by Johanna Klümper
The sound emissions from ultrasonic baths (ultrasonic cleaners) are often an underestimated hazard. However, they must be measured and evaluated in accordance with applicable legal requirements. To this end, a number of guide and threshold values have been defined in Germany but they mainly refer to the audible frequency range and do not take into account large amounts of the sound emitted by ultrasonic baths. The properties of ultrasound also raise technical issues, which is why conventional measuring equipment cannot be used and a tailored approach must be taken for measurements. While measurements show that ultrasonic baths of normal size should not generally pose a hazard to workers, there are mechanisms that could potentially cause harm as the size increases or several units are operated simultaneously. However, if necessary, sound reduction measures can be incorporated into operations relatively easily.
The hazards to which the Reprocessing Unit for Medical Devices (RUMED) personnel are exposed when using ultrasonic baths have generally been thoroughly investigated and are well known. These include the harmful effects of reaching into the ultrasonic bath during operation, in addition to the fact that at temperatures above 50 C aerosols can be formed in the cleaning solution which can potentially transmit infectious pathogens. For this reason, the German Technical Regulation TRBA 250 (Technical Rules for Biological Agents in Healthcare and Welfare Facilities) stipulates the use of a lid or suction system when using ultrasonic baths. Conversely, the noise pollution generated by ultrasonic baths and its potentially harmful effects on staff are often overlooked. On the one hand, this is due to the fact that the audible cavitation noise exposure is subjectively assessed as being rather low, while the ultrasound components above 16 kHz are naturally not perceived by the human auditory system and their potential hazardousness are thus underestimated. On the other hand, occupational safety and health (OSH) regulations often exclude ultrasound, which creates legal uncertainties as to whether and how it should be assessed. Besides, the damaging effects of ultrasound on human hearing is a controversial topic that to date has been researched relatively little. Besides, measurements of conventional sound in the audible frequency range below 16 kHz are also distorted by ultrasound components, while exposure in the ultrasound range cannot be measured by conventional, handheld sound level meters, because these are not designed to measure the special properties of ultrasound.
This paper summarizes the harmful effects of ultrasound on humans and describes the legal and normative requirements for sound level measurements, in relation to the special features of ultrasound, as well as protective noise reduction measures. It also presents the results of an exemplary measurement to illustrate and better characterize this issue.
Potential health hazards
Noise can cause both aural and extraaural negative health effects in humans. Extra-aural noise effects are all effects on humans that are not directly related to hearing, such as high blood pressure, stress, or impaired concentration. With regard to ultrasound, employees sometimes report subjective effects such as nausea, dizziness, headaches, etc. However, it is debatable to what extent ultrasound is actually responsible for the extra-aural effects, or whether sound in the high audible frequency range could be the cause. It is also possible that the experienced symptoms are of a psychosomatic nature .
Aural effects are those that directly affect the inner ear. Damage to the auditory system can be caused by permanent exposure to noise as well as by a single episode of very short and loud sound events. Bang and explosion trauma caused by single sound events destroy hair cells of the inner ear within a few milliseconds, while permanent noise exposure causes gradual noise-induced hearing loss. This stems from sensory cell fatigue, leading to what is termed temporary threshold shift (TTS); the auditory faculties are fully restored by taking a sufficiently long break from noise exposure. If the sensory cells cannot be regenerated because of permanent exposure to sound, a noise-induced permanent threshold shift (NIPTS) presents. Since ultrasound cannot generally be perceived by the human auditory system, it is not possible to simply extrapolate substantiated research findings on the development of noise-induced hearing loss from the auditory frequency to the ultrasound range.
For a detailed list of studies on the aural effects of ultrasound, please refer to the various literature reviews [2-5]. In summary, it can be stated that continuous exposure to ultrasound is thought to cause some damage to the human auditory system. While ultrasound does not appear to cause permanent threshold shifts in the speech frequency range, it can cause temporary threshold shifts in the high-frequency range, which in turn are a potential trigger for permanent damage. It is unclear to what extent the ultrasound itself is the cause of the observed damage, or whether conversely this is caused by noise in the high-frequency range that normally accompanies the ultrasound cavitation or subharmonics. Besides, no statement can be made about dose-effect relations, which must be regarded as a basic requirement for effective stipulation of threshold values. Therefore, there is a considerable need for further research in this area.
Legal, normative and technical requirements
In Germany, the legal situation as regards the assessment of ultrasound in terms of occupational safety and health is accordingly rather patchy. Notwithstanding this, the risk assessment enshrined in the German Occupational Safety and Health Act (ArbSchG) can be applied to ultrasonic baths. The German Noise and Vibration Occupational Safety and Health Regulation (Lärm-VibrationsArbSchV) sets out in more specific terms the provisions of the Occupational Safety and Health Act with regard to the hazards caused by noise or vibrations to the health of employees. Noise is defined in the regulation in relatively broad terms as “any sound that can impair hearing or otherwise directly or indirectly endanger the safety and health of employees” ; this also includes ultrasound. With regard to risk assessment, the Noise and Vibration Regulation stipulates that the exposure of employees be determined and assessed. To that effect, the manufacturer documentation or other sources should be consulted (e.g. publications by the Employer’s Liability Insurance Association). However, such sources are usually not available for ultrasonic systems because emissions depend very much on the properties of the ultrasound prevailing in the respective workplace. The regulation also specifies lower and upper exposure action values, i.e. threshold values, which, when reached or exceeded, require the implementation of measures to protect employees. The threshold and guide values applicable in Germany for noise that is hazardous to health are shown in Table 1. The weighting filters used for these threshold values should be borne in mind and which, depending on the intended use, serve to simulate human hearing and thus provide information about the effect of sound on it. The A-weighted daily noise exposure level LEX,8h, which calculates the sound exposure of employees during an 8-hour work shift, leads to problems here since it is designed for measurements in the auditory frequency range and produces inaccurate results as soon as ultrasound components come into play. The C-weighted peak sound pressure level LpCpeak, whose weighting filter is designed for the detection of loud sounds and is thus used for the detection of single, very loud sound events by displaying the absolute sound peak during a measurement, is also unsuitable for the measurement of ultrasound, since ultrasonic baths generally generate constant sound emissions and sound peaks are therefore of subordinate importance for evaluation.
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