RECORDS ADMINISTRATION
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`ACLZ
`ULTRASOUND: BIOLOGICAL EFFECTS AND
`INDUSTRM HTGIENE CONCERNS
`by
`Christopher Wiernicki and William J. Karoly
`E. I. du Pent de Nemours & Co.
`Savannah River Plant
`Aiken, South Carolina 29808
`A paper proposed for publication in the
`American Industrial Hvziene Journal
`This paper was prepared in connection with work done under Contract
`No.DE-AC09-76SROOO01 with the U.S. Department of Energy.By ac-
`ceptance of this paper, the publisher andlor recipient acknowledges
`the U.S.Government’s right to retain a nonexclusive, royalty-free
`license in and to any copyright covering this paper, along with the
`right to reproduce and to authorize others to reproduce all or part
`of the copyrighted paper.
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`This document was prepared in conjunction with work accomplished under Contract No.
`DE-AC09-76SR00001 with the U.S. Department of Energy.
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`DP-MS-84-36
`ULTRASOUND: BIOLOGICAL EFFECTS AND
`INDUSTRIAL HYGIENE CONCERNS
`by
`Christopher Wiernicki and William J. Karoly
`E. I. du Pent de Nemours & Co.
`Savannah River Plant
`Aiken, South Carolina 29808
`ABSTRACT
`Due to the increased use of high intensity ultrasonic devices,
`there is now a greater risk of worker exposure to ultrasonic radiation
`than there was in the past.Exposure to high power ultrasound may
`produce adverse biological effects.High power ultrasound, character- _
`ized by high intensity outputsat frequencies of 20-100 kHz, has a wide
`range of applications throughout industry.Future applications may
`involve equipment with higher energy outputs.Contact ultrasound,
`i.e., no airspace between the energy sourceand the biological tissue,
`is significantly more hazardousthan exposure to airborne ultrasound
`because air transmits less than one percent of the energy.This paper
`discusses biological effects associated with overexposure to ultra-
`sound, exposure standards proposedfor airborne and contact ultrasound,
`industrial hygiene controls that can be employed to minimize exposurej
`and the instrumentation that is required for evaluating exposures.
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`INTRODUCTION
`Ultrasound has been virtually ignored as a potential hazard by the
`industrial hygiene community.In the past this may have been due to
`the low power output of ultrasonic devices and the small number of
`workers with a potential for exposure. However,since the late 1960’s
`there has been a sharp increase in the production and use of industrial
`ultrasonic equipment.(1) Industrial applications of ultrasound use
`frequencies ranging from 10 kHz to greater than 10 MHz, and intensities
`between 10-3 and 105 W/cmz. Ultrasound is generally divided into
`low power applications and high power applications. Applications of
`low power ultrasound in medicine, nondestructive testing, control
`applications, and delay lines make use of frequencies in the megahertz
`range and power intensities in the milliwatt range.High power ultra- -
`sound,characterized by lower frequencies and higher power outputs
`(Figure 1),has applications in cleaning, welding, impact grinding,
`drilling, atomization, sonar, and other processes.(1,3) Most high
`power applications occur at frequencies between 20 and 60 kHz.4
`Literature from the medical field, while often ambiguous, indicates
`that a variety of undesirable effects may be elicited by high power
`ultrasound.
`The purpose of this paper is to summarize the state of knowledge
`concerning the biological effects of ultrasound.In so doing, an
`attempt was made to separate known verified effects from ungrounded
`speculation, in order to determine if any possible workplace hazards
`exist.Exposure standards, industrial hygiene controls available for
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`minimizing the exposure potential and instrumentation for measuring
`ultrasound are discussed.
`BIOPHYSICS
`Ultrasound is defined as mechanical vibrations propagated at fre-
`quencies above the upper limit of human hearing.‘5) The upper
`limits of useful ultrasound are 106 Hz in gases and 109 Hz in
`solids and liquids.Lower limits for ultrasound are not well defined
`for two reasons:first,upper limits for human hearing are quite
`variable;and second, the term ultrasound is occasionally used to
`designate frequencies within the range of human hearing.(5,6)
`Ultrasound can be propagated as continuous or pulsed waves.out-
`put is generally expressed as temporal average, but with pulsed waves,
`instantaneous peaks can be an order of magnitude or more greater than
`this average.Much of the early literature failed to differentiate
`between continuous or pulsed modes; therefore, all exposures are given
`as temporal averages.The majority of high-power industrial applica-
`tions use the continuous wave mode.
`The physical properties of ultrasound are basically those of
`audible sound.Ultrasonic wave interactions result in reinforcement,
`annihilation,or standing waves.Velocity is a function of density and
`elasticity of the medium as well as wave form.Ultrasound intensity is
`attenuated as the distance from the source increases.This is due both
`to geometrical factors (the inversesquare law) and the scattering and
`absorption of energy.At the interface between two mediums, ultrasonic
`waves may be absorbed,transmitted, or reflected.(5)
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`Ultrasound of sufficient intensity interacts with biological
`tissues to produce lesions thermally, by mechanical disruption, or bY
`cavitation.
`1.Thermal Effects
`Exposures of several seconds or more and intensities of greater
`than 100 mW/cm2 produce lesions resulting from the absorption of
`acoustical energy and a concomitant rise in temperature. Experiments
`have
`heat:
`ante
`vers:
`demonstrated that identical lesions can be produced by direct
`ng by passing an electrical current through an implanted resist-
`wire. A threshold temperature must be exceeded before any irre-
`ble damage will be seen. This threshold temperature is inversely
`related to the log of the exposure time, varying from 66.5°C for a
`0.3-second exposure to 43°C for a 900-second exposure in the mammalian
`brain.(7,8,9)
`2. Mechanical Disruption
`When an ultrasonic field impinges on an object with a density
`different from the surrounding medium,a force called radiation torque
`will be exerted on the object.’11) An ultrasonic wave with a fre-
`quency of 1 MHz can produce tissue displacement ranging from 18-1,800A
`and acceleration ranging from 1,400-740,000 g’s.Ultrasonically
`induced shearing stresses cause stretching, twisting, and rupturing of
`biological membranes.These shearing stresses have been implicated as
`a mechanism for inducing biological damage.(1) Mammalian brain
`lesions are characterized by an immediate loss of nerve electrical
`activity, a ten-minute development period before the lesion is
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`histologically visible,and the appearance of lesions at the focus of
`the ultrasonic waves.White matter is more sensitive than gray matter.(7,10)
`3.Cavitation
`Alternating phases of rarefaction and compression result in dense
`zones of gas bubbles that grow and collapse.Bubbles grow during
`rarefaction and then collapse during compression to produce an expand–
`ing shock wave (3,7,8,12).The cavitation threshold for water is
`frequency-dependent ranging from 1 W/cm2 at 10 kHz to 500 W/cm2 at
`1 MHz.Two forma of cavitation are recognized.Transient or collapse
`cavitation occurs at relatively high pressure amplitudes; in this case
`the gas
`in loca
`at much
`bubble will collapse in a fraction of a single wave resulting
`ized high pressure and temperature.Stable cavitation occurs
`lower pressure amplitudes,resulting in alternating compression
`and expansion of the bubbles. Cavitation can produce heating, mechani-
`cal stress, and ionization.(11) In the mammalian brain,cavitation
`lesions are characterized by instantaneous histological appearance,
`discernible temperature rise,and lesions that do not always appear
`the focus of the ultrasonic waves.Gray and white matter appear to
`equally susceptible. (7,10)
`no
`at
`be
`Ultrasonic energy is absorbed by biological tissues.The lowest
`absorption coefficients are found in soft tissues such as liver,
`kidney, and brain.Fatty tissue coefficients are about 10% lower than
`for other soft tissues;the highest values are found in striated
`muscle. Some absorption coefficients are shown in Figure 1.(13,14)
`Higher absorption coefficients are found in skin, tendon, and bone;
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`these tissues will be heated at a faster rate than soft tissue. Bone
`in particular can produce very high local temperatures when subjected
`to ultrasound.In addition,bone produces a shadowing effect so that
`distal tissues remain cool.(13,14) Proteins are largely responsi-
`ble for absorption of ultrasonic energy by biological tissues at the
`molecular level.Individual molecular constituents display a range of
`absorption coefficients dependent on pH and molecular integrity.14
`A proton-transfer mechanism has been postulated to explain the observa-
`tion that absorption peaks occur at both high and low pH values. Blood
`circulation is an important factor in determining the heating effect of
`ultrasound ; efficient circulation will minimize spot heating(15,16).
`Ultrasonic exposure can occur from direct contact with a solid or
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`liquid medium, or through the air.However,ultrasound is transmitted
`much more efficiently through a solid or liquid medium than through air
`(Table 2).The biological effects of contact and airborne ultrasound
`have been investigated and are summarized below.It must be emphasized
`that by definition,contact exposure to ultrasound indicates that there
`is absolutely no air space between the biological tissue and the
`ultrasound source.
`BIOLOGICAL EFFECTS
`Nervous System
`OF CONTACT ULTRASO~D
`During the 1950’s, investigators reported that ultrasound produced
`paraplegia in mice.Paraplegia and hindlimb dysfunctions were observed
`in cooled neonate mice exposed to beam intensities between 54 and 154
`W/cm2.(17~18)Paraplegia and hemorrhage into the spinal cord
`resulted when mice were exposed to a field of 25 or 50 W/cm2 peak
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`intensity for 300 seconds at 0.5-6 ~z;damage resulted at time average
`intensities as low as 2.5 W/cm*.A 5.5-11.0°C temperature rise was
`recorded in damaged areas of the spinal cord.(19) Central nervous
`system lesions were produced in cat brains by cavitation (25 to 200
`millisecond exposures to 5000 W/cm* at 1 and 3 ~z).The lesions
`were characterized by blood vessel destruction, disrupted neurons
`lacking cytoplasm glial cell detritus,and a dispersed or disarrayed
`matrix.10 Focal lesions were produced in cat brains at intensities
`as low as40W/cm2 when ultrasound was applied to the exposed brain or
`spinal cord.20
`Stolzenberg demonstrated evidence for autonomic nervous system
`damage as well as central nervous system damage.(21) Ultrasound,
`(at 2 ~z, 1 W/cm2, and 80-200 seconds in duration), produced hind-
`limb dysfunction, a distended bladder syndrome, and intestinal paral-
`ysis in mice.Dose response damage thresholds were 140 and 120 seconds,
`respectively.Damage to the spinal cord and adjacent ganglia, bone
`marrow, and dorsal skeletal muscle were also noted.Wile mammalian
`brain lesions have been produced when a specific time-temperature
`threshold is exceeded, the specific mechanism by which morphological
`damage is produced is unknown.Damage thresholds range from 66.5°C
`(for a 0.3 second exposure) to 43.0°C (for a 900 second exposure).
`Lesion size is a function of peak intensity and exposure
`duration.(8,12,22)
`Ear
`Barnett determined that direct ultrasonic irradiation of the
`vestibule and cochlea of cats and guinea pigs via the sound window
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`will produce histological and cellular damage.Test animals were
`subjected to intensities between 5-10 W/cm2 for 20 minutes at
`3.5 MHz.Histological damage, manifested as severe balance dysfunc-
`tion,was seen throughout the vestibular labyrinth.The cochleas
`suffered cellular damage over an area greater than two cochlea turns.
`Inner hair cells appeared to be less sensitive to ultrasound than outer
`hair cells.(23)
`Eye
`Baum determined that all optical damage was reversible at a level
`of 0.25 W/cm2 (1 MHz) for exposures up to five minutes.(24)6
`Other investigators demonstrated reversible changes for exposures of
`0.2 W/cm2 and no effect when 0.0337 W/cm2 was applied for four
`hours .(25,26) The threshold for cataract formation in rabbit eyes
`has been reported to be in the range of 30-400 W/cm2 at frequencies
`ranging from 1-9.8 MHz.(27~28)
`Olson, et al., using an intensity of 3.4 mW/cm2 and exposure
`times from 10 seconds to 5 minutes produced discrete lesions in the
`corneal endothelium of adult Dutch rabbit eyes when the ultrasonic
`probe was activated inside the anterior chamber.The theorized
`reaction sequence was:“(l) Cytoplasmic disruption near the basement
`membrane; (2) cellular condensation and contraction of apical
`membranes; (3) rupture of apical membranes and cytoplasmic loss;
`(4) increase in peripheral involvement
`endothermal sloughing.”The extent of
`time of exposure.(29)
`and cell loss; (5) gross
`the damage was related to the
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`Testes
`O’Brien, et al.,exposed mice testicles to two levels of ultra–
`25 W/cm2 for 30 seconds and 10 W/cm2sound at a frequency of 1 MHz;
`for 30 seconds. Both levels resulted in marked disruption of the testi-
`cular tissue affecting both spermatocytogenesis and spermiogenesis.
`The high exposure level produced detectable damage immediately, whereas
`the lower exposure level required several days for damage to appear.
`The lower intensity produced a 2°C rise in temperature, to 39°C, while
`the high intensity exposure produced intrascrotol temperatures of
`47-50”C.In addition to thermal effects, cavitation could have
`produced damage at the higher intensity.(30)
`Teratology
`Female mice were exposed to 2.0 or 2.5 W/cm2 of ultrasound (at
`1 MHz) for a period of three minutes on the eighth day of gestation.
`Exenchalies, an anomaly not found in normal or control animals, were
`(31)Fry, et al., determined thatproduced in irradiated fetuses.
`there was a significant reduction in litter size for female mice
`irradiated with a 2 mm diameter beam at an intensity of greater than
`45 W/cm2.(32) In addition, a significant rise in abnormalities
`concomitant with a reduction in average pup weight was seen when
`pregnant mice wereirradiated with a beam intensity of 50 W/cm2 or
`greater.Lele(31) states that a 2.5°C temperature increase can be
`produced in anesthetized mousefetuses in situ when using an ultrasound——
`beam of 2.7 MHz and 200 mW/cm2 and exposure times of 30 minutes or
`more.It is further stated that such hyperthermia, produced during
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`fetal organogenesis, would be sufficient to produce an excess of
`abnormalities .Systemic hyperthermia, 2.5-5.O”C, for one hour or
`longer during fetal organogenesis (rat, guinea–pig, sheep) produced a
`number of teratological effects including fetal resorption, growth
`retardation, exencephaly, tail defects, limb defects, and palate
`defects (see Table-111).(33) Abnormalities were reported when
`9-day-old rat fetuses were exposed above a threshold beam intensity of
`3.0 W/cm2 for 5 or 15 minutes at frequencies of 0.71 or 3.2 MHz
`continuous wave or 2.5 MHz pulsed wave.Gross and microscopic heart
`abnormalities were observed. A recent review of medical
`ultrasound indicated that with pulsed ultrasound exposures it may be
`the peak power which is the determining factor in the reported
`effects.(34)
`Genetic and Cellular
`In 1970 Macintosh and Davey reported that ultrasound from an
`ultrasonic fetal heart detector produced an increased frequency of
`chromosome and chromatid irregularities in human blood cultures.(36)
`The author concluded that ultrasound was potentially mutagenic to
`humans.Several later studies have refuted the claims of Macintosh and
`Davey; some investigators have claimed that a toxin, given off from the
`polythene culture bags upon exposure to ultrasound, was the cause of
`increased chromosome irregularities.(37-41) GalPerin-Lemaitre, et
`al., reported that ultrasonic intensities utilized for therapy
`(200 2,mW/cm2,1 W/cm2 and 1.5 W/cm2) broke down all of the DNA
`molecules exposed.However, DNA was not damaged by a 20 mW/cm2
`beam. (42) In contrast, Prasad, et al.,reported that DNA synthesis
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`was reduced in cultures of hela cells irradiated by an ultrasonic beam
`of only 4 mW/cm2.(43) Cellular inactivation by ultrasound,
`measured by plating efficiency, was reported by Li, et al.(44) The
`mechanism inactivating mammalian cells is apparently non-thermal.Two
`recent papers by Liebeskin,et al., investigated the effects of ultra-
`sound (15 mW/cm2 temporal average intensity) on cell cultures. Hela
`cells demonstrated increased immunoreactivity to antinucleoside anti-
`bodies in G1 cells,indicating single strand breaks or unwinding of the
`helix.C3H mouse cells demonstrated a loss of contact inhibition,
`while surface membranes of cultured babl/c 3T3, clone 1-13 cells showed
`increased densities of microvilli. (45>46)
`BIOLOGICAL EFFECTS OF AIRBORNE ULTRASOUND
`Machines or processes employing ultrasound sources may emit high-
`level,random, and audible noise, possibly as subharmonic of the
`fundamental frequency.This noise can lead to temporary hearing thres-
`hold shifts,permanent hearing loss,and subjective effects such as
`fatigue,nausea, and headaches.High-frequency audible sound in the
`upper region of human hearing has been reported to produce tinnitus,
`ringing in the head,and a sensation of pressure in the ears.Parrack
`reported that subharmonicof frequencies between 9.2 and 37 kHz at
`levels of 148-154 dB produced temporary threshold shifts.
`Hearing loss has been documented at frequenciesUp to 14 kHz. (48)
`The biological effects of noise are well documented and will not be
`considered further here.It is assumed that exposure to pure ultra-
`sound at levels less than 140 dB will not produce even temporary
`threshold shifts.(47,49,50)
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`Acton(50) reviewed the physiological effects of airborne
`ultrasound in animals and man.These effects are summarized in
`Figure 2.Fatal body temperature rises occured between 144 and165 dB
`at1-30kHz for mice, rats, guinea pigs and rabbits.The calculated
`lethal whole-body exposure dose for man is at least 180 dB at
`20 kHz.(48) Documented human responses include mild warming of the
`body surface at 159 dB and loss of equilibriumand dizziness at
`160-165 dB (20 kHz). (51$52) Acton stated:“In the case of air-
`borne ultrasound, the acoustic mismatch between the air and tissue
`leads to a very poor transfer of energy.The effects on small fur-
`covered animals are more dramatic because the fur acts as an impedance
`matching device; they have a greater surface area to mass ratio; and
`they have a much lower total body massto dissipate the heat generated
`than man.Furthermore, the lower ultrasonic frequencies may well be
`audible to these animals, and the exposures have been to high sound
`pressure levels. Therefore,the effects on small laboratory animals
`cannot be extrapolated directly to the human species.”
`EXPOSURE STANDARDS
`Three frequency dependent exposure standards for airborne
`ultrasound were proposed in the 1960’s.These are summarized in
`Figure 3.Grigor’eva experimented with both audible sound and airborne
`ultrasound.She suggested acceptable limits for airborne ultrasound
`and also for one-third octave bands in the audible sound spectrum.
`Exposure time limits were not specified.Grigor’eva concluded, in
`1966, that:“The experiments lead one to believe that airborne
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`ultrasound is considerably less hazardous to man by comparison with
`audible sound. Also bearing in mind the data available in the litera-
`ture, 120 dB may be adopted as an acceptable limit for the acoustic
`pressure for airborne ultrasound. The possibility of raising this
`level should be tested experimentally”.(52) In 1968 Acton proposed
`a level of 110 dB in the one-third octave bands centered on 20, 25,
`and 31.5 kHz to prevent both auditory and subjective effects, “I.e.,
`fatigue, loss of equilibrium, nausea, etc., in the majority of the
`population exposed over a single work day.(47>49) In 1969 a sub-
`group of ANSI Standards Working Group S3-W40 chaired by Parrack made
`recommendations based on biological effects at sound frequencies just
`below and in the ultrasonic range. The proposed criteria, which were
`never published, were designed to prevent subjective and audible sound
`effects over an eight-hour work day for five or five and one-half days
`per week.(52)
`More recently, two additional exposure standards have been
`proposed for airborne ultrasound.In 1980 Benwell and Rapacholi of the
`Radiation Protection Bureau, Department of National Health and Welfare,
`Canada recommended a maximum permissible exposure level which is shown
`in Figure 4.A TL@ (the time-weighted average limit for a normal
`eight-hour day and a 40-hour work week,to which nearly all workers may
`be repeatedly exposed, day after day, without adverse effects) for
`airborne upper sonic and ultrasonic acoustic radiation has been
`(53)proposed by the ACGIH (Figure 5),.
`These standards are consistent in that they present exposure
`limits to prevent subjective effects at one-third octave bands centered
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`at or below20kHz and exposure limits for one-third octave bands at
`20 kHz and above to prevent thermal effects and hearing loss from
`possible subharmonic.
`The only exposure standard available for contact ultrasound was
`proposed by Nyborg in 1978 and recommended by Benwell and Repacholi in
`1980.(53,54)The recommended standard shown in Figure 7 was set to
`prevent reported biological effects that are considered hazardous.One
`hundred mW/cm2is considered a threshold below which no adverse
`biological effects are seen.Exposures to intensities greater than
`10 W/cm2 should not be allowed.
`MEASUREMENT OF
`Equipment
`ing accurately
`AIRBORNE ULTWSOUND
`used in measuring ultrasound must be capable of measur- -
`the frequencies of interest.Measurements should be
`made in one-third-octave–bands.The microphone used should have a flat
`response over the frequency range.Commercial equipment is available
`from Bruel & Kjaer, General Radio,and other manufacturers to meet the
`needs for obtaining accurate and reliable measurements.
`The audible sound alone should be measured by adding to the exist-
`ing A-weightingresponse of the sound level meter a low-pass filter
`(with a relatively sharp cutoff) to rejectthe ultrasonic frequencies
`at 20 kHz and above. Commercial equipment is available to measure
`ultrasound in the frequency range of20 kHz to at least 50 kHz.
`Complete calibration should be performed by a qualified laboratory
`or the equipment manufacturer when needed.Frequency of calibration
`depends on the extent of use in certification and routine field use
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`conditions.Although the field calibrator check (calibrated couplers
`operate at about 2000 Hz) does nottest the high frequency measuring
`system, it nevertheless is highly recommended. Usually the instrumen-
`tation either performs correctly at all frequencies or
`all frequencies.
`High frequency sound waves are highly directional
`malfunctions at
`and therefore
`are easily attenuated by barriers. The existing radiation field may be
`very complex.Constructive or destructive interferences between waves
`may occur over short distances so that experimentation is required for
`placement of the microphone.Conduct preliminary measurements to
`assess the noise field; then select the location and orientation of the
`microphone.Use of a rotating microphone boom will facilitate this
`test.(52)
`MEASUREMENT OF LIQUIDBORNE ULTRASOUND
`Acoustic power and intensity are the parameters that have been
`specified in most equipment performance standards.(55,56) Basically
`there are two types of measuring instruments available:those that
`measure total power,and those that measure “point” quantities, i.e.,
`intensities over areas small compared to the dimensions of the ultra-
`sound field.(57)
`Total power is generally measured by the radiation force method.
`lt is relatively simple, accurate, frequency independent, and an
`absolute method for determining total power.Momentum is transported
`in a traveling plane wave ultrasonic field.If momentum is transfered
`at a constant rate to a reflecting or absorbing target, the target will
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`respond as if acted on by a steady force, which is the radiation
`force.(58) This method can be used when the entire ultrasonic
`field is intercepted or when only a portion is intercepted. Absolute
`calibration can be done by weight substitution.The literature con-
`tains descriptions of several systems for measuring radiation force;
`however, this equipment is not readily available for workplace
`measurements .(57)
`“Point” quantities are generally measured using miniature piezo-
`electric hydrophores.There are some drawbacks:not all commercially
`available units are frequency independent and most units have resonance
`in the relevent frequency range distorting ultrasonic pulses. The
`piezoelectric polymer, polyvinylidene fluoride shows promise as a
`broadband acoustically transparent receiver.Calibration techniques
`have been described. Ultrasound dosimetry is not available at
`the present time.
`INDUSTRIAL HYGIENE CONTROLS
`1.Since high power ultrasound can cause a temporary or permanent
`physical change in a system, the following control measures are
`recommended.
`● Avoid direct contact at all times.
`● Equipment should be operated only by qualified personnel,
`knowledgeable
`● Warning signs
`contains high
`about potential harmful effects.
`should be placed at the entrance to any area which
`power ultrasound equipment or applied to each such
`device with appropriate precautionary statements for safe use.
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`● Ultrasonic cleaning tanks should have precautionary labels
`cautioning operators from immersing hands or parts of the body
`into the tank while it is operating.
`– turn off ultrasonic generators when loading/unloading parts
`- if not possible to turn off, place parts in sieves having non
`rigidly fastened handles coated with elastic covering.Sieve
`handles should not come in contact with liquid or sides of
`bath.
`2. Low power ultrasound is used for non-destructive testing without
`inducing temporary or permanent changes in the system.
`● There is little or no chance of harm from contact; however,
`direct contact should be avoided as a matter of good practice _
`(biological data inconclusive, some effects may occur).
`● Equipment should be operated by qualified personnel.
`● Precautionary signs on or near equipment to indicate presence
`of device and to caution workers to take appropriate action.
`3. Airborne Ultrasound
`● Adhere to proposed exposure guidelines.
`● Use total or partial enclosures, baffles, and absorbers to
`reduce sound levels.
`● Use hearing protection.
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`CONCLUSIONS
`The majority of research concerning biological effects of ultra-
`sound deals with diagnostic or low power ultrasound. Several thorough
`reviews concluded that low power ultrasound is relatively harmless when
`applied with discretion. (32)34359~60)The American Institute of
`Ultrasound in Medicine (August 1976;revised October 1978) has endorsed
`the following statement:“In the low megahertz frequency range there
`have been (as of this date) no independently confirmed significant
`biological effects in mammalian tissues exposed to intensities below
`100 mW/cm2. Furthermore, for ultrasonic exposure times less than
`500 seconds and greater than one second, such effects have not been
`demonstrated even at higher intensities, when the product of intensity
`and exposure times is less than 50 joules/cm2,1!(61) Epidemeological -
`studies have shown no adverse effect due to obstetric or clinical
`ultrasound.(61)
`The picture for high power ultrasound is less clear.Actually,
`the biological effects of true high power ultrasound, characterized by
`high intensity outputs at frequencies of20-60kHz, have not been
`investigated with regard to contact ultrasound.In the case of
`airborne ultrasound, the majority of investigations were done in the
`1940’s, 50’s and 60’s when industrial sources of ultrasound were
`generally much less powerful than those employed today.Until data are
`collected on true high power ultrasound, it must be assumed that the
`biological effects of high-frequency, high-intensity ultrasound can be
`extrapolated to the lower frequenciesused in industrial applications.
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`Exhibit 1013
`Page 20 of 40
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`However,frequency may be related to ultrasonic penetration of biologi-
`cal tissues and this may significantly change the type and extent of
`effects.Direct contact between ultrasonic sources and solid or liquid
`transmitting mediums and biological tissues produce a significant
`hazard because there is an efficient transfer of energy.In industrial
`settings the major concern would be with exposure of the hands and
`arms.Direct contact between ultrasound and the eyes,ears,testes,
`etc. appears very unlikely. Airborne ultrasound appears less hazardous
`because of its inefficient transfer through air (see Table 2).(62)
`However, powerful industrial ultrasonic equipment may be able to
`produce relatively high ultrasound intensities for short distances
`around equipment.For example, an aircraft take-off from an aircraft
`carrier can produce nearly 150 dB around flight deck personnel.(64) -
`Equipment capable of generating 160 dB would expose people to 10 W/cm2,
`an energy level capable of producing biological effects on contact.
`Early investigators of the biological effects of airborne ultrasound
`did not consider many of the effects now being investigated with low
`intensity contact ultrasound.Some hazards generally associated with
`airborne ultrasound may be the result of audible subharmonic
`frequencies.
`The literature reviewed here documents a number of effects result-
`ing from exposure to high intensity (contact and airborne) ultrasound;
`however, there is insufficient data to quantify dose–response
`relationships. In addition, there are problems associated with
`extrapolating animal effects to humans.No epidemiological studies
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`Exhibit 1013
`Page 21 of 40
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`have been conducted on workers exposed to high power ultrasound.Until
`the hazards associated with high power ultrasound are more completely
`understood, a cautious approach should be taken in its use.This is
`particularly true with pregnant women because fetal effects have been
`documented at relatively low intensities.The relevence of in vitro
`genetic effects to workplace exposures is unknown.
`a tool that may help in elucidating the mechanism
`.—
`Phantom dosimetry is
`and effects of
`exposure to high power ultrasound (both contact and airborne).
`Industrial hygienists should be aware of the potential hazards of
`ultrasound,j