Potential Health Hazards and Their Prevention
December 4, 1979
The National Institute for Occupational Safety and Health (NIOSH) and the Occupational Safety and Health Administration (OSHA) are concerned about potential health hazards to workers exposed to radiofrequency (RF) energy emitted from RF dielectric heaters (more widely known as RF sealers and heaters). RF energy is nonionizing electromagnetic radiation and should not be confused with X-rays and other ionizing radiation. RF energy, when absorbed in sufficient amounts by workers, may produce adverse thermal effects resulting from heating of deep body tissue which may include potentially damaging alterations in cells. Absorption of RF energy may also result in "nonthermal" effects on cells or tissue, which may occur without a measureable increase in tissue or body temperature. "Nonthermal" effects have been reported to occur at exposure levels lower than those that cause thermal effects. While scientists are not in complete agreement regarding the significance of reports of "nonthermal" effects observed in laboratory animals, NIOSH believes there is sufficient evidence of such effects to cause concern about human exposures. NIOSH and OSHA recommend that precautionary measures be instituted to minimize the risk to workers from unwarranted exposure to RF energy. Section V of the Appendix to this Bulletin lists engineering controls, such as shielding, and other immediate actions that should be taken. Also, NIOSH and OSHA are issuing this Bulletin to alert employers and workers to the potential for overexposure of workers to RF energy from RF sealers and heaters, and to recommend control technology that can prevent unwarranted exposures.
Workers near RF sealers may be unaware of their exposure to RF emissions, because the RF energy from sealers and heaters can penetrate deeply into the body without activating the heat sensors located in the skin. A false sense of employee safety may exist; in many instances, worker exposures to RF energy may not have been properly assessed. This has been due, largely, to the complex problems of measurement and thus the misapplication of the instruments available for monitoring RF energy levels. Recently, monitoring instruments that facilitate accurate measurement of worker exposure have been developed. Federal survey teams, equipped with these new instruments, have detected excessive exposures of workers to RF energy.
NIOSH is seeking additional information about the adverse effects of RF energy and effective control technology. The Institute would appreciate receiving information concerning adverse health effects among workers which might be associated with their exposure to RF energy in the workplace and information on methods for retrofitting existing RF sealers and heaters in order to control the emission of RF energy. NIOSH and OSHA request that manufacturers, distributors, and users of RF sealers and heaters transmit the information in this Bulletin to their customers and employees, and that professional societies, trade associations, and unions inform their members.
RF sealers are used to heat, melt, or cure materials such as plastic, rubber, or glue. Specific uses include: &mbsp 1) the manufacture of many plastic products such as toys, vinyl loose-leaf binders, rain apparel, waterproof containers, furniture slip covers, and packaging materials; 2) wood lamination and veneer processes including glue setting; 3) embossing and drying operations in the textile, paper, plastic, and leather industries; and 4) curing of various materials including plasticized polyvinyl chloride, wood resins, polyurethane foam, concrete binder materials, rubber tires, and epoxy resins. An extensive list of occupations involving the use of RF sealers and heaters is presented in Section II of the attached Appendix.
Experiments in animals suggest that the potential consequences of absorbing excessive amounts of RF energy may include changes in: &mbsp the eye, the central nervous system, conditioned reflex behavior, heart rate, chemical composition of the blood, and the immunologic system. Effects on reproduction and on the development of offspring of females exposed during pregnancy have also been reported.
As previously mentioned a false sense of employee safety may exist in many industrial settings because worker exposures to RF energy may not have been properly assessed. The recent development of monitoring instruments that facilitate accurate measurement of worker exposure to RF energy allowed for a series of studies at workplaces where RF sealers and heaters are used. The results of a NIOSH study indicate that the majority of the workers surveyed were exposed to RF energy at levels exceeding values citable by OSHA.1 RF energy in the immediate area of a worker has been measured at levels as great as ten times the values citable by OSHA. 1,2 A list of manufacturers of instruments suitable for measurement of RF energy is presented in Section III of the attached Appendix.
This Bulletin will provide an overview of the potential adverse health effects associated with the use of RF dielectric heaters. The Appendix contains technical information to assist research, engineering, and manufacturing personnel in evaluating this potential hazard and for initiating appropriate modification and controls to prevent unwarranted worker exposure.
Absorption of RF energy may also result in "nonthermal" effects on cells or tissue, which may occur without a measurable increase in tissue or body temperature. "Nonthermal" effects are reported to occur from exposure to RF energy at field strengths lower than those necessary to cause thermal effects.4,5 While scientists are not in complete agreement regarding the significance of reports of "nonthermal" effects observed in laboratory animals, NIOSH believes there is sufficient documentation of such effects to cause concern.
For radiation frequencies similar to those commonly used with RF sealers and heaters, reported observations at relatively low energy levels in laboratory rats or rabbits included changes in: electroencephalographic (EEG) recordings of electrical activity of the brain,6 conditioned reflex behavior6,7 chemical composition of the blood,6 the endocrine (hormonal) system,6,8 and the immunologic (infection defense) system.6,9 Details of these experiments are summarized in Section IV.E of the attached Appendix. For the frequencies at which these observations have been made the rates of energy absorption in man are much greater than in the laboratory animals.10 Therefore, the biological effects observed in the laboratory animals may occur in humans at exposure levels even lower than those reported for the animals.
Other adverse health effects on the eye, heart rate, and the central nervous system have been observed in laboratory animals exposed to electromagnetic energy at higher frequencies in the microwave region of the electromagnetic spectrum (see Section IV.A of the attached Appendix). The extent to which these latter effects may also be caused by absorption of energy at the lower frequencies employed by RF sealers is not known.
There is no convincing evidence to indicate that RF energy can cause cancer in humans.4 Reports have described chromosomal abnormalities in animal and human cells cultured in the laboratory after exposure to RF energy.11,12 However, the relevance of such studies to humans is not known and must be determined through additional research.
There have been reports which suggest an association between RF exposure and reproductive damage in animals and humans. These reports, primarily from Eastern Europe and the Soviet Union, list a variety of reproductive and developmental effects resulting from occupational exposures of workers and experimental exposures of laboratory animals to electromagnetic energy at frequencies in the RF and microwave ranges. Reported effects from exposure of women to fields of relatively high intensity RF and microwave energy have included changes in menstrual pattern, increased incidence of miscarriage, and decreased lactation in nursing mothers.13 Retarded fetal development and increased congenital anomalies have been noted among exposed offspring.13 Laboratory studies have shown that exposure of pregnant rats to RF energy (at levels believed to have been relatively high) resulted in numerous fetal malformations including abnormalities of the central nervous system, eye deformities, cleft palate, and deformation of the tail.14
There is a report of changes in spermatogenesis (production of male germ cells in the testicles) among workmen exposed to nonionizing electromagnetic energy.15 Reproductive effects in male experimental animals, including testicular damage, debilitated or stillborn offspring and changes in spermatogenesis, have been reported to be related to exposure to electromagnetic energy at microwave frequencies.16,17 Similar studies have not been reported for the lower frequencies of RF sealers and heaters.
NIOSH surveys indicate that a large majority of the workers using RF sealing and heating equipment are women of child-bearing age.18 NIOSH is beginning an epidemiologic study of potential reproductive effects among operators of RF sealers, and is conducting laboratory research to study the possibility that teratogenic effects (malformations) in animals may result from exposure to RF radiation.
Existing national health standards for RF energy have been based on evidence of the thermal effects which result from the body's absorption of RF energy and the subsequent heating of deep body tissue. However, in recent years since the development of existing national standards, concern has increased over reported "nonthermal" effects, which may occur at exposure levels lower than those causing measurable thermal effects.
The extent to which biological effects attributed to the absorption of RF energy by animals reflect an occupational hazard to workers is not fully known. There are uncertainties in extrapolating experimental results from animals to humans and to frequencies other than those used in the experiments. These problems have been compounded by the difficulty in properly measuring near-field RF energy exposures, which has been only recently resolved. NIOSH recommends that future research projects dealing with RF energy meet requirements for: 1) better exposure dosimetry and quantification of biological results, 2) use of adequate experimental controls, and 3) uniform reporting of experimental parameters and results.
While scientists are not in complete agreement on the interpretation of available data on biological effects, NIOSH believes there is sufficient evidence of such effects to cause concern about human exposures. NIOSH and OSHA recommend that precautionary measures, as listed in Section V of the attached Appendix, be instituted to protect workers from unwarranted exposure to RF energy.
[signature] Anthony Robbins, M.D. Director National Institute for Occupational Safety and Health | [signature] Eula Bingham, Ph.D. Assistant Secretary of Labor Occupational Safety and Health Administration |
II. Occupations Which May Involve Use of Radiofrequency Sealing and Heating Equipment
III. Manufacturers of Instruments for Measurement of Radiofrequency Energy in the Near Field
IV. Supporting Technical Information
References
Chemetron Corp. (Voltator), | Louisville, Kentucky |
Compo Industries, Inc. | Waltham, Massachusetts |
Cosmos Electronic Machine Corporation | Farmingdale, New York |
Duomatic Electronics Corporation | Brooklyn, New York |
Gallery Services Fara-Dine | So. El Monte, California |
Gerling Moore, Inc. | Santa Clara, California |
Guild Electronics, Inc. | Brooklyn, New York |
Hall Dielectric Machine Company | Deer Park, New York |
J.A. Callanan Company | Chicago, Illinois |
Kabar Manufacturing Corporation | Farmingdale, New York |
Lepel High Frequency Laboratories, Inc. | Maspeth, New York |
Mann-Russell Electronic Devices, Inc. | Tacoma, Washington |
Pillar Corporation | Milwaukee, Wisconsin |
P.S.C. Inc. | Cleveland, Ohio |
Radio Frequency Company, Inc. | Medfield, Massachusetts |
Seal-Pac Services and Machine, Inc. | Brooklyn, New York |
Solidyne, Inc. Divisions: Thermatron, Sealomatic, Faratron, Stanelco, Ltd, Colpitt, B.V. | Bay Shore, New York |
Thermo Dielectric Machine Company, Inc. | Brooklyn, New York |
Welduction, Inc. | Plymouth, Michigan |
W.T. LaRosa and Associates, Inc. | Troy, New York |
*This list is complete and accurate to the best knowledge of NIOSH; however there may be other manufacturers of this equipment of which the Institute is not aware. [return to text]
Furniture and wood workers: decking assembly, door lamination, fabrication of posts and rafters, fiberboard fabrication, laminated beams, lumber edge glueing, plywood panel patching, plywood or particleboard scarf glueing, ski lamination, veneer panel glueing.
Glass fiber workers: Drying and curing sizing on machine packages, drying coatings on continuous moving strands, drying glass fibers on forming tubes, drying roving packages.
Paper product workers: Correcting moisture profile on continuously moving webs, drying resin coatings, drying twisted twine packages, gluing paper, heating coating on continuous webs.
Plastic heat-sealing workers invovled in the manufacture/fabrication of: Acetate box covers, advertising novelties, appliance covers, aprons, baby pants, beach balls, belts and suspenders, blister packages, book covers, capes, charge cards, checkbook covers, convertible tops, cushions, diaper bags, display boxes, electric blankets, food packages, fountain pens, garment bags, gas masks, goggles (industrial), handbags, hat covers, index cards, lampshades, liquid containers, luggage, machine covers, mattress covers, mild cartons, oxygen tents, packages, pharmaceuticals, pillowcases, pillow packages, plastic gloves, pool liners, protective clothing, racket bags, rain apparel, refrigerator bags, shoe bags, shoes, shower curtains, clipcovers, splatter mats, sponge backings, sport equipment, tobacco pouches, toys, travel cases, umbrellas, wallets, waterproof containers, wire terminal covers.
RF/microwave application workers: Advertising - RF-excited gas display signs. Ceramics - Drying of ceramic objects. Chemical - Activation of chemical reactions. Electronics - Tube aging and testing. Laser - RF-excited gas lasers. Medical - Diathermy and (experimental) cancer therapy. Scientific Equipment- Low temperature ashing of samples. Welding - RF-stabilized welding.
Rubber products workers: Drying latex foams, gelling latex foams, preheating prior to curing latex foams, preheating prior to molding.
Textile workers: Drying continuous webs, impregnated or coated yarns, rayon cake packages, slasher coatings, and wound packages.
General Microwave Corp.
Farmingdale, Long Island, New York
2) the frequency of oscillation - the number of complete oscillations per second of the wave
3) the wavelength - the distance between two consecutive peaks of the wave
*Electromagnetic energy can also be described as discrete particles (or quanta) of energy. [return to text]
Electromagnetic energy emitted from a source propagates through space until it is absorbed, reflected transmitted and/or diffracted by objects in its path. When electromagnetic radiation contains sufficient energy (at frequencies much higher than radiofrequencies), it can ionize atoms of the material absorbing the energy (i.e. dislodge electrons from the atoms of the absorbing material). Radiation of sufficient energy to cause ionization of molecules in biological tissue is often referred to as ionizing radiation, whereas radiation of insufficient energy to cause this effect is referred to as nonionizing radiation. The ionizing and nonionizing regions of the electromagnetic spectrum are shown in Figure 1. While nonionizing radiation absorbed by biological tissue is not capable of ionizing atoms or molecules, it nevertheless may produce changes in the vibrational and rotational energies of the biological molecules, leading to changes in the molecules or dissipation of the energy in the form of heat.
The near field comprises distances from the RF source less than about five wavelengths which includes the immediate vicinity of the RF device where most worker exposures to RF energy occur. In the near fields electromagnetic waves have different characteristics than in the far field. Furthermore, in the occupational setting near a RF sealer or heater, the electromagnetic field generally is not uniform, and the energy field incident upon a worker is complex and depends on many factors. A power density monitors designed for use in the far field, is likely to give exceedingly inaccurate measurements in the near field. Further, in the near field, as opposed to the far field, there is no simple mathematic equivalency between values of power density and measurements of either electric or magnetic field strength.
In the past, values of far-field power density have been used in various public health guides and recommendations for exposure limits, including those intended for occupational settings. However, a power density value, which can be measured or calculated for far-field conditions is not appropriate for quantifying near-field exposure of a worker operating an RF sealer or similar device. In the near field, measurements of both the electric and the magnetic fields are necessary to properly characterize worker exposure conditions. Instruments are now commercially available to make near-field measurements of the electric and magnetic fields. Users should follow instrument manufacturers' use instructions carefully to avoid damage of sensitive instrument probes. A list of manufacturers of these instruments is presented in Section III of the Appendix.
Humans can absorb RF energy at the frequencies used by most RF sealers and heaters. In workers who are not in contact with an electrical ground, the highest absorption rates for whole-body irradiation can occur at frequencies between 60 and 100 MHz with a peak at approximately 80 MHz.3,4 These frequencies of high absorption rates are very close to the frequencies used by most sealers and heaters. Hence, workers near RF sealers and heaters can absorb considerable amounts of the stray energy emitted from the RF machines. Effects of directly touching an electrical ground plane can lower, by as much as one half, the frequency at which an irradiated body will maximally absorb energy.3 Contact of the worker with an electrical ground plane can shift the frequency of maximum absorption rate to well within the frequency band of most RF sealers and heaters; this could increase the amount of energy absorbed by the worker and worsen the exposure condition. RF shielding material incorporated into the floor, walls, and ceiling of some RF workrooms could constitute such a ground plane.
Frequency (Wavelength) | Exposure Conditions | Animal | Reported Effects | Reference |
50 MHz | 0.5-6 volts/meter for 10-12 hrs/day; 180 days | rats and/or rabbits | changes in conditioned reflexes changes in encephalograms decreased blood cholinesterase activity increased urine 17-ketosteroids decreased leukocyte count decreased phagocytic activity | 5 |
(4.3 meters) | 150 volts/meter for 60 min/day; 4 months | rats | changes in conditioned reflexes | 6 |
69.7 MHz | 12 volts/meter for 1 hr/day; 1.5 months | rats | increases weights of adrenal and pituitary glands | 7 |
69.7 MHz | 48 volts/meter for 4 hrs/day; 1.5 months | infant rats | decreased weight of thyroid gland increased weight of adrenal gland | 7 |
14.88 | 70 volts/meter | infant rats | decreased weights of thyroid and adrenal glands | 7 |
14.88 | 100 volts/meter for 4 hrs/day; 10 months | rats | changes in phagocytic activity | 8 |
Control of the emission of RF energy from RF sealers and heaters should rely on the application of properly designed and installed shielding material. The shielding should be placed on or around the equipment so as to minimize occupational exposure due to emissions of stray RF energy. All shielding material should be properly grounded. Shielded conductors should be used for conveying RF current, and path impedance should be minimized by using good conductor materials. Many of these control features are available on RF sealers and heaters being marketed new, and some machines already in use can be retrofitted with some of these features. Older machines may require custom modification to control stray emissions.
The distance between the worker and the source of RF energy emission should be maximized. Examples of means to accomplish this include the use of automatic feeding devices, rotating tables, and remote materials handling.
The RF sealing and heating equipment should be electronically tuned to minimize the stray power emitted.
Whenever possible, equipment should be switched off when not being used. Maintenance and adjustment of the equipment should be performed only while the equipment is not in operation.
After the performance of maintenance or repair, all machine parts, including cabinetry, should be reinstalled so that the equipment is intact and its configuration is unchanged.
Warnings and Information
Access to the vicinity of RF sealers and heaters where there may be stray RF energy should be limited as much as possible to the operator and necessary assistants, maintenance personnel, and industrial hygiene or safety personnel. Use of the RF equipment should be restricted to properly trained personnel.
Areas in which exposures to RF energy have been determined to be appreciable should be posted. Any signs should be of such size as to be recognizable and readable from a distance of three meters. All warning signs must be printed in English and in the predominant languages of non-English-reading workers, and should conform to the design recommended by OSHA.9
Areas in which the RF energy is present at levels higher than the permissible exposure limit also should be posted. The warning signs should contain the following additional information: HAZARD--DO NOT ENTER. The sign must be readable from a distance of three meters. The perimeter of the restricted area should be clearly demarcated with signs visible to all personnel approaching the area.
Medical Monitoring
A medical surveillance program, tailored to the expected degree of employee use of RF equipment and potential for exposure to RF energy, should be developed. The program should include preplacement examination of all new employees and an initital examination of all present employees subject to occupational exposure to RF energy. In an effort to identify possible adverse effects associated with exposure to RF energy, annual examinations should be considered for workers who may be exposed to RF energy on a regular, long-term basis. Work histories should be included in all examinations.
Medical histories and physical examinations should give particular emphasis upon target organs potentially affected by RF energy including the eye (cataracts), the central nervous system, the blood (decreased leukocyte count), the immune defense system, and the reproductive system. Adverse reproductive effects may involve both maternal and paternal exposure. For persons occupationally exposed to RF energy, medical records including health and work histories should be maintained throughout the period of employment and for an extended period after termination of employment.
Exposure Measurements
Areas in the occupational environment where levels of RF energy have been determined to be appreciable should be surveyed at regular intervals. Immediately following a physical or electronic alteration of the equipment or an alteration in the process, a complete survey should also be performed. If measurements taken during a survey indicate that occupational exposure exceeds the permissible exposure limit, a second survey should be made on the next workday. If the limit is still exceeded, the use of RF equipment producing excessive values should be prohibited until appropriate controls have been instituted. The survey data sheets should contain all information pertaining to the survey, and should include the date and time of measurement, the type of monitoring equipment used, the employees' names, and the remedial actions taken, if any. These records should be maintained for an extended period of time.
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