits are set at a maximum of 20 percent by weight (of the glaze) for uranium compounds in glazed ceramic tableware (CFR, 1986c).

The acceptability of ceramic glazes for use in food containers is subject to the food additive provisions of the Food and Drug Administration and the use of such glazes is prohibited unless specifically approved.

No approvals for such applications of uranium ceramic glazes have been granted to date.

Measurements using film badges have shown that surface dose rates due to gross beta and gamma radiation from various tableware items glazed with uranium range from 5 to 200 uGy/h (0.5 to 20 mrad/h) (Menczer, 1965). Measured uranium enamel surface dose rates of 37 ?Gy/h (3.7 mradlh) have been reported (USNRC, 1983). In addition to the external exposures, uranium and lead have been found on occasion to be leachable from glazed ceramics at levels of 10 to 55 ppm (concentration in the leach solution) (Kendig and Schmidt, 1972). The latter is equivalent to 630 mBq/ml (17 pCi/ml) which is in excess of the Maximum Permissible Concentration in drinking water for occupational exposures, 220 mBq/ml (6 pCi/ml) (NCRP, 1959). At the measured concentrations, the chemical toxicity of uranium is considered to be a greater hazard than its associated radiation. As a result of these measurements, the only known producer of uranium glazes for use in foodware in the U.S. has ceased operations.

3 2 7.3 Uranium irt Glass Enamel. Uranium has also been used as a coloring agent in various enamel objects, including tableware and jewelry. Problems associated with the use of such items were highlighted by reports in the early 1980's of relatively high exposures associated with the use of cloisonne jewelry.  A surface dose rate of 37 ?Ｇy/h (3.7 mrad/h) from similar sources has been reported by staff members of the U.S.Nuclear Regulatory Commission (USNRC, 1983).

Such jewelry, most of which was being imported, has proven very popular in the U.S, and could be worn in direct contact with the body.

Stimulated by these problems, the U.S. Nuclear Regulatory Commision in 1983 banned the uranium products to be spld in the U.S. (USNRC, 1983, 1984). Products already distributed to retail outlets and consumers were not recalled because of logistical problems and the relatively low estimates of the associated radiological hazard.

3.2.7.4 Dental Products. Porcelain teeth and crowns are composed principally of feldspar minerals that contain small quantities (0.001 percent) of naturally occurring 40K. The practice of adding uranium salts was initiated at least half a century ago when it was discovered that small amounts of the element contributed a natural color and fluorescence to dentures. Restoration of natural appearance is one of the major reasons for using prostheses. Other substances have been found to imitate these characteristics over a broad range of daylight and artificial lighting conditions. The concentrations of uranium required were considered trivial and easily qualified for a licence exempt status when controls were imposed in the 1960's on the use of source material in ceramics. Under regulations of the U.S. Nuclear Regulatory Commission, neither domestic nor imported teeth and powders may contain in excess of 0.05 percent by weight of uranium (CFR, 1986a).

Dental products also contain naturally occurring radioactive potassium but there are no controls over the potassium content in these products.

A study by O'Riordan and Hunt (1974) in Great Britain indicated that porcelain teeth containing 0.10 percent uranium could deliver an annual dose equivalent to the oral mucosa of almost 6 Sv (600 rem) by alpha particles and 0.028 Sv (2.8 rem) by beta particles. This estimate is in close agreement with a more recent study by Papastefanou et al. (1987) in Greece in which it was reported that uranium concentrations of 500 ppm could yield a surface dose equivalent of about 4 Sv (400 rem) per year. In a study of dental products in the U.S. (Thompson, 1976), the highest concentration observed. 0.044 percent, was calculated to deliver an annual mucosal dose equivalent, of 1.3 Sv (130 rem) from alpha emissions. However, the maximum range of alpha particles in tissue is 30 ?m so that most of their energy is expended in the superfrcial cells overlying the sensitive basal layer.

Saliva, dental pellicle, calculus, food, and tobacco residues in the mouth further reduce the intensity of the alpha flux to a level where it does not, appear to present a significant hazard.

Beta particles can penetrate in tissue to a depth of 200 ?m. The combined beta emissions of uranium and potassium-40 for the highest concentration sample observed in a study by the Bureau of Radiologcal Health, were calculated to deliver an annual dose equivalent of 9mSv (0.9rem) to the basal layer.  The average concentration of uranium in U.S. dental porcelain was estimated to be 0.02 percent. This dental corresponds to a uranium beta dose equivalent rate of about 5 mSv (0.5 rem) per year. The potassium-40 contribution generally ranged from 1.4 to 1.9 mSv (0.14 to 0.19 rem) per year.

As of 1971, over 19 million persons in the United States were estimated to wear full dentures and 60 million to wear crowns (DHEW, 1962, 1971). Some 90 million persons were missing at least one tooth although it is not known how many wore bridges or partial dentures.

More recent published estimates are not available; however, knowledgeable sources in the dental industry indicate that 40 percent of new dental prostheses contain porcelain, and that uranium is no longer used in porcelain by domestic manufacturers (ADA, 1986). The balance of dental products are acrylics and do not contain uranium.

If it is assumed that 45 million people are wearing dental prostheses with an average concentration of 0.02 percent uranium, and that only beta dose need be considered, they will receive a dose equivalent to 7 mSv (0.7 rem) to the basal mucosa. The contribution from this source to the average annual population dose equivalent to the basal mucosa of the mouth would be estimated to be about 1.3 mSv (0.13 rem). On the basis of a weighting factor of 0.01 for the human skin, and assuming that irradiation of the basal mucosa is equivalent to irradiation of 1 percent of the skin, the weighting factor for irradiation of the basal mucosa could be estimated to be 0.01 x 1 percent or 10(~4). The resulting annual collective effective dose equivalent to the U.S. population from this source would be 31.5 person-Sv (3,150 person-rem). This dose is expected to decrease over time as porcelain without uranium displaces the old porcelain containing uranium for use in dental prostheses.

3.2.7.5 Uranium and Thorium Impurities in Ophthalmic Glass.

Ophthalmic glass is used to manufacture lenses for eyeglasses and eyepieces. At present, up to 0.05 percent by weight of source material (uranium or thorium or any combination of these materials) may be contained in any chemical mixture, compound, solution, or alloy without NRC regulation or license requirements. There is a further maximum allowable limit of 0.25 percent by weight of source material in rare earth mixtures and products (CFR, 1986c).

Pecora and Munton (1974) have reported that ophthalmic lenses, tinted by adding thorium salts can be a source of radiation. They tested rose-tinted lenses from several manufacturers and concluded that the dose-equivalent rate to the corneal epithelium from alpha radiation was 0.1 to 0.3 mSv/h ( 10 to 30 mrem/h). At a depth of 0.2cm,the beta dose-equivalent rate was calculated to range from 0.7 to 2 ?Sv/h (0.07 to 0.20 mrem/h) with a gamma dose-equivalent rate to the entire eye of' 0.06 to 0.3?Sv/h (0.006 to 0.030 mrem/h). Another study (Yaniv,1974) reported thorium concentrations of up to 0.14 percent by weight in some samples of ophthalmic glass, with large variations in natural thorium and uranium content for different batches of' glass.

Thorium has been shown (McMillan et al., 1975) to exist as an impurity in the rare earth oxides that are used in the manufacture of certain ophthalmic glasses. The thorium content was found to exceed the limit specified in federal regulations (CFR, 1986a) by as much as a factor of ten. These oxides, and their impurities, are generally thought to be the primary source of radioactivity in certain ophthalmic glasses.

Dose calculations by Tobias and Chatterjee (1974) indicate that the annual alpha-particle dose to the critical tissues of the germinal cell layer of the cornea (50 ?m), from eyeglasses containing 0.05 percent by weight of 232Th in equilibrium with its decay products and worn for 16 hours a day, is 2 mGy (0.2 rad) (estimated to be accurate within a factor of two), with an approximately equal absorbed dose from beta particles. Using these data, and applying a quality factor of 20 for alpha radiation, Casarett et al. (1974) estimated that the dose-equiv-alent rate to the germinal cells of the cornea (50 ?m depth) would be approximately 40 mSv/y (4 rem/y). The dose-equivalent rate at 60 ?m tissue depth was estimated to be 10 mSv/y (1 rem/y). The beta dose-equivalent rate would be a small fraction of this, however, because of the much smaller quality factor of this radiation.

The Yaniv (1974) study concluded that the radiation dose rates from the ophthalmic glass could be reduced significantly with better quality control of the rare earth and zirconium oxides. Another problem revealed by this study was that the observed radiation is not directly related to the source material content of the glass, due to widely varying daughter-parent equilibrium conditions. The radiation emissions are, in fact, mainly due to the short-lived decay products of 232Th and 238U, which can be present in glass even after the parent radionuclides are removed. Thus, control of source material content is not sufficient to eliminate radioactive material from glass. Yaniv recommended that new regulations for ophthalmic glass be established on the basis of emission rates rather than on the abundance by percent, of weight of the parent nuclide. The Optical Manufacturers Association, with the assistance of the U.S.Nuclear Regulatory Commission and other governmental agencies, has established a voluntary radiological standard for ophthalmic glass (OMA,1975).

In 1977, about 96,000,0000 persons in the U.S. wore eyeglasses (BOC, 1979). Currently, it is estimated that about half of the eyeglasses in use in the U.S. contain plastic lenses which do not contain radioactive materials. The same is true for plastic contact lenses (Buckley et al., 1980). As a result, the current estimate of the number of people who are wearing eyeglasses with glass lenses in the U.S. totals about 50,000.000. Assuming an annual dose equivalent of 40 mSv (4 rem) to the cornea at 50 micrometers depth, and assuming a tissue weighting factor of =< 10(~4), the annual collective effective dose equivalent to the U.S. population would be about =<200 person-Sv (=<20,000 person-rem).