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Guidelines for the Safe Use of Ultrasound:
Table of Contents
Part II - Industrial & Commercial Applications - Safety Code 24
2. Health Effects of Industrial and Commercial Ultrasound
Contact exposure is exposure for which there is no intervening air gap between the transducer and the tissue. This may be via direct and intimate contact between the transducer and the tissue or it may be mediated by a solid or liquid. Contact exposure can in some cases provide nearly 100% energy transfer to tissue. However, an air gap can diminish the ultrasonic energy transferred by orders of magnitude. For example, if a person's finger is directly irradiated in the water bath of an ultrasonic humidifier, the energy transferred to the bone is approximately 65% of that which is radiated; but if the finger is just outside the water, one million times less ultrasonic energy is transferred to the finger.
The in vivo mammalian biological effects data for ultrasound contact exposure in the low MHz frequency range are summarized by the graph shown in Figure 1 (NCRP 83). The plots show the intensities below which no significant, independently confirmed biological effects have been observed.
Devices such as ultrasonic humidifiers operating in the low MHz frequency range can readily cause tissue injury if and only if there is a contact exposure. It is known from anecdotal reports that at full ultrasonic power, contact exposure of a finger to the ultrasonic beam from an ultrasonic humidifier will cause sharp pain within seconds, likely due to overheating of the bone.
There also exists the potential for hazardous effects below the MHz frequency range with high-power ultrasound. For example, high-power ultrasonic waves are used in ultrasonic cleaners and cell disintegrators because of their destructive and violent effects. It is certainly reasonable to assume that relatively intense cavitation activity occurs in the water (or solvent) baths of such devices (Ac 83; WHO 88). Nonetheless, documented cases of actual tissue damage are rare. In one documented study, exposure to ultrasound in ultrasonic cleaners operating at frequencies between 20 and 40 kHz was reported to have caused pain in the hands of the volunteers. However, exposure to ultrasound in an 80 kHz cleaner led to no immediate observable effects (Fi 68).
Plots showing intensities below which no independently confirmed significant biological effects have been observed in mammalian tissues. The upper plot (FL) applies to focal lesions; the intensities are in situ values. The lower one (AIUM) is a graphical representation of the in vivo mammalian bioeffects statement of the Bioeffects Committee of the American Institute for Ultrasound in Medicine (AIUM). The ultrasonic frequencies are in the range of 1 to 10 MHz and the spatial peak time average intensities, I(SPTA), are measured in water (NCRP 83).
In two reviews of the hazards of industrial ultrasound (Ac 77, Ac 83), Acton was unable to substantiate reports of necrosis or bone degeneration due to persistent exposure to liquid coupled ultrasound. Furthermore, in a recent study by Carnes and Dunn (Ca 86), testicular damage was observed in only 4 of 150 mice exposed to ultrasound from a 25 kHz tissue homogenizer operating at an intensity of 15 W/cm2.
The literature on devices such as ultrasonic cleaners and tissue homogenizers is confusing: these devices do not appear to be as hazardous as expected, given the effects they were designed to create. Nonetheless, although reports of biological effects are surprisingly rare, exposure to the liquid-borne ultrasound from these devices clearly can cause tissue injury, and protection measures are necessary.
The literature indicating the hazards of devices such as ultrasonic bonding machines is even more sparse. However, a recent report (Fe 84) described a thermal injury inflicted by a direct contact exposure with an ultrasonic bonding machine used in the bonding of plastics, operating at 20 kHz. An exposure of only a fraction of a second was enough to cause a serious localized burn on the operator's finger.
Although those who work with an industrial ultrasound device would not, through design of the device, experience direct solid or liquid contact with transducers emitting high power (or high intensity) ultrasound, direct contact exposure can occur through accidents or carelessness. Therefore, appropriate precautions must be taken to avoid accidental exposure.
Concern about the possible effects of exposure to airborne ultrasonic and upper sonic(1) radiation began in the late 1940s with reports of "ultrasonic sickness" in personnel working around jet aircraft (Da 48). These concerns led to research into the auditory and non-auditory biological effects of airborne ultrasound.
The most plausible mechanisms for non-auditory effects of airborne ultrasound on a human are heating and cavitation. The examples of cavitation thresholds given by Neppiras (Ne 80) suggest that airborne ultrasound would not give rise to cavitation except at a sound pressure level (SPL) above approximately 190 dB. This is well above the levels for which heating effects would occur.
A number of studies of effects of airborne ultrasound have been undertaken on mammals and insects; the observed effects were interpreted as being due to heating. At 160 dB(2) at 20 kHz, Allen et al. (Al 48) reported the death of insects and mice as a result of exposures ranging from 10 seconds to 3 minutes. In these experiments it was established that the heating produced by sound absorption was sufficient to cause death. Work by Parrack and co-workers indicated that, at ultrasonic frequencies, most of the absorption found in rat studies was due to the fur on the rat (Gi 49, Gi 52, Pa 66). The ratio of the absorbed to incident intensity for the human body surface appeared to be about an order of magnitude smaller than that for a shaved rat. Danner et al. (Da 54) found that the heating threshold for mice occurred at an SPL of 144 dB for furred mice and 155 dB for shaved mice at frequencies of 18 - 20 kHz. These results suggest that an SPL of at least 155 dB would be necessary to produce a rapid damaging temperature elevation in humans, and Parrack's calculations (Pa 66) suggest that 180 dB would be required to be lethal to a human.
Further information on the ultrasonic heating of humans is found in the incidental human exposures reported in the work of Allen et al., in which an SPL of 165 dB was used (Al 48). Local heating in the crevices between fingers caused burns almost instantly at these levels. Painful heating occurred after several seconds of exposure of broader surfaces such as the palm of the hand. In addition, Acton (Ac 74) has reported on unpublished work by Parrack indicating that mild heating in skin clefts has been observed in the SPL range of 140-150 dB. Other non-auditory effects at these high levels included extremely disagreeable sensations in the nasal passages.
It can be concluded that, at SPLs greater than approximately 155 dB, acute harmful effects will occur in humans exposed to airborne ultrasound, primarily as a result of sound absorption and subsequent heating. It is plausible that chronic lengthy exposures to levels between 145 and 155 dB might also be harmful, as they could raise body temperatures to mild fever levels during the exposure periods. However, as indicated in Section 3 of this guideline, such high sound-pressure levels have never been encountered in either commercial or industrial applications.
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The major effects of airborne ultrasound of concern in practice are the result of reception by the ear. The effects fall primarily into two categories: the so-called "subjective" effects on the central nervous system and damage to the ear. These effects form the basis for exposure guidelines as they occur at lower sound pressure levels than those which give rise to heating.
Two reports have indicated temporary hearing loss at frequencies below 8 kHz due to a high SPL of airborne ultrasound. Parrack (Pa 66) reported that for five-minute exposures at selected frequencies in the range from 17 to 37 kHz at SPLs between 148 and 154 dB, hearing sensitivity was reduced at the subharmonic frequencies. Slight losses also occurred occasionally at the third subharmonic. Recovery from the losses was rapid and complete. Dobroserdov (Do 67) measured reductions in auditory sensitivity at 4, 10, 14 and 15 kHz after one hour's exposure to 20.6 kHz ultrasound at an SPL of 120 dB. No significant effects were observed at SPL exposures of 100 dB by Dobroserdov.
Sound pressure levels lower than 120 dB at ultrasonic frequencies have not been demonstrated to cause hearing losses. In a study of 18 men working with ultrasonic cleaners and other ultrasonic instruments, Knight (Kn 68) found no evidence of hearing loss attributable to ultrasonic exposure. Acton and Carson (Ac 67) found no temporary threshold shifts (TTS) in a study of 31 ears in 16 subjects exposed to SPLs of up to 110 dB in the 1/3-octave bands centred on 20 and 25 kHz. Grigor'eva (Gr 66) exposed five volunteers to 110 - 115 dB of a 20 kHz pure tone for one hour and found no change in auditory sensitivity (or pulse rate or skin temperature). However, a TTS appears to have been observed by Grigor'eva for exposure to pure tones at 16 kHz for SPLs greater than 90 dB.
Recently Grzesik and Pluta (Gr 83) studied the hearing of 55 ultrasonic cleaner and welder operators. No significant differences in thresholds of hearing between exposed and controls were observed at frequencies between 0.5 and 8 kHz. However, the authors claimed significant differences in hearing between exposed and control subjects in the 10 - 20 kHz range. They claimed threshold elevations and a decreasing number of subjects responding to stimuli at the highest audible frequencies. In a follow up of 26 of these workers, Grzesik and Pluta (Gr 86) suggested that a hearing loss of approximately 1 dB/year occurs in the frequency range of 13 - 17 kHz due to the occupational exposure of these workers to the acoustic fields created by the ultrasonic cleaners and welders. The acoustic spectra of these devices (Gr 80, Gr 83) indicated that the SPLs were in the range of 80 to 102 dB at frequencies between 10 and 18 kHz, the upper sonic frequencies, whereas the SPLs were in the range of 100 to 116 dB at frequencies greater than 20 kHz. In the absence of a detailed correlation between the acoustic spectra and the measured effects on hearing, it is impossible to say with certainty which frequencies were responsible for the high-frequency hearing losses. However, it is more likely that the upper sonic rather than the ultrasonic radiation led to the measured hearing losses in these studies since high SPLs at upper sonic frequencies were found more frequently than at ultrasonic frequencies (Gr 80, Gr 83). Also, as noted above, TTSs have apparently been observed for subjects exposed to pure tones at upper sonic frequencies between 10 and 16 kHz, with SPLs greater than 90 dB. Furthermore, there is no other substantiated evidence for effects on hearing below ultrasonic SPLs of 120 dB.
Other physiological effects of airborne ultrasound are likely to occur only at SPLs greater than or equal to those which would lead to TTS. Knight (Kn 68) and Grigor'eva (Gr 66) found no evidence for any physiological effects at ultrasonic frequencies. Dobroserdov (Do 67) found significant loss of balance stability and reduced motor response time for exposures to 120 dB at 20 kHz, but the effects were insignificant at 100 dB at the same frequency.
A number of "subjective" effects have been reportedly caused by airborne ultrasound, including fatigue, headache, nausea, tinnitus and disturbance of neuromuscular coordination (Sk 65, Ac 67, Ac 68, Cr 77, FDA 81(b)). Skillern (Sk 65) measured the 1/3-octave band spectra from 10 to 31.5 kHz from a number of ultrasonic devices and found that subjective effects were associated with devices which produced SPLs greater than 80 dB in this frequency range.
Acton and Carson (Ac 67), investigated effects of exposure to ultrasound on 18 young females working near a bank of "ultrasonic" cleaners. They were exposed to both ultrasound and audible acoustic energy and complained of fatigue, headache, nausea and tinnitus. The same symptoms were found in subsequent laboratory experiments in which human subjects were exposed to high-frequency acoustic radiation with audible components. When these same subjects were exposed to similar high-frequency energies, but without audible components, no complaints occurred, leading Acton and Carson to conclude that audible components had to be present for a subjective effect to be observed. They supported this finding by noting that women complained about these effects more than men. Since the exposed males were older and all had history of noise exposure as well as high frequency hearing losses, they assumed that the exposure radiations were largely inaudible to many of the men.
A detailed analysis of Acton and Carson's data indicated that subjective effects were not found if the 1/3-octave band SPLs were less than 75 dB for centre frequencies up to and including 16 kHz and less than 110 dB for centre frequencies greater than or equal to 20 kHz (Ac 68). Acton indicated that this criterion for the occurrence of subjective effects was also consistent with Skillern's data (Sk 65) and suggested it as an exposure criterion. Acton modified the criterion in 1975 (Ac 75). In the revised criterion, the 75 dB limit was extended to include the 1/3-octave band centred on 20 kHz. This was done when more reports of subjective effects were documented by Acton. He found that subjective effects could still occur below 110 dB in the 20 kHz, 1/3-octave band (Ac 75). This was interpreted as being because the nominal frequency limits of the 1/3-octave band centred on 20 kHz are 17.6 kHz and 22.5 kHz. The lower end of this frequency band was within the upper end of the audible frequency range of a significant proportion of the population and therefore subjective effects could occur at relatively low levels.
Taken together, the results on subjective effects indicate that they are a reaction of the central nervous system to the upper sonic frequencies or ultrasound as they become audible. The shape of Acton's empirically derived criterion suggests that ultrasound could be made audible if the sound pressure levels were high enough and that the threshold of hearing should rise rapidly in the transition from upper sonic to ultrasonic frequencies. This is qualitatively consistent with free-field SPLs for the average threshold of hearing measured by Herbertz and Grunter (He 81, He 84) for sonic and ultrasonic frequencies ranging from 10 to 31.5 kHz. Their values were averaged for two separate studies on a total of 30 subjects with normal hearing who were between the ages of 20 and 41. The average threshold of hearing increased rapidly and monotonically with frequency at a rate of approximately 12 dB per kHz between 14 and 20 kHz leading to an average threshold of hearing of approximately 100 dB at 20 kHz (He 84) and approximately 125 dB at 25 kHz. Above 20 kHz the ultrasound may appear audible because subharmonics are generated in the ear (Gi 50). Such a phenomenon has been observed at levels greater than 110 dB in the chinchilla and guinea pig (Da 66). The hypothesis that ultrasound appears audible due to generation of the subharmonics within the ear is also consistent with Parrack's study referre d to above, involving short-term hearing loss at subharmonics of exposure frequencies after exposure to high SPLs of airborne ultrasound (Pa 66).
To summarize, exposure to ultrasonic radiation, when sufficiently intense, appears to result in a syndrome involving manifestations of nausea, headache, tinnitus, pain, dizziness, and fatigue. The type of symptom and the degree of severity appear to vary depending upon the actual spectrum of the ultrasonic radiation and the individual susceptibility of the exposed persons, particularly their hearing acuity at high frequencies.
A concise summary of the physiological effects of ultrasound with specific stated exposure conditions has been given by Acton. It is shown in a modified form in Figure 2 (Ac 74).
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