Textbook of Maritime Medicine

Noise is a significant stressor on board ships. The fact that facilities are located above the propulsion mechanism, increases in engine power and the emergence of significant vibration all mean that noise reduction has become a matter as much of crew health as it is of onboard comfort.

11.5.1   Main noise sources on board ships

Many authors have studied the problem of noise onboard ships, ever since ships first began to be mechanically propelled (1, 2, 3, 4, 5).

The main noise sources are as follows:

Engines

The vast majority of ships are propelled by diesel internal combustion engines. Based on the revolutions/minute of the engine, a distinction can be made between “slow” engines with a relatively low noise level and "high-speed" or "medium-speed" engines, which are more powerful than other types of engine but which create more noise.      

At equal power levels, airborne noise produced by these engines is proportional to speed of rotation and maximum combustion pressure. Noise is produced by the scavenger and exhaust housing, as well as by the gear case. As well as noise from the combustion process, there is noise created by turbo blowers (this is high-frequency noise). In addition to noise created by the engine itself, we should take into account the noise transmitted via combustion gas exhaust pipes (funnels).

Apart from noise generated by the main engine, there is also noise from secondary engines, such as electricity generators, reducers, ancillary machinery (e.g. winches, hydraulic motors). Mounting an engine or auxiliary motor on silencers does not affect the amount of noise it produces, but can reduce the level of vibration which is transmitted to the ship’s structure and by extension the noise from the acoustic radiation thus produced.

The largest ships (gross tonnage greater than 60,000), particularly oil tankers, are equipped with steam turbines. In general, steam turbines are much less noisy than internal combustion engines, for equal levels of power production. However, steam valves can cause loud noise, particularly at high frequency, when they are open and/or unsophisticated in shape.

In the future there will be electric motors, which cause considerably less noise than any other type of propulsion system. The idea of generalised electrification of ships began in the United States in the early 1980s and was called Integrated Electric Drive. The term electric ship is ambiguous, and does not imply that diesel engines and gas turbines will disappear, at least not for another 20 years. The term denotes an integrated system of electrical energy production and distribution to all users on board. One remarkable consequence of this is that it becomes possible to remove the drive shaft, which are large parts that entail constraints both in design (installation) and use (alignment, watertightness, noise and vibration). Moreover, a naval architect is more able to optimise equipment positioning, for example by placing the gas turbine away from the bottom of the ship and by choosing the positions of the diesel engines wisely. Electrification also enables removal of various fluid systems that are associated with conventional types of architecture. The principle of electrification has already been adopted in civil shipbuilding, particularly of passenger ships, because of the increased comfort it provides: low noise levels, no vibration. A recent example is the “Star Princess”, a cruise ship carrying 1700 passengers built by Chantiers de l’Atlantique (France) and equipped with a diesel-electric propulsion system. Independent propulsion pods have also been developed, which enable energy savings of around 10%. These external pods, which contain electric motors and drive shafts, are positionable, which means that ships equipped with such pods (suspended omnidirectional propulsion system) no longer need rudder blades. The Queen Mary II¸ the largest cruise ship in the world which recently emerged from Chantiers de l’Atlantique, has six of these pods.

Propellers

Noise emitted by a propeller is linked to turbulence created by the phenomenon known as “cavitation” (because of the air bubbles that form on the propeller blades) and by the characteristics of the blades themselves (number, type, surface). Propellers are one of the main sources of noise emitted by the ship, and the noise is particularly obvious and can enable the ship to be identified (Husson6).

On board high-speed ships, water jets created by gas turbines replace propellers, which means that a considerable amount of weight is saved and levels of noise pollution fall.

Ventilation

Noise produced by a ventilation system mainly comes from the ventilators and their drive motors and shafts, and is caused by their shape and circulation speed, and air intake and discharge vents.

11.5.2   Influence of noise sources

Noise generated by engines and ancillary devices tends to spread throughout the ship.

The level of noise in engine rooms mainly comes from the various engines that are housed there. The overall noise level in one location is the sum of the acoustic intensities at that location, caused by each engine in the location, and to which is added any influence of sound reverberation on the walls. In a generally reverberant engine room, as a first approximation it is reasonable to consider that the noise level is the same throughout the room, unless one is next to a particularly noisy engine (less than 2 metres away).

In common areas, most noise is transmitted via partitions, floors and ceilings. Ventilation systems and doors, furniture and partitions that are subject to deformation can have an influence over the level of noise in a particular place by generating parasitic noise. Noise that is transmitted by partitions, floors and ceilings mainly originates from vibration energy produced by the propulsion system and propeller, but also comes from impact and movement of the ship caused by sea conditions. Appliances on tables or fixed to walls are also sources of disruptive noise.

Noise transmitted by the structure in question is reduced in proportion to the distance from the source of excitation and in inverse proportion to the size and transmission coefficient of the surface.

Apart from noise transmitted by the structure, there can also be airborne noise caused by exhaust systems of motors, ventilators and appliances such as hydraulic generators, steam valves etc.

The noise level inside a gangway is often higher than the level measured inside the accommodation. This is generally due to airborne noise from internal combustion engine exhausts, ventilation systems and some ancillary systems such as hydraulic cargo systems, lift machinery and the wind. Some equipment that is located inside gangways (e.g. VHF, BLU) is also a source of noise. In terms of noise from gas exhausts, the position of the upper part of the funnel with respect to the gangway determines the level of noise in the gangway. The sound spectrum of exhaust noise is mostly low-frequency, so glass partitions in the wheelhouse should not be relied upon to provide sound isolation that is sufficient to reduce noise levels noticeably. If ventilation system casings are nearby, this is another very troublesome source of external noise. Ventilator noise is loud and can sometimes reach 120 dB(A). This noise is transmitted directly to the outside via slats, and these slats can cause troublesome noise if air travels through them at high speeds. In addition, on some ships, some ancillary systems such as air conditioning units are found near the gangway. Finally, if the wind is high and reaches speeds of approximately 60 km/h (force 8) with respect to the ship, there can be whistling in the handrails and hoist halyards. In addition, because of the high drag coefficient of the wheelhouse, the wind’s effects on the wheelhouse lead to significant levels of background noise.

11.5.3   Noise levels on board ships

As we have seen, the main source of noise is the propulsion mechanism, and therefore the highest levels of noise are found in its vicinity. In most ships, the noise is greater than 100 dB(A), and can sometimes be as high as 110 dB(A).

Table 11.5.1: Mean noise levels in various types of engine room

In other locations, however, noise levels are generally between 60 and 75 dB(A). Technological progress has ensured that on passenger ships, particularly cruise ships, cabin noise levels are around 40 dB(A).

Fishing vessels are generally smaller than merchant ships, and fishermen spend much longer on board over the course of a year than merchant seafarers do. These vessels pose more acute noise problems. The above noise sources on merchant ships are obviously also present on fishing vessels. In addition, there is the noise of winches used for launch and for lifting fishing equipment. We have reproduced below the noise levels found on board various fishing vessels of the same tonnage:

Table 11.5.2: Noise levels in different compartments of fishing vessels

Noise in engine rooms exceeds 105 dB and is perceptually equivalent to levels found on board merchant ships of any size. The difference between merchant ships and fishing vessels is that there is not always a soundproofed control cabin on smaller fishing vessels. Noise levels in sleeping quarters in fishing vessels less than 30 m in length are very high (because the crew quarters and engine room are so close together). There is also a high level of noise on the fishing deck. Modern fishing trawlers have covered fishing decks, which provide greater safety and comfort than outside work, but which increase noise levels as they act as resonance chambers.  

Some authors (7) have measured median noise levels for ships (it is best not to use the term “mean”, as decibels are logarithmic values which cannot be added arithmetically). For a common type of 24-metre trawler, median noise levels were 76 dB on the gangway, 80 dB in the wardroom, 86 dB in the steerage compartment, 84 dB on the fishing deck, 82 dB in the crew quarters and 106 dB in the engine room.

The most relevant audiometry finding in relation to risks to hearing is the equivalent continuous level (Leq) to which the crew members are subjected during one day and during one trip. Data recorded in situ show that in 55-60 m trawlers, there are equivalent average noise levels of around 85 dB over a 14-day trip.

Equivalent noise level (Leq) considers not only the intensity of noise, but also duration of exposure. Generally, in industry, we calculate the Leq for a period of 8 hours per day, which is a normal working hours in Western countries. The Leq (8h) is regarded as a source of hearing loss when it exceeds 80 dB (A). However, on ships, seafarers are exposed to noise 24 hours on 24. Therefore, we now calculate the Leq (24hrs) for seafarers, which produce a more precise idea of exposure.

As fishermen are on board for 24 hours a day over several days, levels that would be considered to be disease-causing in workers on land cannot easily be applied to them. Much also depends on the job an individual carries out on board.

Table 11.5.3: Average equivalent levels for a while trip for crews of four semi-industrial 34- metre fishing trawlers ( l7)

The only fisherman who is not exposed to a level of noise that causes trauma to the auditory system is the skipper. All other crewmembers are. The recognised threshold above which there is a risk of hearing loss is 80 dB(A), 8 hours per day. However, if the formula for calculating equivalent continuous levels is used, we notice that a Leq(24h) of 82 dB(A) corresponds to a Leq(8h) of 95 dB(A), according to energy equivalence laws. This means that a seafarer who is exposed to a noise level of 82 dB(A) over 24 hours is exposed to a stressor and a risk to hearing that is equivalent to that of a worker who is exposed to 95 dB(A) for 8 hours a day. In this type of fishing, engineers are subject to highly traumatic levels of noise. If we study, as we are currently, the noisiest times of the work day, we see that length of stay on deck (handling fishing equipment, mending nets, processing the catch) means that fishermen are exposed to 85 dB(A) for an average of 13 hours per day. Then there is time spent in the crew quarters (during sleeping periods, around 5-6 hours per day) exposed to 83 dB(A), and meals in the wardroom (81dB for three hours) and gangway watch (73 dB) for 2 hours. For a conventional trawler of between 15 and 25 metres in length, the equivalent continuous noise level over 24 hours has been calculated as Leq(24h) = 83.6 dB(A).

Therefore, we have to consider that fishermen, apart from skippers of large industrial trawlers, are at high risk of hearing damage due to noise, whatever job they do on board.

11.5.4   Effects on seafarers’ hearing

There are few published international studies on the effects of ship noise on seafarers’ hearing (9).

We should remember that if a person is exposed to noise greater than 80 dB(A) for 8 hours per day or more, this can harm the inner ear, bilaterally and more or less symmetrically, and this damage will worsen as the period of exposure lengthens and will affect higher frequencies, primarily 4000 Hz. This is permanent endocochlear perception hearing loss in the context of chronic auditory fatigue.

On board merchant ships, as we have seen, engineers are exposed to the most noise, with equivalent average levels generally being above 85 dB(A). An audiometry study carried out in 1983 (10) concluded that there was a small region of hearing loss at 4000 Hz in merchant ship engineers, which was most noticeable in those over 40. Seafarers involved in other occupations were not affected.

              Fig 11.5.1: audiometric curves of Marine engineers by ages (Median values,
             combined results from both ears).

   At similar ages, onboard engineers had noticeably less hearing loss than did subjects who worked in noise levels of 95 dB or 100 dB 8 hours per day.

In a more recent study, Parker (11) found hearing loss in 26.8% of engineers, compared with 16% of deck crew members and 9.9% of supervisors (these differences were statistically significant).

The moderate levels of hearing loss observed in engineers may be explained by the fact that their exposure to engine noise is tempered by soundproofing of the control cabins in merchant ship engine rooms, which means that exposure to significant noise is confined to routine patrols and maintenance tasks, and the fact that they wear ear protection when carrying out such tasks. If the noise is constant or only slightly fluctuating, this can also moderate its effects. If work is distributed throughout the year in the form of 2-3 months of work followed by a holiday of the same length, this is a significant factor responsible for the moderate nature of this hypoacusis.

Nevertheless, in a recent study (2008), Kaerlev et al. (12) found that, compared to other seafarers, the engine room personnel have a relative risk ratio of 2.39 for hearing loss among Danish seafarers.

The situation on board fishing boats, however, is very different. As we have already noted, Andro and Dorva (l7) have shown that seafarers on board high-sea fishing vessels are subjected to constant noise levels 24 hours a day. If we take a “median” vessel, we see that seafarers are exposed to 84-86 dB when working on deck, 76 dB when on gangway watch and 82 dB when resting in the crew quarters. In parallel with this audiometric study, a similar study was carried out on 113 fishermen on board the same type of vessel.

Fig 11.5.2: Comparison of audiometric curves between Marine engineers and workers exposed to 95dB (by ages)

(13). The results showed that there was noise-related hearing loss with a window of hearing loss at 4000 Hz, which worsened with age and length of service. The hearing deficits were compared with the French standard NF S 31-013 which contains estimations from international epidemiological data concerning hearing in workers over 40 years old who had been exposed to quasi-stable levels of industrial noise, at 90, 95 and 100 dB, for 20 years. Hearing deficit for fishermen (aged 40, 23 years of exposure) lay between levels of deficit for standardised subjects exposed to 90 dB and those exposed to 95 dB in terms of high frequencies, and were still greater for low frequencies.

A continuous noise level of 85 dB 24 hours a day, as experienced by fishermen, is calculated as equivalent to a continuous noise level of 90 dB over 8 hours. In other words, a seafarer who experiences 85 dB 24 hours a day has hearing loss equivalent to that of a worker exposed to 90-95 dB of factory noise 8 hours a day. These results clearly show that high-sea fishing is an occupation that carries a risk of hearing loss due to noise.

   Fig 11.5.3: Comparison of audiometric curves between fishermen, industrial workers
   exposed to 90 dB (8h per day) and witnesses.

This risk has explicitly been recognised in the report by the European Parliament on safety and accidents in sea fishing, dated 12 March 2001: “Incessant noise creates an aggressive climate on board and means that fishermen sleep little and badly, making it difficult for them to obtain the rest they need... A separate topic concerns fishing-related diseases, which are prevalent among fishermen. Furthermore, this risk is explicitly recognised in the European Parliament report dated 12 March 2001 concerning safety and accidents in sea fishing, which notes that incessant noise creates an aggressive climate on board and means that fishermen sleep little and badly, making it difficult for them to obtain the rest they need, and they can also be affected by hearing loss.

The results of the study by F Trecan (14) involving 18,000 audiography tests on French seafarers confirm the findings of the previous studies. This study confirms that fishermen are at greater risk than commercial seafarers. In maritime transport, seafarers on board oil tankers and cargo ships are observed to be at the greatest risk. Kaerlev (12) finds also an increased risk for fishermen.

The problem with noise on fishing vessels is one of individual protection; as we have seen, in current standard practice, vessels do not have specific soundproofing. Individual protection could be effective (15), but the fisherman would have to wear it 24 hours a day, which is not practicable, although it could potentially be possible to wear custom-made earplugs constantly. For all vessels, the only valid improvement would be soundproofing of quarters when the vessel is built.

11.5.5   Non-hearing effects of noise on seafarers

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 11.5.4: Rates of hypertension in engineers and non-engineers in the merchant 
           navy

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.”[1]*   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.

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