HAZARDS AND INJURIES OF MANUAL HANDLING AND LIFTING
What are the Hazards and Possible Injuries of Manual Handling?
The term ‘manual handling’ is defined as the movement of a load by human effort alone. This effort may be applied directly or indirectly using a rope or a lever.
Manual handling may involve the transportation of the load or the direct support of the load including pushing, pulling, carrying, moving using bodily force and, of course, straightforward lifting. Back injuries due to the lifting of heavy loads is very common and several million working days are lost each year as a result of such injuries.
Typical hazards of manual handling include:
S lifting a load which is too heavy or too cumbersome resulting in back injury
S poor posture during lifting or poor lifting technique resulting in back injury
S dropping a load, resulting in foot injury
S lifting sharp-edged or hot loads resulting in hand injuries.
Injuries caused by manual handling
Manual handling operations can cause a wide range of acute and chronic injuries to workers. Acute injuries normally lead to sickness leave from work and a period of rest during which time the damage heals.
Chronic injuries build up over a long period of time and are usually irreversible producing illnesses such as arthritic and spinal disorders. There is considerable evidence to suggest that modern life styles, such as a lack of exercise and regular physical effort, have contributed to the long-term serious effects of these injuries.
The most common injuries associated with poor manual handling techniques are all musculoskeletal in nature and are:
S muscular sprains and strains – caused when a muscular tissue (or ligament or tendon) is stretched
beyond its normal capability leading to a weakening, bruising and painful inflammation of the area
affected. Such injuries normally occur in the back or in the arms and wrists
S back injuries – include injuries to the discs situated between the spinal vertebrae (i.e. bones) and can lead to a very painful prolapsed disc lesion (commonly known as a slipped disc). This type of injury can lead to other conditions known as lumbago and sciatica (where pain travels down the leg)
S trapped nerve – usually occurring in the back as a result of another injury but aggravated by manual
handling
S hernia – this is a rupture of the body cavity wall in the lower abdomen causing a protrusion of part of the intestine. This condition eventually requires surgery to repair the damage
S cuts, bruising and abrasions – caused by handling loads with unprotected sharp corners or edges
S fractures – normally of the feet due to the dropping of a load. Fractures of the hand also occur but are less common
S Work-related upper limb disorders (WRULDs)
S rheumatism – this is a chronic disorder involving severe pain in the joints. It has many causes, one of which is believed to be the muscular strains induced by poor manual handling lifting technique.
WORKS THAT REQUIRE WORK PERMITS BASIC INFORMATION AND TUTORIALS
WORKS THAT REQUIRES WORK PERMIT
What are the Works that Requires Work Permit?
The main types of permit and the work to be covered by each are identified below.
General permit
The general permit should be used for work such as:
S alterations to or overhaul of plant or machinery where mechanical, toxic or electrical hazards may
arise
S work on or near overhead crane tracks
S work on pipelines with hazardous contents
S work with asbestos-based materials
S work involving ionising radiation
S work at height where there are exceptionally high risks
S excavations to avoid underground services.
Confined space permit
Confined spaces include chambers, tanks (sealed and open-top), vessels, furnaces, ducts, sewers, manholes, pits, flues, excavations, boilers, reactors and ovens.
Many fatal accidents have occurred where inadequate precautions were taken before and during work
involving entry into confined spaces. The two main hazards are the potential presence of toxic or other dangerous substances and the absence of adequate oxygen. In addition, there may be mechanical hazards (entanglement on agitators) and raised temperatures.
The work to be carried out may itself be especially hazardous when done in a confined space, for example, cleaning using solvents, cutting/welding work. Should the person working in a confined space get into difficulties for whatever reason, getting help in and getting the individual out may prove difficult and dangerous.
Stringent preparation, isolation, air testing and other precautions are therefore essential and experience shows that the use of a confined space entry permit is essential to confirm that all the appropriate precautions have been taken.
Work on high voltage apparatus (including testing)
Work on high voltage apparatus (over about 600 volts) is potentially high risk. Hazards include:
S possibly fatal electric shock/burns to the people doing the work
S electrical fires/explosions
S consequential danger from disruption of power supply to safety-critical plant and equipment.
In view of the risk, this work must only be done by suitably trained and competent people acting under the terms of a high voltage permit.
Hot work
Hot work is potentially hazardous as a:
S source of ignition in any plant in which highly flammable materials are handled
S cause of fires in all locations, regardless of whether highly flammable materials are present.
Hot work includes cutting, welding, brazing, soldering and any process involving the application of a naked flame. Drilling and grinding should also be included where a flammable atmosphere is potentially present.
In high risk areas hot work may also involve any equipment or procedure that produces a spark of sufficient energy to ignite highly flammable substances.
Hot work should therefore be done under the terms of a hot work permit, the only exception being where hot work is done in a designated area suitable for the purpose.
What are the Works that Requires Work Permit?
The main types of permit and the work to be covered by each are identified below.
General permit
The general permit should be used for work such as:
S alterations to or overhaul of plant or machinery where mechanical, toxic or electrical hazards may
arise
S work on or near overhead crane tracks
S work on pipelines with hazardous contents
S work with asbestos-based materials
S work involving ionising radiation
S work at height where there are exceptionally high risks
S excavations to avoid underground services.
Confined space permit
Confined spaces include chambers, tanks (sealed and open-top), vessels, furnaces, ducts, sewers, manholes, pits, flues, excavations, boilers, reactors and ovens.
Many fatal accidents have occurred where inadequate precautions were taken before and during work
involving entry into confined spaces. The two main hazards are the potential presence of toxic or other dangerous substances and the absence of adequate oxygen. In addition, there may be mechanical hazards (entanglement on agitators) and raised temperatures.
The work to be carried out may itself be especially hazardous when done in a confined space, for example, cleaning using solvents, cutting/welding work. Should the person working in a confined space get into difficulties for whatever reason, getting help in and getting the individual out may prove difficult and dangerous.
Stringent preparation, isolation, air testing and other precautions are therefore essential and experience shows that the use of a confined space entry permit is essential to confirm that all the appropriate precautions have been taken.
Work on high voltage apparatus (including testing)
Work on high voltage apparatus (over about 600 volts) is potentially high risk. Hazards include:
S possibly fatal electric shock/burns to the people doing the work
S electrical fires/explosions
S consequential danger from disruption of power supply to safety-critical plant and equipment.
In view of the risk, this work must only be done by suitably trained and competent people acting under the terms of a high voltage permit.
Hot work
Hot work is potentially hazardous as a:
S source of ignition in any plant in which highly flammable materials are handled
S cause of fires in all locations, regardless of whether highly flammable materials are present.
Hot work includes cutting, welding, brazing, soldering and any process involving the application of a naked flame. Drilling and grinding should also be included where a flammable atmosphere is potentially present.
In high risk areas hot work may also involve any equipment or procedure that produces a spark of sufficient energy to ignite highly flammable substances.
Hot work should therefore be done under the terms of a hot work permit, the only exception being where hot work is done in a designated area suitable for the purpose.
BIOLOGICAL EFFECTS OF IONIZING RADIATION BASIC INFORMATION AND TUTORIALS
IONIZING RADIATION IMPACT TO BODY
What Are The Biological Effects of Ionizing Radiation?
Information on the biological effects of ionizing radiation comes from animal experiments and from studies of groups of people exposed to relatively high levels of radiation. The best-known groups are the workers in the luminising industry early this century who used to point their brushes with the lips and so ingest radioactivity; the survivors of the atomic bombs dropped on Japan, and patients who have undergone radiotherapy.
Evidence of biological effects is also available from studies of certain miners who inhaled elevated levels of the natural radioactive gas radon and its radioactive decay products. The basic unit of tissue is the cell. Each cell has a nucleus, which may be regarded as its control centre.
Deoxyribonucleic acid (DNA) is the essential component of the cell’s genetic information and makes up the chromosomes which are contained in the nucleus. Although the ways in which radiation damages cells are not fully understood, many involve changes to DNA.
There are two main modes of action. A DNA molecule may become ionised, resulting directly in chemical change, or it may be chemically altered by reaction with agents produced as a result of the ionisation of other cell constituents. The chemical change may ultimately mean that the cell is prevented from further division and can therefore be regarded as dead.
Very high doses of radiation can kill large numbers of cells. If the whole body is exposed, death may occur within a matter of weeks: an instantaneous absorbed dose of 5 gray or more would probably be lethal (the unit gray is defined below).
If a small area of the body is briefly exposed to a very high dose, death may not occur, but there may be other early effects: an instantaneous absorbed dose of 5 gray or more to the skin would probably cause erythema (reddening) in a week or so, and a similar dose to the testes or ovaries might cause sterility.
If the same doses are received in a protracted fashion, there may be no early signs of injury. The effect of very high doses of radiation delivered acutely is used in radiotherapy to destroy malignant tissue. Effects of radiation that only occur above certain levels (i.e. thresholds) are known as deterministic. Above these thresholds, the severity of harm increases with dose.
Low doses or high doses received in a protracted fashion may lead to damage at a later stage. With reproductive cells, the harm is expressed in the irradiated person’s offspring (genetic defects), and may vary from unobservable through mildly detrimental to severely disabling.
So far, however, no genetic defects directly attributable to radiation exposure have been unequivocally observed in human beings. Cancer induction may result from the exposure of a number of different types of a cell. There is always a delay of some years, or even decades, between irradiation and the appearance of a cancer.
It is assumed that within the range of exposure conditions usually encountered in radiation work, the risks of cancer and hereditary damage increase in direct proportion to the radiation dose. It is also assumed that there is no exposure level that is entirely without risk.
Thus, for example, the mortality risk factor for all cancers from uniform radiation of the whole body is now estimated to be 1 in 25 per sievert (see below for definition) for a working population, aged 20 to 64 years, averaged over both sexes5. In scientific notation, this is given as 4 10 2 per sievert.
Effects of radiation, primarily cancer induction, for which there is probably no threshold and the risk is proportional to dose are known as stochastic, meaning ‘of a random or statistical nature’.
What Are The Biological Effects of Ionizing Radiation?
Information on the biological effects of ionizing radiation comes from animal experiments and from studies of groups of people exposed to relatively high levels of radiation. The best-known groups are the workers in the luminising industry early this century who used to point their brushes with the lips and so ingest radioactivity; the survivors of the atomic bombs dropped on Japan, and patients who have undergone radiotherapy.
Evidence of biological effects is also available from studies of certain miners who inhaled elevated levels of the natural radioactive gas radon and its radioactive decay products. The basic unit of tissue is the cell. Each cell has a nucleus, which may be regarded as its control centre.
Deoxyribonucleic acid (DNA) is the essential component of the cell’s genetic information and makes up the chromosomes which are contained in the nucleus. Although the ways in which radiation damages cells are not fully understood, many involve changes to DNA.
There are two main modes of action. A DNA molecule may become ionised, resulting directly in chemical change, or it may be chemically altered by reaction with agents produced as a result of the ionisation of other cell constituents. The chemical change may ultimately mean that the cell is prevented from further division and can therefore be regarded as dead.
Very high doses of radiation can kill large numbers of cells. If the whole body is exposed, death may occur within a matter of weeks: an instantaneous absorbed dose of 5 gray or more would probably be lethal (the unit gray is defined below).
If a small area of the body is briefly exposed to a very high dose, death may not occur, but there may be other early effects: an instantaneous absorbed dose of 5 gray or more to the skin would probably cause erythema (reddening) in a week or so, and a similar dose to the testes or ovaries might cause sterility.
If the same doses are received in a protracted fashion, there may be no early signs of injury. The effect of very high doses of radiation delivered acutely is used in radiotherapy to destroy malignant tissue. Effects of radiation that only occur above certain levels (i.e. thresholds) are known as deterministic. Above these thresholds, the severity of harm increases with dose.
Low doses or high doses received in a protracted fashion may lead to damage at a later stage. With reproductive cells, the harm is expressed in the irradiated person’s offspring (genetic defects), and may vary from unobservable through mildly detrimental to severely disabling.
So far, however, no genetic defects directly attributable to radiation exposure have been unequivocally observed in human beings. Cancer induction may result from the exposure of a number of different types of a cell. There is always a delay of some years, or even decades, between irradiation and the appearance of a cancer.
It is assumed that within the range of exposure conditions usually encountered in radiation work, the risks of cancer and hereditary damage increase in direct proportion to the radiation dose. It is also assumed that there is no exposure level that is entirely without risk.
Thus, for example, the mortality risk factor for all cancers from uniform radiation of the whole body is now estimated to be 1 in 25 per sievert (see below for definition) for a working population, aged 20 to 64 years, averaged over both sexes5. In scientific notation, this is given as 4 10 2 per sievert.
Effects of radiation, primarily cancer induction, for which there is probably no threshold and the risk is proportional to dose are known as stochastic, meaning ‘of a random or statistical nature’.
WORKING ON TRANSFORMERS AND CIRCUIT BREAKERS SAFETY PRECAUTION TIPS AND TUTORIALS
SAFETY PRECAUTIONS ON WORKING ON TRANSFORMERS AND CIRCUIT BREAKERS
What Are The Safety Precautions on Working on Power Transformers and Circuit Breakers
Take the following safety precautions when working on transformers and circuit breakers:
• Prevent moisture from entering when removing covers from oil-filled transformers.
• Do not allow tools, bolts, nuts, or similar objects to drop into the transformers. Tie tools or parts with suitable twine.
• Have workers empty their pockets of lose articles such as knives, keys, and watches.
• Remove all oil from transformer covers, the floor, and the scaffold to eliminate slipping hazards.
• Exhaust gaseous vapor with an air blower before allowing work in large transformer cases because they usually contain some gaseous fumes and are not well ventilated.
• Stay away from the base of the pole or structure while transformers are being raised or lowered.
• Ensure that anyone working on a pole or structure takes a position above or well clear of transformers while the transformers are being raised or supported with blocks.
• Ground the secondary side of a transformer before energizing it except when the transformer is part of an ungrounded delta bank.
• Make an individual secondary-voltage test on all transformers before connecting them to secondary mains. On banks of three transformers connected Y-delta, bring in the primary neutral and leave it connected until the secondary connections have been completed to get a true indication on the lamp tests.
• Disconnect secondary-phase leads before opening primary cutouts when taking a paralleled transformer out of service. Do not disconnect secondary neutral or ground connections until you have opened the primary cutouts.
• Do not stand on top of energized transformers unless absolutely necessary and then only with the permission of the foreman and after all possible precautions have been taken. These precautions include placing a rubber blanket protected with a rubbish bag over the transformer cover. Do not wear climbers.
• Treat the grounded case of a connected transformer the same as any grounded conductor. Treat the ungrounded case of a connected transformer the same as any energized conductor because the case may become energized if transformer windings break down.
• Ensure that the breaker cannot be opened or closed automatically before working on an oil circuit breaker and that it is in the open position or the operating mechanism is blocked.
• Ensure that metal-clad switching equipment is deenergized before working on it.
• Ensure that regulators are off the automatic position and set in the neutral position before doing any switching on a regulated feeder.
• Do not break the charging current of a regulator or large substation transformer by opening disconnect switches because a dangerous arc may result. Use oil or air brake switches unless special instructions to do otherwise have been issued by the proper authority.
• Do not operate outdoor disconnecting switches without using the disconnect pole provided for this
purpose.
• Ensure that all contacts are actually open and that safe clearance is obtained on all three phases each time an air brake switch is opened. Do not depend on the position of the operating handle as evidence that the switch is open.
• Do not operate switches or disconnect switches without proper authority and then only if thoroughly
familiar with the equipment.
• Remove potential transformer fuses with wooden tongs. Wear rubber gloves and leather over gloves.
• Do not open or remove disconnect switches when carrying load. First open the oil circuit breaker in series with the switches. Open disconnect switches slowly and reclose immediately if an arc is drawn.
What Are The Safety Precautions on Working on Power Transformers and Circuit Breakers
Take the following safety precautions when working on transformers and circuit breakers:
• Prevent moisture from entering when removing covers from oil-filled transformers.
• Do not allow tools, bolts, nuts, or similar objects to drop into the transformers. Tie tools or parts with suitable twine.
• Have workers empty their pockets of lose articles such as knives, keys, and watches.
• Remove all oil from transformer covers, the floor, and the scaffold to eliminate slipping hazards.
• Exhaust gaseous vapor with an air blower before allowing work in large transformer cases because they usually contain some gaseous fumes and are not well ventilated.
• Stay away from the base of the pole or structure while transformers are being raised or lowered.
• Ensure that anyone working on a pole or structure takes a position above or well clear of transformers while the transformers are being raised or supported with blocks.
• Ground the secondary side of a transformer before energizing it except when the transformer is part of an ungrounded delta bank.
• Make an individual secondary-voltage test on all transformers before connecting them to secondary mains. On banks of three transformers connected Y-delta, bring in the primary neutral and leave it connected until the secondary connections have been completed to get a true indication on the lamp tests.
• Disconnect secondary-phase leads before opening primary cutouts when taking a paralleled transformer out of service. Do not disconnect secondary neutral or ground connections until you have opened the primary cutouts.
• Do not stand on top of energized transformers unless absolutely necessary and then only with the permission of the foreman and after all possible precautions have been taken. These precautions include placing a rubber blanket protected with a rubbish bag over the transformer cover. Do not wear climbers.
• Treat the grounded case of a connected transformer the same as any grounded conductor. Treat the ungrounded case of a connected transformer the same as any energized conductor because the case may become energized if transformer windings break down.
• Ensure that the breaker cannot be opened or closed automatically before working on an oil circuit breaker and that it is in the open position or the operating mechanism is blocked.
• Ensure that metal-clad switching equipment is deenergized before working on it.
• Ensure that regulators are off the automatic position and set in the neutral position before doing any switching on a regulated feeder.
• Do not break the charging current of a regulator or large substation transformer by opening disconnect switches because a dangerous arc may result. Use oil or air brake switches unless special instructions to do otherwise have been issued by the proper authority.
• Do not operate outdoor disconnecting switches without using the disconnect pole provided for this
purpose.
• Ensure that all contacts are actually open and that safe clearance is obtained on all three phases each time an air brake switch is opened. Do not depend on the position of the operating handle as evidence that the switch is open.
• Do not operate switches or disconnect switches without proper authority and then only if thoroughly
familiar with the equipment.
• Remove potential transformer fuses with wooden tongs. Wear rubber gloves and leather over gloves.
• Do not open or remove disconnect switches when carrying load. First open the oil circuit breaker in series with the switches. Open disconnect switches slowly and reclose immediately if an arc is drawn.
HOTLINE TOOLS SAFETY RULES FOR WORKING BASIC INFORMATION AND TUTORIALS
LINEMAN SAFETY RULES FOR WORKING USING HOTLINE TOOLS
Hotline Tools Safety Rules
Follow these safety rules when working with hot-line tools:
• Do not perform hot-line work when rain or snow is threatening or when heavy dew, fog, or other excessive moisture is present. Exceptions to this rule are when conducting switching operations, fusing, or clearing damaged equipment that presents a hazard to the public or to troops.
• Remain alert. If rain or snow starts to fall or an electrical storm appears while a job is in progress, complete the work as quickly as possible to allow safe, temporary operation of the line until precipitation or lightning ceases. Judgment of safe weather conditions for hot-line work is the foreman's responsibility.
• Perform hot-line work during daylight if possible. In emergency situations, work under artificial light if all conductors and equipment being worked on are made clearly visible.
• Do not wear rubber gloves with hot-line tools because they make detection of brush discharges impossible.
• Avoid holding outer braces or other metal attachments.
• Avoid unnecessary conversation.
• Maintain close cooperation among everyone on the job.
• Treat wooden pole structures the same as steel towers.
• Be careful with distribution primaries. When they are located on the same pole with high-tension lines, cover them with rubber protective equipment before climbing through or working above them.
• Do not change your position on the pole without first looking around and informing others.
• Never use your hands to hold a live line clear of a lineman on a pole. Secure the line with live-line tools and lock it in a clamp.
• Stay below the live wire when moving it until it is thoroughly secured in a safe working position.
Take special precautions on poles having guy lines. Do not use a rope on conductors carrying more than 5,000 volts unless the rope is insulated from the conductor with an insulated tension link stick.
Hotline Tools Safety Rules
Follow these safety rules when working with hot-line tools:
• Do not perform hot-line work when rain or snow is threatening or when heavy dew, fog, or other excessive moisture is present. Exceptions to this rule are when conducting switching operations, fusing, or clearing damaged equipment that presents a hazard to the public or to troops.
• Remain alert. If rain or snow starts to fall or an electrical storm appears while a job is in progress, complete the work as quickly as possible to allow safe, temporary operation of the line until precipitation or lightning ceases. Judgment of safe weather conditions for hot-line work is the foreman's responsibility.
• Perform hot-line work during daylight if possible. In emergency situations, work under artificial light if all conductors and equipment being worked on are made clearly visible.
• Do not wear rubber gloves with hot-line tools because they make detection of brush discharges impossible.
• Avoid holding outer braces or other metal attachments.
• Avoid unnecessary conversation.
• Maintain close cooperation among everyone on the job.
• Treat wooden pole structures the same as steel towers.
• Be careful with distribution primaries. When they are located on the same pole with high-tension lines, cover them with rubber protective equipment before climbing through or working above them.
• Do not change your position on the pole without first looking around and informing others.
• Never use your hands to hold a live line clear of a lineman on a pole. Secure the line with live-line tools and lock it in a clamp.
• Stay below the live wire when moving it until it is thoroughly secured in a safe working position.
Take special precautions on poles having guy lines. Do not use a rope on conductors carrying more than 5,000 volts unless the rope is insulated from the conductor with an insulated tension link stick.
RELATIONSHIP BETWEEN NOISE EXPOSURE AND HEARING LOSS BASIC INFORMATION
WHAT IS THE RELATIONSHIP BETWEEN NOISE EXPOSURE AND HEARING LOSS
The relationship between noise exposure and hearing loss.
If hearing damage is to be prevented by limiting occupational noise exposure, then it is necessary to have some quantitative understanding of the relationships between sound pressure level, frequency, exposure time and the degree of damage caused.
Having established that there is a risk to hearing, though, it would be unethical to refrain from taking all reasonable measures to prevent it. During the 1960s a great deal of work was done in the UK and the rest of the world to establish the relationships between noise exposure and noise-induced hearing loss.
At that time it was relatively easy to find populations who had worked at one job, and been exposed to steady noise levels, for a number of years. Since then, social mobility, changing patterns of employment, and indeed government action to limit noise exposure, have made it much harder to find large groups of workers whose noise exposure can be logged over several years.
Information on the precise relationship between the various factors influencing hearing damage is therefore incomplete, and a full understanding of the subject will never be achieved. Full understanding is not required, though.
What is needed is sufficient information to frame legislation and advisory procedures which are capable of being put into practice in such a way that occupational hearing damage is reduced and eventually eliminated, without also making essential industrial processes impossible or uneconomical to carry out. This is itself quite a demanding objective.
In studying the relationship between noise exposure and hearing loss a range of questions can be asked. It can be assumed that louder sounds will result in more damage than quieter ones, but more detailed questions include:
. Is there a sound pressure level below which there is no contribution to hearing damage?
. If so, then what is this level?
. If all noise contributes to damage then what is the trade-off between level and damage?
. Does an extended period of noise exposure do the same amount of damage as a series of shorter exposures at the same level?
. Are particular frequencies or ranges of frequencies significantly more damaging than others?
. Is there a link between the frequencies to which the ear is exposed and the frequencies at which hearing loss occurs?
The answers to these questions and other questions will all have consequences for the way in which noise exposure must be measured. In the European Union, an approach to the assessment of noise exposure has emerged which uses the best available answers to these questions.
Each of the assumptions listed below can be challenged, and together they represent a gross simplification of a very complicated area of knowledge. For the time being, they seem to offer a practical way forward to those working to reduce occupational hearing loss, and as stated above, that is the most that can be asked for.
1. All sound energy received by the ear will, in some degree, contribute to hearing damage.
2. The degree of damage is proportional to the amount of sound energy deposited in the ear. That means that a doubling of exposure time is equivalent to a 3 dB increase in sound pressure level. It also means that the total exposure time at a given level is important; breaking the overall time up into shorter periods has no effect.
3. The A weighting system correctly evaluates the contribution of different frequencies to hearing loss.
4. Very high sound pressures can cause damage which may not be reflected in an equal-energy assessment as described above. An additional limit on peak sound exposure can be used to prevent this.
In the United States, rather different conclusions have been reached, and as a result a rather different trade-off between sound pressure level and exposure time is used. This is based on the assumption that a 5 dB increase in level (rather than 3 dB) is equivalent to a doubling of exposure time.
To add to the confusion, for some purposes in the United States 4 dB (rather than 3 or 5 dB) is assumed to be equivalent to a doubling of exposure time. Those carrying out noise exposure assessments in Europe need to be aware of these different practices in order to avoid being misled by procedures or instrumentation intended for American use.
The current European approach to the prevention of occupational hearing damage is based on the principles listed above. The issue is the subject of continuing debate as research into hearing damage continues. Given the difficulties of generating further large sets of data which can be used to refine our knowledge, it seems likely that for the foreseeable future this approach will continue.
The relationship between noise exposure and hearing loss.
If hearing damage is to be prevented by limiting occupational noise exposure, then it is necessary to have some quantitative understanding of the relationships between sound pressure level, frequency, exposure time and the degree of damage caused.
Having established that there is a risk to hearing, though, it would be unethical to refrain from taking all reasonable measures to prevent it. During the 1960s a great deal of work was done in the UK and the rest of the world to establish the relationships between noise exposure and noise-induced hearing loss.
At that time it was relatively easy to find populations who had worked at one job, and been exposed to steady noise levels, for a number of years. Since then, social mobility, changing patterns of employment, and indeed government action to limit noise exposure, have made it much harder to find large groups of workers whose noise exposure can be logged over several years.
Information on the precise relationship between the various factors influencing hearing damage is therefore incomplete, and a full understanding of the subject will never be achieved. Full understanding is not required, though.
What is needed is sufficient information to frame legislation and advisory procedures which are capable of being put into practice in such a way that occupational hearing damage is reduced and eventually eliminated, without also making essential industrial processes impossible or uneconomical to carry out. This is itself quite a demanding objective.
In studying the relationship between noise exposure and hearing loss a range of questions can be asked. It can be assumed that louder sounds will result in more damage than quieter ones, but more detailed questions include:
. Is there a sound pressure level below which there is no contribution to hearing damage?
. If so, then what is this level?
. If all noise contributes to damage then what is the trade-off between level and damage?
. Does an extended period of noise exposure do the same amount of damage as a series of shorter exposures at the same level?
. Are particular frequencies or ranges of frequencies significantly more damaging than others?
. Is there a link between the frequencies to which the ear is exposed and the frequencies at which hearing loss occurs?
The answers to these questions and other questions will all have consequences for the way in which noise exposure must be measured. In the European Union, an approach to the assessment of noise exposure has emerged which uses the best available answers to these questions.
Each of the assumptions listed below can be challenged, and together they represent a gross simplification of a very complicated area of knowledge. For the time being, they seem to offer a practical way forward to those working to reduce occupational hearing loss, and as stated above, that is the most that can be asked for.
1. All sound energy received by the ear will, in some degree, contribute to hearing damage.
2. The degree of damage is proportional to the amount of sound energy deposited in the ear. That means that a doubling of exposure time is equivalent to a 3 dB increase in sound pressure level. It also means that the total exposure time at a given level is important; breaking the overall time up into shorter periods has no effect.
3. The A weighting system correctly evaluates the contribution of different frequencies to hearing loss.
4. Very high sound pressures can cause damage which may not be reflected in an equal-energy assessment as described above. An additional limit on peak sound exposure can be used to prevent this.
In the United States, rather different conclusions have been reached, and as a result a rather different trade-off between sound pressure level and exposure time is used. This is based on the assumption that a 5 dB increase in level (rather than 3 dB) is equivalent to a doubling of exposure time.
To add to the confusion, for some purposes in the United States 4 dB (rather than 3 or 5 dB) is assumed to be equivalent to a doubling of exposure time. Those carrying out noise exposure assessments in Europe need to be aware of these different practices in order to avoid being misled by procedures or instrumentation intended for American use.
The current European approach to the prevention of occupational hearing damage is based on the principles listed above. The issue is the subject of continuing debate as research into hearing damage continues. Given the difficulties of generating further large sets of data which can be used to refine our knowledge, it seems likely that for the foreseeable future this approach will continue.
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