All signals picked up by the auditory system are transported via the nervous system:
directly, via specific pathways, which link the inner ear with the auditory cortex which takes in the signal and recognises its significance;
or via an indirect route, a non-specific pathway; (these are collaterals of the direct pathways) to reach the reticular activating system (which regulates arousal), which is in turn connected to the limbic system and to other parts of the brain, to the autonomic nervous system and the neuroendocrine system, which play crucial roles in regulation of physiological functions in attention and behaviour.
This explains why an irritant noise, even if it is of low intensity (generally from 60 dB), by introducing a subjective dimension, can cause psychological harm and other problems (a stress reaction) which are not directly or solely linked to the physical properties of the noise. The level of harm to an individual is not fully correlated with the level of noise; there is, however, a correlation between harm to an entire population and noise levels. Noise therefore belongs in the category of environmental stressors. This kind of stress is experienced all the more because the subject has little or no control over the source of the stressor.
Sleep and alertness problems
On board non-soundproofed ships, the greatest risk that is not related to hearing is that sleep is disturbed by noise. A seafarer lives 24 hours a day in the confined environment of the ship, and should experience the good-quality sleep that is essential if the body is to recover from fatigue and maintain proper biological functions.
Slow-wave sleep is involved in repair of tissues that are involved in physical effort.
Rapid eye movement (REM) sleep restores the higher functions of the nervous system (alertness, learning, memory, adaptiveness and intent).
However, noise above 60 dB(A) causes sleep problems in the form of reduced total amount of sleep, reduced duration of REM sleep, and increased occurrence of night-time waking. This sleep disturbance leads to increased fatigue and irritability. The problems are cumulative, and a vicious circle can develop whereby serious sleep dysfunction can arise, leading to physical exhaustion and overwork. Such noise is found very frequently on all types of ships. It is therefore reasonable to think that seafarers in general suffer from sleep problems which worsen general fatigue.
Tamura (16) et al.studied sleep patterns in three subjects who were exposed to 65 dB noise from a ship’s diesel engine for five nights, and their sleep in such conditions was compared to their sleep in a quiet environment. They found that the number of episodes of REM sleep, and the duration of these episodes, were reduced, and that the time between these episodes of REM sleep was increased. They also reported a reduction in subjective sleep quality and difficulties in falling asleep. At noise levels of around 60 dB, the same authors (17) observed that, although seafarers became habituated to such noise levels in terms of subjective sleep parameters, there were still disturbances in the physiological parameters, as measured using actigraphy.
Rabat et al.(18) carried out a sleep study on rats who were exposed to a recording of warship noise for 9 days, and compared the results to those of rats sleeping in a quiet environment. They confirmed that normal sleep structure was distorted, leading to a ten-hour debt of slow-wave sleep (the number of episodes of deep sleep was increased, but the duration was shorter than normal) and also, like Tamura et al. (13) found a six-hour debt of REM sleep (the number of episodes of REM sleep was reduced, as was their duration). The consequences of such sleep disturbance were significant, and appeared after the noise stopped; they involved the ability to commit information to long-term memory, the extent of the memory problems being positively correlated to the extent of the debt of slow-wave sleep. They also demonstrated two types of sleep-related behaviour: one group of rats was resistant, and rats in this group rapidly recovered their ability to memorise, and one group of rats was vulnerable, and had significant problems. These results strengthen the theory that there are differences between individuals in terms of noise sensitivity.
Tirilly (19) studied sleep patterns in coastal sea fishermen and demonstrated the importance of sleep at night, which can be short in duration but which must occur at the same time each day for a given individual (this is known as “Anchor sleep” and maintains biological rhythms). In this study, the mean level of alertness in seafarers fell very soon after leaving port and the sleep deficit was between 60-90 minutes/24 hours, caused by fragmentation of sleep: on average six episodes of sleep were observed, with a total daily sleep period of between 5.5 and 6.5 hours.
Alertness may be defined as maintaining attention during activities requiring prolonged periods on watch (particularly gangway watch). Alertness is reduced in proportion to the intensity of the noise, and this can result in attention problems. Noise also increases the risk of human error (20). Particularly in fishing, this can be a non-negligible cause of accidents.
Intellectual performance seems to be reduced if noise is above 85 dB, in terms of psychomotor ability, reasoning and capacity to commit to memory, which is linked, as we have already seen, to sleep problems. At sound levels above 80 dB, there may also be effects on intellectual capacity, but this depends on the frequency of the noise, whether it is intermittent or not, how long it lasts and its significance. Poulton21 observed that people working in constant noise initially performed better than those in quiet environments, but that there was gradual deterioration in performance if the noise persisted. Poulton thought that the physical intensity of the noise masked the signals produced by the machines, which were used by the operator as a guide to performance when the environment was quiet. When the signals were masked, performance levels worsened. When the noise began, the stressor seemed to cause a sudden burst of physiological and behavioural stimulation which overcame the harmful masking effects of the noise. This effect, however, gradually loses its impact, and subsequently there is an inescapable loss of performance. Another explanation might be that processing of noise using cortical filtering might impose an additional workload on this central brain structure. The capacity devoted to this task would be unavailable for other tasks, which would cause a reduction in the ability to reason and process information.
These problems could cause errors of judgement on board ship, which in some cases could have dramatic consequences: these could include failure properly to understand orders when undertaking difficult manoeuvres, a risk of damage to machinery through negligence caused by reduced judgement or abnormal levels of fatigue.
Noise-induced cardiovascular problems
It is generally agreed that noise causes generalised vasoconstriction. This vasoconstriction persists as long as noise exposure continues (22).
Although this phenomenon has been discussed frequently (23,24), the problem of the link between noise and blood pressure problems has been the subject of many studies. Despite the fact that the methodology of many of these studies has been criticised, 80% of the studies suggest the existence of such a link.
An increase in blood pressure is found in those exposed to noisy conditions and the duration of the increase was correlated to length of exposure to the stressor. The increase depends not only on the level of sound but also on many other factors in the work environment such as the type of work and the category of staff (25, 26;27).
In addition, it has been demonstrated that employees with work-related hearing loss have significantly higher diastolic blood pressure than a control population with no hearing problems (28; 29).
This increase in hypertension is a logical consequence of this latter observation. The link between noise and hypertension was first suspected after it was noted that use of anti-hypertensive drugs was greater in areas near airports than it was in quieter areas (30). Many subsequent studies have confirmed this link (31, 26, 32, 33,34,35,36). There have been several studies of shipping, with similar results (37, 38 ,37 has shown that levels of hypertension are significantly higher among engineers aged over 40 on board merchant ships (18.90%, N=164) than they are among non-engineer personnel of the same age (N=291) of whom 11.68% were hypertensive. This difference was not found in younger subjects. Levels of hypertension in the engineer group were independent of other risk factors such as family history of hypertension, obesity and alcoholism. The relative risk of hypertension due to noise has been calculated at 1.62. This is similar to results of other studies (39). Roodenko et al. (40) also found, in a recent study, increased levels of hypertension in engineers when compared to deck crew and catering personnel.
Fig 4: Rates of hypertension in engineers and non-engineers in the merchant
The difference between engineers and non-engineers aged between 40 and 55 is significant (p=0.05).
Noise can also be responsible for myocardial infarction (41).
Effects on vision
Subjects who are regularly exposed to noise experience a reduction in nocturnal visual acuity and difficulties with depth perception, associated with a narrowing of the visual field. This narrowing can be as much as 10° at the red end of the spectrum. These abnormalities can be very troublesome when on gangway watch at night (when night vision is needed, and when the ambient lighting is red) but usually only occur if noise levels are above 100 dB. Noise-induced stress seems to reduce dopamine synthesis; dopamine is a neurotransmitter that is used by the retina.
Effects on the endocrine system
Stress caused by noise (60 dB or above) causes the same type of endocrine problem that is seen in all types of stress (catecholamines and cortisol). A recent study (42) has shown that three days of noise exposure appears to cause significant increases in corticosteroid and adrenaline levels. An effect on immune functions has also been observed, with an increase in oxidative stress.
Indirect noise-related effects
Noise limits people’s ability to communicate, and contributes to isolation which is already a significant phenomenon on board merchant ships. Intelligibility of a conversation reduces in proportion with the increase in background noise and the distance between the interlocutors. At a distance of one metre, communication is only possible if the noise level is lower than 75 dB.
A high level of noise may mask a warning or alarm indicating danger, or may lead to incorrect interpretation of instructions. Noise may be a direct cause of accidents. In 1955, Sir Lionel Heald confirmed that “men who worked on the flight decks and were exposed to tremendous noise from aircraft and ventilating machinery became extremely careless, got into the way of the planes, fell over things and got themselves injured.”* According to Poulton (20), noise has a masking effect on non-intentional auditory signals and on the “internal monologue” that each individual uses to overcome deficiencies in short-term memory. More generally, it is possible that noise, by masking a whole range of auditory signals that are characteristic of an environment, creates an impression of isolation, which leads to lack of attention and negligence. It is therefore essential, when choosing alarm equipment, to check that the acoustic power and frequency of the signal (low-frequency sounds mask high-frequency sounds) are adequate for the planned area of use.
In addition, it has been demonstrated (43, 44) that ambient noise increases the risk of accidents, particularly in subjects with hearing loss.
The problem of multiple stressors
Noise is just one of many stressors that affect personnel on board ships. Among others, there is also vibration, and heat in some cases. The question arises as to whether these stressors interact with each other. This area is very complex, and little is known about it. In the maritime field, which is greatly affected by this problem, the scientific literature is particularly sparse.
Some studies (45, 46, 47 ) suggest that whole-body vibrations play a role in the aetiology of noise-induced hearing problems. Effects on low frequencies seem to be increased if there is combined exposure to noise and vibration. Okada (48) and Manninen (49) considered that, while vibrations play a role in noise-induced hearing problems, this would only be of the order of 5 dB at TTS² (auditory fatigue, measured 2 minutes after exposure). Pyykko (50) indicates that whole-body vibrations of between 2 and 10 Hz at 10 ms-2 seem to increase auditory fatigue (TTS) when noise levels are above 90 dBA.
Several authors have attempted to show that there is an antagonist interaction between noise and heat. However, it seems as though the effects of associated noise and heat may be synergistic, antagonistic or negligible, depending on the intensity of the stressors, type of work and length of exposure. According to Pekkarinen (33), high temperatures increase auditory fatigue in the presence of vibration.
It has recently been established that there is a synergistic effect between exposure to noise and exposure to various solvents (toluene, styrene, xylene and trichloroethylene in particular) as well as carbon monoxide, which increases ototoxicity and therefore also hearing loss. (51, 52, 44).
Smoking can increase the ototoxicity of noise (53). However, Bur (54) finds that smoking is an independent risk factor for incidence of hearing loss.