Are we being exposed to potentially harmful airborne ultrasound?
Published in Proceedings of the Royal Society A, the study suggests the general public are unaware that they are being exposed to very high-frequency (VHF) sound and ultrasound (US) at levels over the current guidelines.
Such exposure could be putting people's health at risk, a UK researcher claims, causing headache, nausea, dizziness, migraine and tinnitus - ringing in the ears. These are symptoms that have been reported in occupational settings among workers exposed to high sound frequencies through drilling, for example, or industrial cleaning devices.
In the UK, current guidelines state that humans should not be exposed to ultrasound greater than 20 kilohertz (kHz), which is the highest frequency of sound that humans can hear.
However, study author Prof. Tim Leighton, of the University of Southampton in the UK, notes that such guidelines are only applicable to workplaces in which employees are aware of their ultrasound exposure, enabling them to protect themselves against any possible health implications.
"The guidelines are also based on an insufficient evidence base, most of which was collected over 40 years ago by researchers who considered it insufficient to finalize guidelines, but which produced preliminary guidelines," notes Prof. Leighton.
"This warning of inadequacy was lost as regulatory bodies and organizations issued 'new' guidelines based on these early guidelines, and through such repetition, generated a false impression of consensus."
Ultrasound levels over 20 kHz in many public places
For his study, Prof. Leighton used a smartphone/tablet application that produced a spectrogram - a visual representation of sound frequencies - based on microphone readings.
Sound frequencies were recorded in a variety of public buildings in the UK, including train stations, libraries, museums, schools and sports stadiums - places where there had previously been reports of people experiencing symptoms that could be related to ultrasound exposure.
Prof. Leighton notes that there are a variety of ultrasound sources present in public places, including loudspeakers and door sensors.
The buildings assessed in the study were occupied by hundreds of people at the time of recording, he notes.
Data showed that the general public occupying the buildings were being exposed to VHF/US at levels over 20 kHz, which Prof. Leighton says has the potential to be a public health concern.
He admits that there is insufficient evidence to reach a definitive conclusion as to whether exposure to such sound frequencies is harmful to health. "However," adds Prof. Leighton, "it is important that sufferers are able to identify the true cause of their symptoms, whether they result from VHF/US exposure or not."
He notes that people who are unlikely to be aware of their ultrasound exposure are complaining of a number of health conditions, and he points out that recent research has indicated that "1 in 20 individuals aged 40-49 years have hearing thresholds that are at least 20 decibels (dB) more sensitive at 20 kHz than the hearing thresholds of the average 30-39-year-old."
"Moreover," says Prof. Leighton, "5% of the 5-19 year age group is reported to have a 20 kHz threshold that is 60 dB more sensitive than the median for the 30-39-year age group."
New guidelines for public ultrasound exposure required
Based on his findings, Prof. Leighton says further investigation into the potential health implications of airborne ultrasound is warranted.
In particular, he believes studies should assess "whether current audiological practices, equipment and standards are suitable for the VHF and ultrasonic regime," and they should also identify strategies to combat any shortfalls.
Furthermore, he says future research should include a survey of modern devices and the sound frequencies they emit.
Prof. Leighton also calls for new guidelines that address the potential harms of ultrasound exposure among the general public.
These new guidelines should not be based solely on selection of ultrasound levels referred to in older guidelines, he says, nor should guidelines for occupational ultrasound exposure be applied to exposure in public places.
New guidelines should also account for people who are "long-term guests" in public buildings where ultrasound exposure may be high, such as in schools, hospitals and prisons.
Last year, Medical News Today reported on a study that suggested people exposed to higher levels of noise pollution are more likely to have a bigger waist size.