Ultrasound in Air: Safety and Regulations

This is a post for references, there's minimal commentary, just links for ultrasound in air background reading, regulatory and safety and a few notes on each. I've previously commented on ultrasound in air and safety considerations across multiple posts, but I wanted to put together a post that will contain relevant information in a single place. I intend this to be a 'living post' and will be updated as more documents/regulations are included, or regulations are updated - so it's a work in progress. If you're knowledgeable in the field and have a reference I should include, please let me know by email or in the comments.

General Reading

Are some people suffering as a result of increasing mass exposure of the public to ultrasound in air? by T.G. Leighton. A great starting point as it covers the history of ultrasound limits in air, details of possible adverse effects, and with recommendations for further study and safety considerations. An extensive list of this author's work, including further research on ultrasound in air, can be found here.

Wireless Power Transfer to Millimeter-Sized Nodes Using Airborne Ultrasound by Angad S. Rekhi, Butrus T. Khuri-Yakub, and Amin Arbabian. Detailed, peer reviewed paper published in Transaction on UFFC in 2017, best paper to date on the topic. Indicates microWatt level power transfer viable.

Damage to human hearing by airborne sound of very high frequency or ultrasonic frequency by B.W. Lawton for the Health and Safety Executive in the UK, a literature and standards survey in 2001.

A Review of Current Ultrasound Exposure Limits, Carl Q. Howard, Colin H. Hansen and Anthony C. Zander. A 2004 study which concludes "Until more definitive data become available, it is recommended that the more conservative standard proposed by the Health Canada [13] and listed in Table 1 be adhered. This means that sound pressure levels should be less than 110dB above 25kHz, regardless of the exposure duration, to prevent the undesirable subjective effects of ultrasound."

Effects of Ultrasonic Noise on the Human Body—A Bibliographic Review Bożena Smagowska. A review of other ultrasound safety papers, with the following quote "According to Allen, Rudnik and Frings, a mouse dies from overheating after 10 s to 3 min of exposure to a signal of 20 kHz and level of 160 dB [10]. According to Danner, a lethal level for signals of 18–20 kHz for an unshaven mouse were 144 dB and for a shaven mouse 155 dB [21]. Acton obtained similar results and extended studies to larger animals such as guinea pigs and rabbits [22]."

Guidelines for the Safe Use of Ultrasound: Part II – Industrial and Commercial Applications by Environmental Health Directorate Health Protection Branch, Canada  Canadian guide to potential harmful effect of ultrasound, with the quotes "In addition, Acton (Ac 74) has reported on unpublished work by Parrack indicating that mild heating in skin clefts has been observed in the SPL range of 140-150 dB...  It is plausible that chronic lengthy exposures to levels between 145 and 155 dB might also be harmful, as they could raise body temperatures to mild fever levels during the exposure periods. However, as indicated in Section 3 of this guideline, such high sound-pressure levels have never been encountered in either commercial or industrial applications... However, in the ultrasonic frequency range, if potential problems due to heating are to be avoided, total linear measured SPL exposure to other parts of the body must never exceed 137 dB. This value is based on the lowest value (140 dB) (see Figure 2) which allegedly has led to mild heating of skin clefts. A safety factor of 3 dB (a factor of 2 in energy) should ensure that no significant heating of a human could occur."

Environmental Health Criteria: Ultrasound. 1982 WHO summary of ultrasound limits and safety studies. 

Regulatory

OSHA and Global dB limits. Covers Occupational Safety and Health considerations, and shows a limit of 115 dB above 40 kHz in the USA for the workplace (not home, consumer etc exposure). No mention of a 145 dB limit. Notes limits around the world are between 110 and 115 dB.

Underwriters Labs (UL) Testing. From UL's website "UL helps companies demonstrate safety, confirm compliance, enhance sustainability, manage transparency, deliver quality and performance, strengthen security, protect brand reputation, build workplace excellence, and advance societal wellbeing". This is a standards document you need to purchase to read in its entirety, the relevant section is 12.5.2, a pic of that section is below. Limits SPL to 110 dB in useable area of most devices.

FDA Requirements for Radiative Devices. Even for non-medical use, any radiation emitting device, including ultrasound, can fall under the jurisdiction of the FDA. An abbreviated report, needs filled out, and cannot be done until there is a product as it requests brand, model number, output levels, frequency, operating conditions etc. Anyone claiming that the FDA does not regulate ultrasound in air devices should be asked to demonstrate why this does not apply. There are also details  under the Code of Federal Regulations Title 21 particularly in section 1002 that states the main exceptions are if the product is only for export or:

"Manufacturers of electronic products listed in table 1 of this section if such product is sold exclusively to other manufacturers for use as components of electronic products to be sold to purchasers"

essentially, if you sell parts to other companies, you don't need to go through the FDA, but whoever sells the final product will.

International Non-Ionizing Radiation Committee of the International Radiation Protection Association. (INIRC-IRPA). 1984. Interim guidelines on the limits of human exposure to airborne ultrasound, and concludes the general public should not be exposed to ultrasound at levels above 100 dB regardless of frequency.

Groups/Societies/Investigative Bodies
Health Effects of Ultrasound in Air (HEFUA). "HEFUA is the UK consortium (encompassing researchers, clinicians, policymakers, and the public) that addresses the fact that humans are being increasingly exposed to ultrasound in air through commercial devices. There is insufficient understanding of how these devices affect health, even when exposures are known." This page also has links to multiple documents on ultrasound in air.

Biological Effects

More specific papers on ultrasound effect on biology

Subharmonic Distortion in Ear Canal Pressure and Intracochlear Pressure and Motion by Stanley Huang, Wei Dong, and Elizabeth S. Olson  which looks at the generation of subharmonics, that is frequencies lower than the transmitted sound, in the eardrum from high intensity ultrasound.

Weaponizing Sound

Acoustic Weapons - A Prospective Assessment by Jürgen Altmanna. A good summary of ultrasound used as a weapon.

WIP