Guidelines for the Safe Use of Ultrasound:
Part II - Industrial & Commercial Applications - Safety Code 24

4. Guidelines for Safe Use

4.1 Contact Ultrasound

Contact exposure to high-power ultrasound must be avoided at all times.

The following steps are recommended to ensure the safe use of high-power ultrasound, which is generally employed in order to bring about a permanent physical change in a system:

1. Limited occupancy -
   Only operators qualified to use high-power ultrasound equipment or persons under strict supervision should be allowed within the boundaries of the controlled area while the equipment is operating.
2. Responsibility of personnel -
   Personnel using high-power ultrasound, and safety inspectors in industry, should be knowledgeable about the possible harmful effects of ultrasound and necessary protective measures.
3. Warning signs -
   The warning sign for ultrasound radiation recommended for use is shown in Figure 3. Warning signs should be placed at the entrance to any area which contains high power ultrasound equipment or applied to each high power ultrasound device. Accompanying each warning sign there should also be a statement indicating the precautionary measures to be taken while the ultrasound power is on.
4. Ultrasonic cleaning tanks -
   Labels should be placed on all ultrasonic cleaning tanks cautioning nearby personnel not to immerse hands or other parts of the body in the tank while it is operating.

Figure 3.
Ultrasound Radiation Warning Sign.

ULTRASOUND

DANGER

For large cleaning baths, which cannot be easily turned off when objects are removed or introduced, objects can be suspended from hooks or in baskets. If these protective measures cannot be incorporated and immersion of hands and arms is required, then, as a last resort, gloves lined with flock or fur should be used. They can provide adequate protection since the trapped air layer provides a sufficient acoustic impedance mismatch to reduce the ultrasonic energy transmitted to the body. However, the gloves must be long enough and in good condition (Ac 77), as loss of this air layer would destroy the protection.

5. Other high-power ultrasound devices -
Any high-power ultrasound device which might cause a person to receive contact exposure to the ultrasound should carry a label specifying that the device, or a certain part of it, should not be touched while it is operating. Where required, solid sources of ultrasound can be manipulated with suitable tools.
The levels of low-power ultrasound as used in non-destructive testing are such that, in general, there is little chance of harm occurring from brief accidental contact exposure. However, since biological effects data are still inconclusive, unnecessary contact exposure should be avoided.

4.2 Airborne Ultrasound

4.2.1 Human Exposure Limits

Based on slightly differing interpretations and analyses of the biological effects studies described in Section 2, a number of similar human exposure limits have been recommended in several different countries for upper sonic and ultrasonic radiation. These are summarized in Table 4 (Ma 85).

Two other, quite different criteria have also appeared. The U.S. Air Force (USAF 76) retains an 85 dB limit for each 1/3-octave band from 12.5 to 40 kHz. Based on comparisons to some of Skillern's and Acton's spectra giving rise to subjective effects, these limits may be too high in the 16 and 20 kHz bands to adequately protect against subjective effects and unnecessarily restrictive in the 25 to 40 kHz bands. The other guideline has been presented by auf der Maur in his review article (Ma 85). This proposal, which is based on the AU weighted filter suggested by Herbertz (He 84), has apparently been under consideration by several organizations. However, the high levels it allows, such as 105 dB in the 16 kHz band, 120 dB in the 20 kHz band and 140 dB for the 31.5 kHz band, appear to be excessively high when seen in relation to the data on subjective effects. In addition, the levels permitted in the 16 and 20 kHz bands by this guideline are greater than or equal to those for which Grigor'eva and Dobroserdov appear to have observed TTS in their exposed subjects (Gr 66, Do 67). The value of 140 dB is at the lower limit of the range in which observations of mild heating of skin clefts have been reported (Ac 74).

At present, Acton's modified 1/3-octave criterion appears to be the most suitable as a basis for exposure limits. It is based on the most detailed published comparison available of 1/3-octave band spectra and the presence of subjective effects.

The Canadian recommended exposure limits are given in Table 5. These are given in 1/3-octave bands from 16 kHz to 50 kHz. The exposure limits are independent of time as subjective effects can occur almost immediately (IRPA 84). Below the 16 kHz 1/3-octave band, the sound must clearly be classified as audible and should be assessed through criteria given in the appropriate existing federal, provincial or municipal noise legislation. The large step between the 1/3-octave 20 kHz band and the 1/3-octave 25 kHz band is based on the empirical analysis of industrial exposure by Acton. It is qualitatively consistent with the steep rise in the average threshold of hearing measured by Herbertz and Grunter (He 81, He 84), from approximately 75 dB at 17.5 kHz to 115 dB at 22.5 kHz.

Table 4. Examples of Occupational Exposure SPL Limits (in dB). The SPLs are for 1/3-octave bands.

Frequency kHz	Proposed By*
	1	2	3	4	5	6
8	90	75	-	-	-	-
10	90	75	-	-	80	-
12.5	90	75	75	-	80	-
16	90	75	85	-	80	-
20	110	75	110	105	105	75
25	110	110	110	110	110	110
31.5	110	110	110	115	115	110
40	110	110	110	115	115	110
50	110	-	110	115	115	110

*Legend:

1. Japan (1971)
2. Acton (1975)
3. USSR (1975)
4. Sweden (1978)
5. American Conference of Governmental Industrial Hygienists (ACGIH 89)
6. International Radiation Protection Agency (IRPA 84)

Adapted from (Ma 85)

Table 5. Canadian Exposure Guidelines for Airborne Ultrasound. The SPLs, for 1/3-octave bands, are independent of time of exposure as subjective effects can occur immediately

Frequency (kHz)	SPL(dB)
16	75
20	75
25	110
31.5	110
40	110
50	110

The limits recommended in this guideline may be exceeded for occupational exposure, if reduction of the SPL via engineering controls is not possible (see 4.2.2. below) and workers are provided with ear protectors that reduce ultrasound levels at their ears to the sound pressure levels given in Table 5. 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.

4.2.2. Protective Measures for Airborne Ultrasound Exposure

Safety procedures for the protection of personnel are similar to those used for audible noise. The objective is to ensure that ambient sound pressure levels do not exceed the recommended maximum permissible exposure level. This is achieved first by measuring the SPL (see Appendix 1 for measurement techniques), and reducing exposure levels where required, preferably with engineering controls, reducing sound at its source or in its path by installing sound-absorbing material and containment baffles. If engineering controls are not possible, then ear protection should be used.

Protective measures are considerably simpler and more economical in the ultrasonic range than in the audible range since the higher ultrasonic frequencies tend to be much more readily absorbed and reflected inward by enclosures. In addition, they are less liable to diffraction through orifices. Gold et al. (Go 84) reported that cardboard 3 mm thick was adequate to reduce SPLs by 70 dB between 20 and 50 kHz. Crabtree and Forshaw (Cr 77) constructed relatively simple enclosures for several ultrasonic cleaners which had yielded airborne ultrasound SPLs greater than Acton's criterion. One enclosure, made of 3/4" plywood, lined with one-inch styrofoam and fitted with a top lid and front panel hinged with piano hinges, was effective in attenuating the SPL from 85 to 55 dB in the 16 kHz 1/3-octave band. Rooms constructed for large cleaners in a hangar were made with simple construction materials such as 1/2" gypsum board and 1/4" plywood. Sealing of the doors was entirely unnecessary and attenuation was at least 40 dB. Acton (Ac 67) described simple protective measures for a bank of ultrasonic cleaners. The enclosure was constructed from polyvinyl chloride and Perspex, with stainless steel runners on the outside of the enclosure for the doors. (These materials were used because they were resistant to the corrosive fumes generated during the washing process.) Although the enclosure was far from perfectly sealed, Acton and Carson (Ac 67) were able to achieve attenuation of approximately 10 dB at 20 kHz and 15 dB at 40 kHz with this shielding, enough to eliminate the subjective effects.

In those cases where engineering controls are not feasible, reduction of sound at the receiver by ear protection for ultrasonic frequencies is simple and effective. The protection for ultrasonic frequencies is expected to be at least 14 dB for ear muffs and rubber ear plugs, and 24 dB for foa m ear plugs (Ac 83).