Ultrasound in air: Today's guidelines have an insufficiently solid basis for today's exposures

Today we see a proliferation of technology that purports to use the low ultrasonic frequency range (~20-40 kHz) in air for a wide variety of purposes: pest deterrents; through-air electrical charging; haptic feedback; acoustic spotlights; etc. These exposures are in addition to the inadvertent exposures of humans to ultrasound in air from cleaning baths, dental treatments etc. that have occurred for decades. New forms of exposure might possibly occur in future as more technology is introduced into homes, workplaces and classrooms. Whilst the vast majority of humans have not reported ill effects from this, some have, although there have not been the resources for widespread testing of the validity of these claims. However the dozens of national and international guidelines for such exposures are not currently adequate for the task of offering guidance for public exposures, since they are based on a very sparse dataset (of observations of primarily adult males), and all but one are for occupational exposure, and so cannot cover the un-monitored exposures of, say, infants taken into public location by adults who potentially have different susceptibilities to possible adverse effects.