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A Review of Current Ultrasound Exposure Limits
Carl Q. Howard, Colin H. Hansen and Anthony C. Zander
School of Mechanical Engineering,
University of Adelaide,
South Australia, 5005, Australia
September 8, 2004
Abstract
Research has indicated that airborne ultrasonic sound impinging on the ear
drums of humans has the potential to cause undesirable effects. The United States
of America’s Occupational Health and Safety Administration have changed their
guidelines to permit an additional 30dB increase in the acceptable ultrasonic am-
plitudes under certain conditions. This paper contains a review of current rec-
ommended acceptable exposure limits from standards organisations around the
world.
1 Introduction
Exposure limits for ultrasonic noise were developed in the late 1960s and one standards
organisation in the United States of America has recently changed their recommended
exposure limits. This paper contains a review of the current recommended noise expo-
sure limits from standards organisations around the world. The limits are set based on
current knowledge, so as to ensure the safety of people from the potential hazards of
exposure to high levels of ultrasonic sound.
Ultrasonic sound is beyond the range of normal human hearing and is usually de-
scribed as sound that has a frequency above 20kHz. There are no reports of hearing
loss due to ultrasound exposure [1, 2], although there is a report of temporary thresh-
old shift in subjects exposed to frequencies of 18kHz at 150dB for about 5 minutes
[1]. Research has shown that airborne ultrasound has the potential to cause nausea,
fatigue, and headaches [3–8].
Recently, devices have become commercially available, which indirectly generate
audible sound by initially generating ultrasound. The details of the method by which
1
audible sound is generated from ultrasound can be found in Refs [9–12]. The potential
use of these devices to act as focussed control sources in an active noise control system
was the impetus to conduct a review of the literature to find current recommendations
for the exposure limits of airborne ultrasound impinging on the human ear.
There is a general consensus amongst standards organisations on the exposure lim-
its for ultrasound. The limits were derived by three independent research groups who
arrived at very similar findings for acceptable exposure limits [5]. The exception to
the general consensus are the guidelines from the US Occupational Health and Safety
Administration (OSHA), which in 2004 voted to adopted the recommendations from
The American Conference of Governmental Industrial Hygienists, which permit an in-
crease in the exposure limits by 30dB under certain conditions [4] that are described
later in this paper.
The general consensus is embodied in a Health Canada report [13], which is based
on the findings from the International Radiation Protection Agency (IRPA) [8], which
provides recommendations to the World Health Organisation, listed in Table 1. These
limits are applicable for continuous occupational exposure to airborne ultrasound. The
IRPA guidelines allow for an increase in the exposure limits if the exposure duration is
less than 4 hours per day; however Health Canada [13] does not support this recom-
mendation and it should be noted that the IRPA recommended limits for continuous
exposure for airborne ultrasound to the public are lower than those shown in Table 1.
Table 1: Recommended ultrasound exposure limits in octave bands.
Frequency Sound Pressure Level
(kHz) (dB re 20µPa)
8 -
10 -
12.5 -
16 -
20 75
25 110
31.5 110
40 110
50 110
2
2 Standards on Ultrasound Exposure
2.1 Effects
There are many medical products that use ultrasound in the mega-Hertz frequency
range for such purposes as imaging, destruction of kidney stones, and others. There is
a great deal of literature available that discusses the occupational risks of using such
equipment and the possible damage that can occur to practioners, patients or foetuses
[14]. This research is not relevant to the current discussion as typically the frequency
range is higher than the frequency range of interest here, the amplitudes are much
greater than proposed here, and the method of conduction is with direct skin contact
travelling through water or the body. The literature review discussed here is focused
on the effects of ultrasound that is less than 50kHz, travels through air, and impinges
on the ear drums.
Parrack found that slight heating of the skin could occur when exposed to sound
pressure levels of 140-150dB at ultrasonic frequencies [15]. Parrack has also calculated
theoretically that a dose of more than 180dB would be lethal to humans [16].
Von Gierke and Nixon [5] have a concise description of the effect of ultrasound;
the effects of “... ultrasonic energy at frequencies above about 17kHz and at levels in
excess of about 70dB may produce adverse subjective effects experienced as fullness in
the ear, fatigue, headache, and malaise”.
The National Occupational Health and Safety Commission’s Annual Safety Report
[3] says that “... there is less evidence on the specific adverse affects of ultrasound.
However, it is becoming a suspect factor in harmful effects on the human genome,
spontaneous abortion, congenital malformation, chromosomal aberrations, and even
cancers.”
Schust [17] wrote that “The human ear may perceive auditory sensations up to
at least 40kHz. In laboratory experiments a temporary threshold shift by ultrasound
could be demonstrated. Some epidemiologic studies point to the fact that an impair-
ment of high-frequency hearing above 8kHz may not be excluded by long-term ultra-
sound exposure”.
The generation of ultrasound is often accompanied by high amplitude sound pres-
3
sure levels of sub-harmonic frequencies in the audible frequency range. In addition,
research has shown that the ear drum vibrates non-linearly and can generate sub-
harmonic vibration when exposed to sound pressure levels in the range from 110-
130dB [18]. The amplitude of the sub-harmonics was the same order as the amplitude
at the fundamental frequency and could possibly damage the ear.
The subjective effects that are attributed to ultrasound are usually caused by the
sound energy in the audible frequency range. When the sound energy in the audible
frequency range is reduced, it is usually accompanied by a reduction in the subjective
symptoms [3]. Criteria have been developed to limit the levels of ultrasound to control
auditory and subjective effects. The criteria were developed from the independent
investigations of three scientists [5], and are discussed in the next section.
2.2 Exposure Limits
Several standards exist that specify acceptable ultrasound exposure limits. The pre-
scribed limits vary between countries and a summary of the exposure limits is shown
in Table 2, which was adapted from a table in a Health Canada report [13].
Table 2: Guidelines for the safe use of ultrasound [13].
Proposed By*
Frequency (kHz) 1 2 3 4 5 6 7
8 90 75 - - - - -
10 90 75 - - 80 - -
12.5 90 75 75 - 80 - -
16 90 75 85 - 80 - 75
20 110 75 110 105 105 75 75
25 110 110 110 110 110 110 110
31.5 110 110 110 115 115 110 110
40 110 110 110 115 115 110 110
50 110 - 110 115 115 110 110
*Legend: 1. Japan (1971); 2. Acton (1975) [19]; 3. USSR (1975); 4. Sweden (1978); 5.
American Conference of Governmental Industrial Hygienists (ACGIH 89) [20] and US
Department of Defense (2004) [21]; 6. International Radiation Protection Agency (IRPA
1984) [8]; 7. Canada (1991) [13].
The International Commission on Non-Ionising Radiation Protection (ICNIRP) is
an independent scientific organisation responsible for providing guidance and advice
4
on the health hazards of non-ionising radiation exposure. ICNIRP provides recom-
mendations to the World Health Organisation (WHO) for ultrasound exposure limits,
and are listed in column 6 of Table 2.
Note that for the data listed in Table 2, some have exposure time limits and oth-
ers do not. The amplitude limits prescribed by Health Canada [13] in column 7 are
independent of time as the subjective effects of high amplitude ultrasound can occur
immediately.
The exposure limits proposed by the American Conference of Governmental Indus-
trial Hygienists (ACGIH) have recently changed the permissible levels, and hence it is
worthwhile highlighting the changes.
“The American Conference of Governmental Industrial Hygienists (ACGIH)
has established permissible ultrasound exposure levels (Table III:5-4). The
latest edition of the ACGIH publication, 1998 Threshold Limit Values (TLV’s)
and Biological Exposure Indices (BEI’s), adopted TLV’s that had been pro-
posed in the 1997 publication as a “nature of intended changes.” There-
fore, Table III:5-4 represents the newly adopted TLV’s for ultrasound. These
recommended limits (set at the middle frequencies of the one-third octave
bands from 10kHz to 50kHz) are designed to prevent possible hearing loss
caused by the subharmonics of the set frequencies rather than the ultrasonic
sound itself.” [4].
Table 3 lists the ultrasound exposure limits that were described in Table III:5-4 from
the above quote. The 1997 reference in the quote refers to a publication of the American
Conference of Governmental Industrial Hygienists 1997, page 81.
The accompanying Note 2 is confusing as the title of the column describes the ultra-
sound exposure limit when the noise is “Measured in Air” and “Head in Air”. How-
ever Note 2 says that “These values assume that human coupling with water or other
substrate exists.” It is unclear how the effects of ultrasound are modified when a person
with their head in the air has contact with water or not.
The second statement implies that the values may be raised by 30dB, and hence
the limits are 145dB, which is 30dB greater than the limits proposed by other countries
5
Table 3: Ultrasound exposure limits from OSHA [4]: Table III:5-4. TLV’s for Ultra-
sound, Notice of Intended Change - Ultrasound.
Measured in Air Measured in Water
in dB re 20µPa in dB re 1µPa
Head in Air Head in Water
Mid-Frequency of Ceiling 8-Hour Ceiling
Third-Octave Values TWA Values
Band (kHz)
10 105188 167
12.5 105189 167
16 105192 167
20 105194 167
25 1102– 172
31.5 1152– 177
40 1152– 177
50 1152– 177
63 1152– 177
80 1152– 177
100 1152– 177
Notes:
1. Subjective annoyance and discomfort may occur in some individuals at levels
between 75 and 105 dB for the frequencies from 10 kHz to 20 kHz, especially
if they are tonal in nature. Hearing protection or engineering controls may be
needed to prevent subjective effects. Tonal sounds in frequencies below 10 kHz
might also need to be reduced to 80 dB.
2. These values assume that human coupling with water or other substrate exists.
These thresholds may be raised by 30 dB when there is no possibility that the
ultrasound can couple with the body by touching water or some other medium.
[When the ultrasound source directly contacts the body, the values in the table do
not apply. The vibration level at the mastoid bone must be used.] Acceleration
Values 15 dB above the reference of 1g RMS should be avoided by reduction of
exposure or isolation of the body from the coupling source. (g = acceleration due
to the force of gravity, 9.80665 meters/second; RMS = root-mean-square.)
6
as listed in Table 2. One would have expected that the exposure limits are based on
measurements at the listener’s ear when their head was in air, and that the limit values
would be unmodified if the person was in contact with water or a substrate. Further
clarification of this table is necessary to determine if the clauses are relevant to the
application where the ultrasound travels through air and the listener is not in contact
with water.
The changes made to OSHA’s exposure limits have also caused concern amongst
hearing conservationists who predict that the changes made to acceptable levels in the
audible frequency range (<20kHz) are likely to cause a substantial increase in the
number of workers in hearing conservation programs [22–24].
Results of tests conducted on two devices which emit ultrasound to generate highly
directional audible sound has found that the ultrasonic noise levels are about 130-
140dB at 1 metre, which exceeds the recommendations described in Table 2. One man-
ufacturer has published data to show that their product generates ultrasonic sound
pressure levels around 130dB [25]. Another manufacturer of the ultrasonic devices
writes in a whitepaper [26] that the ultrasound “... thresholds may be raised by 30dB
when there is no possibility that the ultrasound can couple with the body by touching
water or some other medium,” to claim that their device is safe for humans.
3 Conclusion
The United States of America’s OSHA recommendations appear to be inconsistent
with the ultrasound exposure limits proposed by other countries. In 2004 OHSA have
increased the permissible levels, under certain conditions, and their justification is un-
clear.
Until more definitve data become available, it is recommended that the more con-
servative standard proposed by the Health Canada [13] and listed in Table 1 be ad-
hered. 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.
7
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