Ultrasound in public places can be generated from a number of sources including loudspeakers, door sensors and public address systems. (Representational Image)
Increasing exposure to airborne ultrasound in locations such as railway stations, museums, libraries, schools and sports stadiums can lead to nausea, dizziness, migraine, fatigue and tinnitus in people, researchers report.
The research team from University of Southampton has shown that the members of the public are being exposed without their knowledge to airborne ultrasound.
Ultrasound in public places can be generated from a number of sources including loudspeakers, door sensors and public address systems.
Using smart phones and tablets equipped with an app, professor Tim Leighton collected readings of very high frequency/ultrasonic VHF/US fields in several public buildings.
The findings were then calibrated with two or three independent microphone and audio data systems.
Study author Professor Tim Leighton found that members of the public were exposed to VHF/US levels over 20 kHz, which is the threshold of the current guidelines.
"Individuals who are unlikely to be aware of such exposures are complaining, for themselves and their children, of a number of negative conditions," he wrote in a paper published in the journal Proceedings of the Royal Society.
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, five percent of the 5 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 professor said.
According to Leighton, current guidelines and research knowledge for occupational safe levels are inadequate to cope with the current mass exposure of large numbers of people.
"Existing guidelines are insufficient for such large public exposures as the vast majority refer to occupational exposure, where workers are aware of the exposure, can be monitored and can wear protection," he said.
Furthermore, the guidelines are based on the average response of small group, often of adult males.
For a number of years, workers who have been regularly exposed to occupational ultrasound through industrial devices for cleaning and drilling have reported similar negative effects.
"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," Leighton stressed.