High-frequency sounds emitted by door sensors, public address systems and various other devices could be contributing to a wide range of health problems, according to a new study.
Dizziness, migraines, fatigue and tinnitus are just some of the health problems that might be triggered by the ultrasound in the air. Though not audible by the human ear, this noise, however, can affect how people feel, said Professor Tim Leighton, from the University of Southampton, who led the study.
Leighton and his team from the University of Southampton’s Institute of Sound and Vibration measured levels of high-frequency ultrasonic fields in public areas, including libraries, railway stations, schools and sport stadiums.
Their data suggested the public was regularly exposed to ultrasound frequencies of 20kHz, which, coincidentally, is currently the threshold for guidelines limiting exposure to ultrasound in workers handling ultrasound-emitting equipment.
"Individuals who are unlikely to be aware of such exposures are complaining, for themselves and their children, of a number of negative conditions," said Professor Leighton, whose findings are reported in the journal Proceedings of the Royal Society A.
"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 20kHZ than that of the average 30-39 year old. Moreover, five per cent of the five to 19-year-old age group is reported to have a 20kHz threshold that is 60dB more sensitive than the median [middle of the range] for the 30-39 year age group."
A frequency of 20kHz is at the highest end of a young person's hearing range. Generally people hear their highest sounds within the 15-18kHz range.
"The lack of research means that it is not possible to prove or disprove the public health risk or discomfort,” Professor Leighton said. “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."
The existing guidelines for occupational exposure, however, are not suitable to govern public or residential exposure to ultrasound, said Professor Leighton, calling for more research and new guidelines to be established.
"The guidelines are 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,” he said. “This warning of inadequacy was lost as regulatory bodies and organisations issued 'new' guidelines based on these early guidelines and through such repetition generated a false impression of consensus."
Many researchers, however, disagree with Leighton’s conclusions.
"The amount of physical energy in these ultrasounds is absolutely tiny and almost none of those tiny amounts of sound energy will penetrate from the air into our bodies,” said Professor Jan Schnupp, from Oxford University. “It will just bounce harmlessly off our skin. Such a phenomenally tiny amount of ultrasound energy from the air will penetrate our bodies that it is quite hard to envisage it could do any serious damage."
According to Bernard Katz, Professor of biophysics at the University College London Ear Institute, the human ear is not able to pick up any airborne sounds above 18kHz.
“If there are indeed real experimentally reproducible physiological effects, then indeed more research needs to be done, but it might be more profitable to look for other mechanisms, for example on the vascular system, before going for this," he said.