Exposure to airborne ultrasound - high frequency sounds beyond the range of human hearing - from public address systems, loudspeakers and door sensors may be making people ill, a new study has claimed.
Increasing exposure to ultrasound in the air was recorded in railway stations, museums, libraries, schools and sports stadiums in which it is claimed people have complained of nausea, dizziness, migraine, fatigue and tinnitus.
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, researchers said.
While there has been insufficient research to confirm or deny a link, study author Professor Tim Leighton from the University of Southampton in UK said that 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," said Leighton.
Furthermore, the guidelines are based on the average response of small group, often of adult males.
"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 finalise guidelines, but which produced preliminary guidelines," said Leighton.
Using smartphones and tablets equipped with an app that produced a spectrogram of the microphone reading, Leighton collected readings of very high frequency/ultrasonic fields (VHF/US) in a number of public buildings, at a time when they were occupied by hundreds of people.
The findings were then calibrated with two or three independent microphone and audio data systems.
Leighton found that members of the public were exposed to VHF/US levels over 20 kHz, which is the threshold of the current guidelines.
He is now calling for further research and the production of a new set of guidelines based on this research.
"Individuals who are unlikely to be aware of such exposures are complaining, for themselves and their children, of a number of negative conditions. 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," Leighton said.
"Moreover, five per cent 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," he said.
