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suggested physiological mechanism is an autonomic response to a perceived threat, leading to increased release of stress hormones and other stress-related responses (Murphy, 2017). It is plausible that exposure to an unacceptable sound could lead to anxiety and stress, particularly if the source is unidentified and its existence is disputed by other people (Leighton, 2018). However, to date, few epidemiological studies have been conducted on the prevalence of adverse effects in people exposed to ultrasound. The choice of population studied is impor- tant because we know the effect will be on a minority of the general population. Still, for public exposures, this might constitute millions of people, and so we do not wish the data from this minority to be lost through aver- aging (Leighton, 2016a, 2017; Leighton et al., 2019).
Some members of the public have anecdotally reported adverse symptoms that they attributed to ultrasound even though the sound was inaudible and identified only by seeing a sound source and detecting ultrasound using a smartphone. We therefore tested whether adverse effects could be provoked by inaudible ultrasound in a double- blind study (Fletcher et al., 2018b). Two groups of subjects were recruited: those who self-reported previously find- ing VHFS sounds aversive and those who did not. Each subject’s HTL was measured at 20 kHz, and the SPL of the ultrasound was set at least 15 dB below this to ensure that the ultrasound remained inaudible and hence was not immediately distinguishable from a sham exposure condi- tion (e.g., no ultrasound). SPLs ranged from 57 to 88 dB (the ethical upper limit) depending on the subject’s HTL. Subjects then experienced both genuine and sham expo- sures, each being presented continuously for 20 minutes.
No adverse effects of ultrasound were found compared with sham exposure in either subject group. Neither were the subjects able to correctly distinguish genuine from sham ultrasound after a 20-minute period. This does not mean that adverse effects are impossible, but rather that they could not be provoked in this participant sample and at the amplitudes, frequencies, and durations that we were ethically permitted to use. A longer term sin- gle-blind study of inaudible ultrasound also found no evidence of symptoms of genuine compared with sham ultrasound exposure (EARS II, 2019). Both studies found some evidence of nocebo effects (an adverse response arising from the subject’s erroneous belief that they are being exposed; Fletcher et al., 2018b; EARS II, 2019).
Claims of Attacks on United
States Embassies
We have not had access to the raw data on the claims of ultrasonic attacks on embassies in Cuba and China. Consequently, we could only make an assessment based on the information published in the media and in peer- reviewed journals (Leighton, 2018; Swanson et al., 2018). We concluded that the evidence suggests that it is unlikely that ultrasound or VHFS exposure (whether perceptible or imperceptible) caused the ill effects that were reported in brain injury tests some 200 days later (Leighton, 2018).
It would be easy to hide a pest-scarer in a room that could disturb a minority of the population in that room only. However, the embassy staff reportedly targeted would likely be robust against such attacks because they were typically over 30 years old. Those susceptible would most likely be children, and it would therefore make an odd choice for targeting an embassy. It would be technically far more chal- lenging to project ultrasound over long distances or through a substantial wall. The science underpinning the neurologi- cal damage reported by the mass media has been challenged (Della Sala and Cubelli, 2018; Leighton, 2018).
Conclusions: What Do We Think Is Really Going On?
At the levels of high-frequency sound to which the public are currently exposed, most people will be unaffected, although a minority will be too disturbed to complete tasks and (pos- sibly) a small number will experience headaches, nausea, tinnitus, and other problems. The effects are likely to dis- appear if the exposure ceases. That is not to underplay the significance because these are avoidable exposures that can be unpleasant to a minority and detrimental to the quality of life if they persist. Although most people will feel no effect at those levels, if the site is a railway station or a mall, the minority who are affected may number thousands each day, some of whom might work there for several hours.
It is important to consider the mechanisms through which headaches and other symptoms might arise. The evidence remains inconclusive. However, we suggest two potential causes. The first is a stress response to audible VHFS or ultrasonic stimuli, which appear to be particu- larly intrusive (at least for some people).
The second is anxiety arising from the sufferer’s emo- tional or cognitive response either to audible ultrasound
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