Airborne ultrasound in public places may be making people ill, with reports of nausea, dizziness and migraines at museums, train stations and sports stadiums. A team of researchers from the University of Southampton say ultrasound in public places comes from a number sources, such as loudspeakers, door sensors and public address systems, and that guidelines and research on its effects are lacking. It is defined as sound waves with frequencies higher than the upper audible limit of human hearing.

Tim Leighton, author of the study published in the Proceedings of the Royal Society A, says that while there is not enough research to confirm or deny a link between ultrasound and sickness, workers who have been exposed to it through industrial devices have reported similar health effects in the past.

"For over 40 years, there have been reports of hearing threshold shift and a range of subjective effects (nausea, dizziness, migraine, fatigue, tinnitus and 'pressure in the ears') from ultrasound in the air to which workers have been routinely exposed," they wrote.

Current guidelines regarding exposure are based on little research, most of which was collected decades ago. The authors warn that a better understanding of the possible risk is essential – as current guidelines are not designed for the mass exposure of large numbers of people.

To test airborne ultrasound, the researchers used smartphones and tablets with an app that let them produce a spectrograph of the reading from the microphone. They collected readings of very high frequency or ultrasonic fields in public areas at times when there were hundreds of people there.

Findings showed the public were exposed to levels over the threshold of the current guidelines (20 kHz): "Individuals who are unlikely to be aware of such exposures are complaining, for themselves and their children, of a number of negative conditions," Leighton said.

"Recent data suggests that one in 20 people aged 40-49 years have hearing thresholds that are at least 20 decibels (dB) more sensitive at 20 kHZ than that of the average 30-39 year old. Moreover, 5% of the five to 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.

"The lack of research means that it is not possible to prove or disprove the public health risk or discomfort. However, it is important that sufferers are able to identify the true cause of their symptoms, whether they result from VHF/US exposure or not."