Ultrasound may disrupt the balance centres of the inner ear
Some people may be more sensitive to ultrasound
"Ultrasound in public places could be triggering sickness," the Daily Mail reports.
Ultrasounds are high frequency sound waves used by a wide range of devices, and are thought to be inaudible to most humans.
A review has highlighted how many public places are now exposed to ultrasound, and there is a knowledge gap about what effect it has on our health.
What is the basis for these reports?
The news stories are based on a report by a scientist at the University of Southampton published in the peer-reviewed journal, Proceedings of the Royal Society.
The report comprises results from new investigations, as well as a narrative review focusing on the available evidence on ultrasound exposure and human health.
It has been made available on an open-access basis, so you can read the report for free online.
The author, Professor Timothy Leighton, said he recorded ultrasound in a number of public places, including a large library, a major railway station and a large swimming pool.
These places were chosen because people using them had reported a number of symptoms, including feeling sick, dizzy, tired, getting headaches and a feeling of pressure in the ears.
People have also reported getting vertigo, a combination of symptoms such as severe dizziness, loss of balance, feeling sick, being sick and headaches.
The report points out there is very little evidence to show the potential effects of ultrasound on people's health, and current guidelines are based on a few small studies from the 1960s.
These studies were done to assess the effect of ultrasound used in industry on workers' hearing, and did not consider wider issues such as public exposure.
Professor Leighton says the guidelines are not adequate to apply to ultrasound in public spaces, which people may be completely unaware of and affect large numbers of people for long periods of time.
What is ultrasound?
Ultrasound is sound at very high frequencies, above those most people can hear – usually above 20kHz.
It can be generated by most activities – for example, rubbing our hands together generates ultrasound – but some technologies emit constant ultrasound at higher volumes.
Examples cited in the report include public address systems, which emit a constant high-frequency noise when left switched on, and automatic door sensors. Professor Leighton said he recorded high-frequency ultrasound in these places at between 63 and 94 decibels (dB).
Some guidelines use a cut-off of 65dB for exposure to ultrasound noise at work, although the guidelines vary considerably. There are also problems comparing how ultrasound was measured when the studies were carried out and how it is measured today.
Pest control systems, which are designed to deter animals that can hear ultrasound, are another example of devices that emit ultrasound.
So too is the Mosquito device, designed to deter young people from gathering in public places by emitting an unpleasant high-pitched noise most adults cannot hear. Industrial devices that emit ultrasound include ultrasonic cleaning baths.
And, perhaps most controversially, many law enforcement agencies now employ what are known as long range acoustic devices (LRAD) to control crowds.
These are essentially next-generation loud speakers that can deliver intense beams of sound over a large distance – usually warning messages that a crowd should disperse.
An LRAD was deployed by the Metropolitan Police during the 2012 London Olympics, but was never used.
What is the evidence that ultrasound causes harm?
There is very little evidence on the effect of ultrasound on human health, either to show that it does or does not cause harm.
This report does not provide any new evidence about possible harms from ultrasound, either. It only shows some public places have volumes of ultrasound comparable to volumes covered in industrial guidelines.
We do know high-frequency ultrasound may damage people's hearing. The industrial guidelines were intended to avoid hearing damage at the lower frequencies we use for hearing speech.
These were based on the average hearing of a small group of men in their 40s. The effects on other groups – such as women, children or older people – may be different.
The study says: "Lack of research means that it is not possible to prove or disprove public health risk or discomfort."
All we know is some people exposed to ultrasound in industrial settings reported symptoms such as nausea, dizziness, headaches, tiredness and sensations of ear pressure.
We don't know whether the problems were caused by ultrasound or something else altogether. Most people exposed to ultrasound in public places are likely to be unaware of it.
Professor Leighton's report goes on to say: "There are no records of large numbers of complaints from the public, and this might be because only a small number are affected, or it might be because there has been no awareness of exposure and no route by which to complain."
We also don't know if there's a plausible way ultrasound could cause symptoms such as nausea, tiredness, dizziness and headaches.
Professor Leighton speculated symptoms could be caused by confusion in the brain, which perceives vibration from the eardrum, but does not get signals from the nerve that transmits sound.
He pointed out a similar confusion in the brain caused by disconnection between signals from balance receptors in the ear and what the eye can see, which is thought to be the cause of travel sickness, a condition with similar symptoms.
How does this report affect you?
The report does not provide new evidence that ultrasound in public places causes harm to human health.
Its main message is that as technologically generated ultrasound is becoming more common in public places, we should be carrying out more research into its effects on our health.
The study also calls for the existing guidelines on ultrasound and health to be completely revised, based on new research. Due to the ubiquitous nature of ultrasound in the modern environment, these calls seem prudent.
Possibly a useful first step in terms of research would be to carry out a double blind randomised controlled trial involving people who think they are sensitive to ultrasound.
We could then see if their reported symptoms correspond to ultrasound exposure, or whether they also occur when they are exposed to other noise frequencies or no sound at all