Report for Defra by Dr Geoff Leventhall
Assisted by Dr Peter Pelmear and Dr Stephen Benton
Dr Geoff Leventhall
Consultant in Noise, Vibration and Acoustics
150 Craddocks Avenue,
Ashtead, Surrey, KT21 1NL
Tel: 01372 272 682
Fax: 01372 273 406
Department for Environment, Food and Rural Affairs (DEFRA)
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Workers exposed to simulated industrial infrasound of 5 and 10Hz and levels of 100 and 135dB for 15 minutes reported feelings of fatigue, apathy and depression, pressure in the ears, loss of concentration, drowsiness, and vibration of internal organs. In addition, effects were found in the CNS, cardiovascular and respiratory systems (Karpova et al., 1970). In contrast, a study of drivers of long distance transport trucks exposed to infrasound at 115 dB found no statistically significant incidence of such symptoms (e.g. fatigue, subdued sensation, abdominal symptoms, and hypertension (Kawano et al., 1991).
Danielson and Landstrom (Danielsson and Landstrom, 1985) exposed twenty healthy male volunteers to infrasound in a pressure chamber and the effects on blood pressure, pulse rate and serum cortisol levels of acute infrasonic stimulation were studied. Varying frequencies (6, 12, 16Hz) and sound pressure levels (95, 110, 125dB) were tested. Significantly increased diastolic and decreased systolic blood pressures were recorded without any rise in pulse rate. The increase in blood pressure reached a maximal mean of about 8 mm Hg after 30 minutes exposure. Lidstrom (Lidstrom, 1978) found that long-term exposure of active aircraft pilots to infrasound of 14 or 16Hz at 125dB produced the same changes. Additional findings in the pilots were decreased alertness, faster decrease in the electrical resistance of the skin compared to unexposed individuals, and alteration of hearing threshold and time perception.
In several experiments to assess cognitive performance during exposure to infrasound (7 Hz tones at 125, 132, and 142dB plus ambient noise or a low frequency noise up to 30 minutes), no reduction in performance was observed in the subjects (Harris and Johnson, 1978). Sole exposure to infrasound at 10 to 15Hz and 130 to 135dB for 30 minutes also did not produce changes in autonomic nervous function (Taenaka, 1989). The ability of infrasound (5 and 16Hz at 95dB for five minutes) to alter body sway responses suggested effects on inner ear function and balance (Tagikawa et al., 1988).
To study vestibular effects in humans, both a rail-balancing task and direct nystagmus (involuntary eye movements) measurements have been used. In the balancing task subjects were required to balance on narrow rails while being presented with various acoustic stimuli. The task results indicated that humans were affected in the audible range as low as 95dB. For frequencies of 0.6, 1.6, 2.4, 7 and 12Hz, aural stimulation at levels as high as 14 dB, either monaural or bilateral, did not significantly affect rail-task performance (Harris, 1976; von Gierke, 1973). However, Evans (Evans and Tempest, 1972) examining the effect of infrasonic environments on human behaviour found that 30% of normal subjects exposed to tones of 2 ñ 10Hz through earphones at SPLs of 120 ñ 150Hz had nystagmus within 60 seconds of exposure to the 120dB signal, with 7Hz being most effective in causing it. Higher intensities resulted in faster onset of nystagmus, but there were no complaints of discomfort from any of the subjects at any SPL. Subsequently, Johnson (Johnson, 1975), who investigated nystagmus in many experiments under different conditions with aural infrasound stimulations from 142 to 155dB had negative results. For example, an investigator stood on one leg with his eyes closed, listening aurally to 165dB at 7Hz and 172dB at 1 to 8Hz (frequency sweep) without effect.
Research on the effect of infrasound on mental performance has also shown negative results. For example, infrasound at 125dB (7Hz) did not significantly affect subjectsí ability to perform a serial search, a mental task requiring searching and linking pairs of numbers together into a progression (Harris and Johnson, 1978). Because of the lack of CNS effects in controlled studies, the reports of fatigue, drowsiness, or sleepiness have generally been discounted as unimportant. ACGIH believes these are the consequence of the simple relaxation effects of infrasound rather then any adverse health effect (ACGIH, 2001).
Although the effects of lower intensities are difficult to establish for methodological reasons, evidence suggests that a number of adverse effects of noise in general arise from exposure to low frequency noise: loudness judgements and annoyance reactions are sometimes reported to be greater for low frequency noise than other noises for equal sound pressure level; annoyance is exacerbated by rattle or vibration induced by low frequency noise; and speech intelligibility may be reduced more by low frequency noise than other noises except those in the frequency range of speech itself, because of the upward spread of masking. Intense low frequency noise appears to produce clear symptoms including respiratory impairment and aural pain. On the other hand it is also possible that low frequency noise provides some protection against the effects of simultaneous higher frequency noise on hearing (Berglund et al., 1996).