Airborne ultrasonic standards for hearing protection , 2008

The American Conference of Governmental Industrial Hygienists (ACGIH) increased the threshold level Values (TLVs) for airborne ultrasound and these levels were adopted by Occupational Safety and Health Administration (OSHA) in 2003. The European community, Australia and Canada have retained the more stringent levels that stemmed from reports of “ultrasonic sickness” in the 1960s. Ultrasound can be solely airborne, or it can be fluid and/or solid-coupled. The substantial impedance mismatch between the air and the body prevents absorption of most of the airborne ultrasound energy. When airborne ultrasound impacts on human skin, less than 0.1 % of the energy is absorbed (Wiernicki & Karoly 1985). Partially on this basis, the ACGIH voted to raise the TLVs by 30 dB; unless solid or liquid coupling is also possible allowing increased transmission through these paths into the body. A source of risk in industrial applications is the unintended transfer of acoustic energy into the human operator through solid or liquid coupling.