Ultrasound is included under the heading of noise for the reason that, even though it cannot be heard, it is airwave pressure (like audible sound) but with a frequency greater than the upper limit of human hearing (approximately 20 kHz). Ultrasound is used in many different fields, typically to penetrate a medium and measure the reflection signature or supply focused energy. The reflection signature can reveal details about the inner structure of the medium. The most well known application of ultrasound is its use to produce pictures of foetuses in pregnancy but there are many other biomedical, industrial and laboratory applications as well, including
· Ultrasonic testing of testing of metals, plastics, composites, construction materials etc to identify flaws or measure thickness
· Heat transfer in liquids
· Ultrasonic cleaning
· Ultrasonic disintegration and dispersal
· Ultrasonic welding
Health effects can result from exposure to ultrasonic noise such as localised heating from direct contact and potential hearing loss. Anyone whose work involves use of ultrasound should consult the separate section which goes into more detail about carrying out risk assessments and control measures.
This guidance is intended as a supporting document for risk assessors when assessing the risk of workers exposure to hazards when using devices with ultrasonic frequencies.
Research has indicated that ultrasonic noise has little effect on general health unless there is direct body contact with a radiating ultrasonic source. Contact exposure is when there is no air gap between the ultrasonic source and the tissue. This may be direct contact or when transferred through a solid or liquid medium. Contact exposure can in some case transfer nearly 100% of the energy to tissues.
An air gap can reduce the energy transfer significantly. For example, if a person’s finger is in the water bath of an ultrasonic cleaning device the energy transfer to the bone is approximately 65%, but if the finger is just outside of the water the energy transfer is 106 less.
The American Conference of Government Industrial Hygienists (ACGIH) has established permissible ultrasound exposure levels. These recommended limits (set at the middle frequencies of the 1/3 octave bands from 10kHz to 50kHz) are designed to prevent possible hearing loss cause by the sub harmonics of the set frequencies, rather than the ultrasonic sound itself. The HSE have not set any exposure action values or limits for ultrasound, they consider that such effects from exposure to upper audible or ultrasonic frequencies are as yet unproven.
Within the University typical uses for ultrasonic frequencies include; cleaning, dissolving, dissolution, degassing, non-invasive testing and imaging etc. These applications may produce sub harmonic frequencies in the audible region; the noise levels associated with these sub harmonics are significantly below the exposure action values as defined in the Control of Noise at Work Regulations 2005. The risk to an individuals hearing of this type of device is typically not significant, but the sub harmonic noise may cause a “nuisance”. For these instances the position of the ultrasonic device and any shielding will help to reduce the propagated noise.