Control technology for occupational safety and health
Berenice I.F. Goelzer
ILO Encyclopedia of Occupational Health and Safety, 1983, p.538-546
Control technology in occupational health comprises a set of measures and techniques which aim at the elimination or reduction of exposures to harmful agents in the working environment. The ultimate objective is the prevention of adverse health effects among workers. The recognition of occupational hazards involves the determination of potentially harmful agents and associated health effects; the evaluation of occupational hazards involves the determination of the degree and conditions of exposure, as well as the comparison of such data with associated health effects and accepted standards. However, these two steps per se cannot ensure a healthy working environment, which can only be achieved through adequate control technology. For example, whereas the knowledge that carbon monoxide or benzene or arsine is harmful to health and is present in the workroom air in dangerous concentrations cannot prevent the resulting ill effects, control measures can. Standards for occupational exposure should not only be established but also achieved. The tremendous resources spent in setting up, for example, maximum permissible levels for occupational exposure to chemical agents, would be practically wasted if these values were not put into practice, that is, were not translated into control measures at the workplace.
The correct recognition and careful evaluation of hazards are extremely important and constitute the basis for the design of adequate control measures, which must then be properly implemented, well utilized and maintained. The efficiency of control measures should be periodically checked through routine air monitoring: in certain cases, automatic detectors and alarm systems may be used, particularly when there is danger of a sudden build-up of hazardous concentrations (e.g. leaks). Besides, techniques for the early detection of health impairment due to occupational exposures and biological monitoring may also be used, when possible, as complementary tools in evaluating the efficiency of control measures.
The ideal is that control technology be incorporated into a workplace at the planning and design stage, and implemented during its construction.
Before planning for a control programme it is essential to study and clearly define all possible sources of health hazards (operations, storage of chemicals, transfer of materials, leaks, etc.). Although many techniques are available and are being successfully used at present for the control of hazards in the work environment, more research and applications are still needed in this field. which requires not only technical knowledge but ingenuity as well.
Although each case can be considered as unique, there are basic principles of control technology which can be applied, either alone or in combination, to a great number of workplace situations. To start with, there are certain questions to be asked, the answers to which are likely to indicate the way to the most suitable and feasible control technology (see OCCUPATIONAL HYGIENE SYSTEMATIC APPROACH AND STRATEGY). These questions include the following:
Which are the potential hazards, their sources and location? - Can the hazard sources be removed or completely isolated?
Can the presence of the hazard, or the possibility of its release, be avoided?
Is there a way to prevent or reduce the amount of the hazardous agent coming into contact with the workers (through ventilation, isolation, enclosures) or the workers coming into contact with the hazardous agent (distance, special cabins, personal protection)?
Can the duration of the exposure be minimized (through adequate work practices or administrative controls)?
The control measures for occupational hazards can be grouped into two main categories: 1-Environmental control measures; 2-Personal control measures.
The most important measures for the control of chemical agents and particulates in the work environment are hereby briefly discussed and exemplified. (Control measures for physical agents and for biological agents are discussed under relevant separate articles.)
Environmental control measures
These include changes in the work processes and/or working environment with the objective of controlling the health hazards either by eliminating the responsible agents or reducing them to levels believed not to be harmful to health, as well as by preventing them coming into contact with the workers.
Adequate design and lay-out
The ideal is that health and safety aspects be accounted for at the design stage of equipment, processes and workplaces. After a workplace is established it is usually difficult to make changes in order to reduce health hazards. Therefore, when selecting processes or equipment then relative hazard producing characteristics should be weighed together with the other factors affecting the final decision. For example, if a dust-free process can be used instead of a dust producing one, or a closed system to handle chemicals instead of an open process, the first one should be chosen even if initially it is more expensive. Further need for control measures (e.g. ventilation) may prove to be even more expensive than an eventual difference in cost between the two alternatives. Technical knowledge which can be used for this purpose is available concerning specific branches of activity.
With reference to plant design, in addition to such aspects as illumination and ventilation, lay-out should be carefully considered. An example of the importance of lay-out is the placement of washer and dryer (dry cleaning industry) as close as possible to each other, in order to reduce the time during which the damp load can release solvent vapors to the workroom air. The same reasoning can be applied to the transfer of intermediate products such as molten metal (from furnace to molds), paint and coating mixtures, etc
Elimination or reduction of the harmful agent at its source
This can be achieved through such measures as the following:
Discontinuation of the process. A process which utilizes produces or leads to the formation of the agent may be discontinued (e.g discontinuation of benzidine production), or a certain agent banned (eg banning of benzene as a solvent, beta-naphthylamine, in the manufacture of dyestuffs and as antioxidant for rubber, and, the removal of beryllium phosphors from fluorescent lamps).
This may prove difficult in practice, owing to considerations of, for example, a socioeconomic nature: however, for certain highly toxic, carcinogenic, mutagenic or teratogenic chemicals it is undoubtedly the safest solution.
Substitution of materials used. Substitution of materials (i.e. solvents, fuels, raw materials, etc) can be a very effective way of controlling a hazardous exposure, it may involve or not a change in the work process. As classical examples can be mentioned the substitution of phosphorus sesquisulfide for white phosphorus, which eradicated the serious occupational disease known as "phossy jaw", common among match makers in the past; the substitution of mercury-free carroting materials for the mercury compounds previously used in the felt hat industry, which was followed by the disappearance of the "mad hatter", and the use of tritium-activated instead of radium-based paint for watch and phosphort dials, which has greatly reduced the hazards instrument associated with this manufacture.
In order to plan for a substitution, the possible alternatives should be investigated and factors such as technological and economic feasibility, as well as availability of the substitute material in the market should be considered.
Substitution, although possibly one of the best control measures, can also be misleading. The main pitfall to be avoided is the introduction of a new hazard while removing the old one, as would happen if a dry cleaning solvent of low toxicity but high flammability were utilized in place of another with high toxicity and low flammability. Both toxicological and safety aspects must be taken into account. Even considering the toxicological properties of a substitute material, care must be taken not to drop a chemical of known toxicity (therefore used under strict control) in favor of another whose toxicological properties are not so well known on the assumption that it is of lower toxicity. This second chemical may prove to be even more dangerous than the first one and may have been used without the required precautions. The substitute material must be of proven much lower toxicity and should not introduce new or higher safety hazards.
As examples of substitution can be mentioned the use of:
less hazardous solvents instead of toxic ones, eg. trichloroethane (methyl chloroform), dichloromethane or a fluoro chloro hydrocarbon instead of carbon tetrachloride and toluene, cyclohexane or certain ketones instead of benzene;
solvents and other chemicals with lower vapor pressures and with higher boiling points instead of equivalent ones (including in degree of toxicity) with higher vapor pressures and with lower boiling points, in order to decrease vaporization;
detergent-and-water cleaning solutions instead of organic solvents;
natural rubber cements with aliphatic hydrocarbon solvents instead of benzene cements;
water emulsion coatings (e.g. acrylic latex) instead of those containing organic solvents, as well as water based paints instead of solvent-based paints (this may involve a change in the process);
argon instead of chlorine in the degassing of aluminum (foundries);
clean scrap instead of oily and/or painted scrap in foundries;
freon instead of methyl bromide and chloride as a refrigerant;
leadless glazes in the ceramics industry;
leadless paint pigments (e.g. titanium dioxide, zinc oxide, etc.);
fiberglass instead of asbestos;
non-silica molding aggregates instead of quartz sand in foundries;
steel shot, corundum or silicon carbide instead of quartz sand for abrasive blasting (however, the silica dust hazard would still exist if the parts to be cleaned were, for example, silica-coated castings, regardless of the abrasive used);
magnesite or aluminum oxide bricks instead of silica bricks for the lining of furnaces and ladles in steel works;
synthetic grinding wheels (e.g. aluminum oxide, silicon carbide) instead of sandstone wheels.
Modifications in the process and/or equipment. In this category are included modifications in processes, operations or equipment, leading to appreciable reductions in contaminant generation (e.g. reduced temperatures or speed), the elimination or decrease in the formation of undesirable by-products, the elimination or minimization of physical contact between workers and hazardous agents (e.g. use of mechanical aids such as tongs, mechanization, etc.). As in the case of substitution of materials, the new process, operation or equipment, must not introduce a new hazard, and it must be technically feasible and acceptable. Examples include the use of:
electric motors instead of internal combustion engines: mechanical (single or double) pump seals instead of gasket pump seals;
toxic solids in pellet form rather than as a powder;
chemicals to suppress or decrease the formation of undesirable agents (e.g. the use of urea as a chemical suppressant of nitrogen dioxide gas formed when nitric acid is utilized in operations such as bright dip and pickling);
mechanical gauges instead of mercury-containing types: "dust-free" cutting equipment in the printing industry;
airless spray techniques instead of hand-spraying;
solvents at temperatures as low as technically feasible, in order to decrease vaporization;
dip or brush instead of spray painting (it should be kept in mind, however, that this may reduce the hazard during the painting operation while, during drying, the hazard from the evaporated solvent would be the same for both);
floating plastic balls on open-surface tanks (degreasing, leather finishing, dyeing, etc.), in order to decrease the evaporation surface;
refractory bricks already purchased in the required dimensions so that sawing in the workplace is not necessary;
catalysers (e.g. which transform carbon monoxide into carbon dioxide) at the exhaust outlet of internal combustion engines, and air purifiers in compressed air outlets;
mechanization of operations (e.g. machine application of lead oxide to battery grids);
covered containers to carry materials and products which give off air contaminants.
Maintenance of equipment. This aspect is very important since well maintained, well regulated processes and equipment usually give off less air contaminants. As examples can be mentioned:
adequate regulation of internal combustion engines, which reduces the amount of carbon monoxide produced;
best possible combustion in furnaces and ovens since the more complete the combustion, the less the production of carbon monoxide;
prevention of leakages in closed systems, valves, pumps, etc.;
prevention of leakages in drying ovens or dry cleaning units.
Isolation
The harmful agents can be isolated in order not to come into contact with the workers. This can be achieved by interposing, between the agent and the worker, a barrier or a shield (closed systems, enclosures, separating walls), distance or time.
Closed systems. Many toxic chemicals can be used rather safely in closed systems. For example, in the textile industry, the chlorine hazard (dyeing) can be appreciably reduced if the bleaching vats are constructed as closed vessels (with adequate vents which allow a minimum of chlorine escape); bis-chloromethyl ether while used in "open kettle" operations was associated with the occurrence of a characteristic carcinoma which, however, has not yet been reported among workers dealing with it in closed systems. In the chemical and petroleum industries, the isolation of processes in closed systems is the usual practice, therefore many hazards are reduced. There are many processes which by nature require a closed system; whenever this is not the case but there is a choice, closed systems should be preferred.
Nevertheless, routine air monitoring is of paramount importance in order to detect any leaks, which should be immediately repaired, otherwise serious hazards may be created. Critical points for leaks are flanges, valves, vents (from process vessels, pneumatic product transfer systems, etc.), relief valves, seals on pumps, compressors, agitator shafts and manways. It may happen that valves, supposedly shut, leak continuously leading to a build-up of air concentrations of chemical agents. In fact, whenever technically feasible, these systems should be in open air in order to reduce this possibility of accumulation of air contaminants.
Vent stacks and relief valves may need to be modified or relocated in order to prevent chemicals re-entering the workplace. In the case of toxic chemicals, care must be taken not to cause air pollution problems by discharging them directly to the atmosphere; systems can be equipped with vent scrubbers or similar installations. All efforts should be made to control any fugitive emissions from a closed system. This matter is well discussed in the specialized literature.
Enclosures. An entire process, part of a process or specific contaminant sources (e.g. pumps that usually leak) can be enclosed to prevent the escape of contaminants into the workroom air. Enclosed spaces should be kept under negative pressure. Enclosures, combined with local exhaust ventilation, constitute one of the best solutions available for the control of very hazardous air contaminants.
Processes or operations which need to be completely enclosed can be mechanized or automated and performed through remote control, or can be handled by means of gloved inlets.
Attention should be to the following aspects:
in a complete enclosure, heat build-up may be a problem depending on the process and this fact should be taken into consideration when the ventilation for the enclosure is designed;
maintenance and repair work inside enclosed spaces requires special procedures, including the use of adequate personal protective equipment.
Whenever total enclosures are not feasible, partial enclosures may be used; the approach to their design is to imagine a total enclosure and then remove the minimum possible to permit the performance of the operation. These are effective in combination with local exhaust ventilation systems; in fact, the partial enclosure constitutes the hood for the system. Examples of enclosures for different operations and as part of ventilation systems can be found in the specialized literature.
Separating walls (isolated areas and cabins). Whenever there are, in a workplace, operations that are more hazardous than others, they should be localized and separated through adequate isolation.
Hazardous areas can be restricted to a few workers who are then adequately protected (personal protection, limitation of exposure, etc., and subjected to medical supervision). Besides, in a restricted area it may be easier to control the exposures.
Certain highly toxic agents or suspected carcinogens should only be utilized in completely isolated areas which are then marked by signs of "controlled area", indicating the nature of the hazard. Only authorized personnel, adequately equipped, individually protected and under strict medical supervision should be allowed to enter. Special procedures for entry and exit should be carefully followed including adequate locker rooms, shower facilities and used garment disposal. The pressure in such isolated areas should be negative so that air can only come in and not out.
The isolation of workers is also possible as is the case with control cabins where a positive pressure (by introduction of clean air) ensures that air contaminants do not enter and special walls and windows keep out agents such as radiant heat and noise.
Distance. It may be desirable to perform operations which create health hazards at a distant location. Then the only workers present would be those involved with the operation and they should be individually protected. This is not always technically possible and, even when it is, the problem of environmental pollution should not be overlooked.
Time. It may be desirable to perform certain hazardous operations out of the regular shift hours, when the workers not involved with it do not need to be present. Those performing the hazardous task should be individually protected, and should never be alone. In order to use this measure, the process must permit displacement in time and, besides, there must be administrative provisions for the special work schedule.
Ventilation
Ventilation in workplaces can be used for one of the three following purposes:
to ensure conditions of thermal comfort;
to renew the workplace air, therefore diluting eventual air contaminants to acceptable levels;
to prevent hazardous air contaminants from reaching the workers' breathing zone.
Because of its importance and its specialized nature, industrial ventilation is discussed separately in more detail under VENTILATION, INDUSTRIAL. Only some basic considerations on this extensive subject are presented here.
General or dilution ventilation. From the point of view of the control of air contaminants, general or dilution ventilation aims at the renewal of the air in the work environment so that the possible contaminants are diluted to levels considered to be not harmful to health. However, general ventilation, as a means of controlling exposures to air contaminants, has limitations and can be accepted provided that:
the contaminants in question are of low degree of toxicity or constitute only a nuisance (control of toxic chemicals and particles cannot be achieved only through dilution ventilation);
the quantity of contaminants generated in the workplace is not too great (otherwise the air volumes required for adequate dilution would be impractical);
workers are far enough from contaminant sources, unless contaminants are given off at very low concentrations (anyway the air concentrations at the breathing zone of workers should be below the maximum permissible levels);
the evolution of contaminants is reasonably uniform.
Whenever general or dilution ventilation is being planned, besides the calculations such as required air volumes, fan power, etc., important points to consider are the following:
lay-out of equipment and operations in relation to air inlets and outlets, or vice-versa (depending on when the system is designed), should be such that the air contaminants are always swept away from the breathing zone of workers;
the relative location of air inlets and outlets should never permit either short-circuits of air or the formation of strong cross-draughts; the latter may not only cause discomfort but also disturb the performance of local exhaust ventilation systems (if there are);
the quality of the air introduced in the workplace should be considered, from the point of view of both eventual pollution and temperature (these factors may need to be corrected).
Local exhaust ventilation. Local exhaust ventilation aims at the removal of the air contaminants from the working environment before they can reach the breathing zone of workers in harmful concentrations. Particularly in combination with adequate enclosures, it is the most efficient engineering control measure for airborne chemical agents and particulates in the working environment. Local exhaust ventilation is usually complemented by general ventilation. Since appreciable amounts of air are removed from the workplaces through ventilation. adequate make-up air should be supplied. In fact, supply and exhaust may be combined into "push-pull" systems which can be quite efficient, although expensive.
The basic elements of a local exhaust ventilation system (see EXHAUST SYSTEMS) are: hood, ductwork, fan and collector. Hoods should enclose as much as the process permits (see "partial enclosures"). The most efficient are the enclosing hoods (air contaminants are generated within the confines of the hood while the worker(s) is (are) outside). Whenever exterior hoods are used, capture distances should be as short as possible.
In certain situations, where it may seem difficult to install a ventilation system because of the mobility of the process and/or equipment, flexible ducts may be the solution. There are also portable exhaust systems (including the air mover) which can be used to ventilate, for example, tanks which contained chemicals before cleaning, underground sewer passageways which need to be inspected through the manholes, welding operations in confined spaces, underground operations that may give off chemicals, etc. (For most of these operations the use of adequate respiratory protection is required in addition.) Whenever this type of portable exhaust ventilation system is utilized, care must be taken with the disposal of the polluted air brought out, there may be a need for the use of air cleaning devices. Such portable exhaust units should be used when there is the possibility of throwing the exhausted air outside, and not in situations where the exhausted air would be discharged indoors, since it would be extremely difficult to clean this air to an acceptable standard, particularly when dealing with contaminants in the gaseous state.
Before designing a ventilation system, all hazard sources must be well defined, critical points being the generation of contaminants (e.g. open tanks) and points of dispersion such as transfer of materials (e.g. bag and for drum filling and emptying, pouring of molten metal, transfer of solids from conveyor belts to hoppers, etc.) and opening of units such as drying ovens or closed vessels (e.g. reactors).
Types of hoods for specific operations and the design of ventilation systems are well described in the specialized literature. However, the actual design of a ventilation system should be the responsibility of specialized engineers. Errors at this stage will either be noticed after the implementation of the system and, therefore, expensive to repair or, worse, will not be noticed until the manifestation of health effects resulting from exposure to hazardous agents believed to be under control.
In addition to considerations such as hood design adequate air exhaust volumes, duct velocities, capture velocities, etc., certain aspects should not be overlooked for example:
hot processes require special ventilation designs which account for this factor;
air currents (cross-draughts) may disturb ventilation patterns. When dealing with canopy hoods, for example, even relatively light cross-draughts may require an appreciable increase in duct velocities in order to maintain the required hood suction power. Open windows in summer months may alter the performance of an otherwise efficient exhaust ventilation system;
there should never be the possibility for the breathing zone of workers to remain between the point of contaminant generation or release and the point of collection. Should this be absolutely necessary, for instance for a repair, either the operation should be stopped or adequate individual protection provided:
new additions to old systems should be carefully studied and the modified system should be recalculated; even if additions are possible, changes in the fan may be necessary. Also, the need for additional make-up air should not be overlooked:
it is very unlikely that a system designed to handle air contaminants in the gaseous state could be used for particulates without modifications, since high velocities are necessary for an air stream to hold particles;
as is the case with any control measure, air monitoring should be carried out periodically in order to ensure the continuous efficiency of the system;
adequate maintenance is also essential; ducts may become perforated due to rust or corrosive atmospheres, dust may accumulate at elbows, air movers may become clogged, etc.;
harmful agents removed from the working environment should not be discharged into the general environment, therefore exhaust ventilation systems must include appropriate collectors (see AIR POLLUTION).
Wet methods
The control of dust dispersion into the working environment can be successfully achieved, in some instances, through the use of water and wetting agents.
Wet methods are particularly efficient when the water is introduced at the point of dust generation so that the particles become wetted before having a chance to disperse into the ambient air. This is the case of wet drilling which has been widely used to reduce dust exposures in, for example, mines and quarries. Many studies have shown sharp decreases in the occurrence of silicosis in granite quarries and in mines in the years d following the introduction of wet drilling. Whenever a choice is possible, wet drilling should be selected over dry drilling. A great variety of wet drills is available in the market, as well as pneumatic jackhammers with continuous-flow water attachments. However, even when drilling is wet, there may be some dust exposure due to the fact that the dust, which is originally dry, is not always completely wetted and retained; besides, for certain positions of the drill (e.g. overhead drilling), the amount of water in the drilling hole may not be sufficient. Therefore, ventilation may be needed as a complement. Also, whenever using wet methods, the evaporation of the dust-laden water may constitute a secondary dust source which must be considered and avoided or controlled.
Another type of wet method is the use of water sprays, which cause the dust to agglomerate in heavy particles and deposit. The water droplets should not be too large in relation to the dust particles (usually not more than 100 times) in order to ensure a good contact. Water sprays are used, for example, in mines after blasting, over rocks and ores which must be transported, in crushers, over transfer points of dusty materials, as a "curtain" to confine dust to certain areas and prevent it from dispersing over large portions of the working environment, and other critical points..
However, the use of water sprays is not always effective, particularly for the control of the very fine "respirable" dust. One problem is that it is difficult to obtain an intimate contact between dust particles and water droplets (unless the dust is coarse); besides, due to the movement of the dusty material (e.g. crushed ores transported on conveyor belts), dry areas may become continuously exposed and dust may be liberated before becoming wet. In such cases continuous application of the water spray as the material moves and dry dust is likely to be released may help the control of, particularly. less fine dust. Whenever the dust in question may cause a serious health effect by penetrating into the alveoli (e.g. silicosis), the control of the "respirable" fraction is essential and air monitoring should be carried out even if the visual impression is that the water spray does suppress the dust.
Among other techniques which have been successfully used, wet abrasive blasting should be mentioned. On the other hand, wet grinding is not always efficient to control dust which can escape before becoming adequately wetted, due to the velocity of its generation; in addition, there is the problem that the dust-laden water is thrown off as fine droplets which can evaporate before falling to the floor, thus liberating the dust.
The use of water is very important in the cleaning of dusty workplaces particularly when vacuum cleaning equipment is not available.
Some aspects to be considered when planning the use of wet methods are the following:
technical feasibility, which also includes the non interference of water with the process;
the dust should be "wettable”;
thermal environment, since the increase in the ambient humidity due to the use of wet methods can create or aggravate heat stress problems;
adequate disposal of the dust-laden water which would eventually evaporate and release the dust thus constituting a secondary dust source.
The efficiency of wet methods depends on how completely the wetting of the particles is achieved. Wetting agents, which improve the spread of water over a surface, can be used. However, monitoring of airborne dust is an essential complement of this method, it should be kept in mind that the most difficult portion of dust to control by wet methods is the "respirable" fraction, which is also invisible to the naked eye and, usually, the most harmful to health. When necessary, the use of wet methods should be complemented by other control measures, mainly ventilation.
Good housekeeping, storage and labeling
Good housekeeping and maintenance. This includes cleanliness of the workplace and machinery as well as adequate waste disposal, and may contribute appreciably to keeping down the exposures to chemical agents and dust (see HOUSEKEEPING AND MAINTENANCE. MAINTENANCE OF MACHINERY AND EQUIPMENT)
As examples of such practices the following can be mentioned:
cleaning up of spills before they have a chance to evaporate into the workroom air;
adequate and immediate disposal of solvent-soaked rags as well as of empty containers and bags likely to contain residues of toxic chemicals and dusts;
periodic cleaning of the workplace (with water or vacuum) in order to avoid accumulation of dust on beams, machinery, window sills, etc.;
keeping all containers with volatile chemicals tightly closed;
organization and general cleanliness, avoidance of obstructed passages and adequate disposal of trash.
Non-compliance with such fundamental and apparently simple good housekeeping rules can lead not only to health but also to safety and fire hazards, besides having a negative effect on the morale of the workers. As an example can be mentioned the case of a plant where a high prevalence of dermatitis was observed among control cabin operators who apparently were not in contact with any dermatitis-producing agents; however, the railings of the stairs used to reach the cabins were covered with grease, due to poor housekeeping.
The importance of good housekeeping cannot be overemphasized and provisions to enable its practice should be made preferably at the building stage and should include:
smooth surfaces (walls and floors); porous surfaces should be avoided and cracks immediately repaired, particularly where chemicals are used;
facilities for adequate cleaning. e.g. water supply. steam (if needed), vacuum cleaning (dust should never be blown with compressed air or removed with dry sweeping);
slightly slanted floors and canalisations to collect washing waters adequately;
facilities for adequate waste disposal with special provisions for chemical and toxic wastes (e.g. reactive chemicals should not be thrown out together. toxic effluents should be treated, etc.).
Another aspect which must be considered is the maintenance of the workplace, equipment, machinery, etc., as well as of the control measures, it is not enough to build a safe system: it also has to be kept safe.
Storage. Storage of raw materials, chemicals and products in appropriate places and adequate containers is essential both for health and safety reasons (see DANGEROUS SUBSTANCES, STORAGE OF). Containers should be preferably unbreakable. Another consideration is to avoid any chemical reactions and/or leakages. Particularly for volatile chemicals (e.g. solvents), containers should have well fitted lids.
Whenever storing chemicals, special attention should be paid to the possibility of accidental chemical reactions; for example, cyanides and acids should never be stored together.
It is a mistake to assume that storage rooms are hazard free areas since, depending on the materials or products stored, there may be a build-up of airborne chemicals to dangerous concentrations, particularly if the ventilation is poor (which is often the case). As examples can be mentioned the formaldehyde released during the storage of durable press fabrics which were treated by the "glyoxal-formaldehyde process" or the vinyl chloride (unpolymerised) released during the storage of granulated PVC. Unless the storage area is well ventilated, adequate precautions should be taken when entering. particularly if the chemicals likely to be released are fast acting and have ceiling values for permissible exposure levels. Intermittent exhaust ventilation, to be turned on some time before entering the storage area, can be a solution. Another solution would be the use of personal protection.
Labeling. Adequate labeling of any chemical agent container is of the utmost importance. Labels should indicate, clearly and in the language of the users, the degree of toxicity of the chemical in question, possible routes of entry, main symptoms, safety and fire hazards, possible dangerous reactions, main precautions for use, and first-aid procedures in the case of overexposure or ingestion. Adequate symbols (e.g. fire, corrosive liquids, explosives, etc.) and other visual messages on labels are very important (see DANGEROUS SUBSTANCES, LABELING AND MARKING OF).
Personal control measures
Work practices
These include specific work procedures designed to minimize the generation of and/or the exposure to hazardous agents in the working environment. Work processes and associated health hazards should be carefully studied with the objective of determining which exposures result from workers' carelessness or mistakes, and which procedures could be changed, and how, in order to decrease the hazards. Experienced workers, if adequately instructed on the potential health hazards, can make valuable contributions to the selection of safe work practices.
Although work practices depend a great deal on the workers' training and collaboration, the responsibility for them lies also with management since there may be need for administrative provisions to allow workers to carry out their tasks in the safest possible way, which is not necessarily the fastest.
Basic principles for good work practices include the following:
minimisation of the time during which a chemical agent has the possibility to evolve into the workroom air, for example:
(a) by reducing to a minimum the time during which any container with volatile materials, or reactors (polymerisation), or drying ovens, or dry cleaning washers and dryers, etc., remain open;
(b) by reducing as much as possible the time during which materials and products which give off air contaminants remain exposed to the workroom air (e.g. in dry cleaning operations, the faster the "damp load" is carried from the washer to the dryer, the better);
removal of certain products and wastes which liberate air contaminants from the working environment as soon as the process permits, for example:
(a) in foundries, pre-cleaning of castings by shot blasting (adequately controlled) right after the "shake-out", so that there will be less chance of dust being released into the workroom air during the transportation and handling of castings;
(b) in certain plastics industries, immediate removal of any crust and other scrap material cleaned from the reactors to an adequate disposal place outside the workroom (concentrations of vinyl chloride up to 100 ppm have been observed in cases when the reactor cleaners just piled up this type of scrap material in the workplace);
avoidance of possible undesirable chemical reactions and accidental formation of toxic by-products. For example, caustic soda should not be mixed with acid solutions; cleaning agents containing ammonia should not be used together with chlorine bleaches nitric acid should not come into contact with organic matter, such as wood; arsenic-containing materials should not come into contact with strong acids (unless special exhaust ventilation is provided), in pickling (acid) operations, acid should always be added to water (not the contrary), etc.;
avoidance of carelessness in closing containers. valves, ovens, etc.; container lids should be tightly closed and kept on while not in use, doors should be completely shut, etc.;
adequate handling of materials, particularly chemicals, for example, transfers from one container to another as well as transport of such containers, should be carried out with strict precautions in order to avoid losses to the workroom air or accidental skin and/or clothing contacts, thus introducing or aggravating a health hazard. It has occurred that workers who usually carry out a well controlled operation were overexposed to hazardous chemicals during sporadic careless handling (e.g. weighing. loading transfers, etc.). In such situations, it may happen that the exposures are evaluated during the normal operation with satisfactory results, while serious overexposures occur at particular times;
suitable speed in performing certain tasks, for example the removal of the basket with cleaned metal parts from the vapor zone in a degreasing should be very slow so that solvent vapor is not entrained into the workroom air;
leaving adequate time, for instance before opening a drying oven or a dryer (time should be set taking into account the drying time for the conditions in question), or before re-entering a certain area in a mine after blasting (time should be allowed for the dust to settle);
avoidance of skin contact with chemicals, especially those which can affect the skin (cancer, dermatoses), or which can penetrate through intact skin.
Practices already mentioned under good housekeeping (e.g. immediate cleaning of spills), as well as strict personal hygiene and the adequate use of required air personal protection must be part of the work practices adopted.
Training workers in adequate and safe work practices should be a responsibility of management and given during working hours. As already mentioned, administrative controls are also essential.
Personal protective equipment
The worker can be isolated from the hazardous environment by means of personal protective equipment, which may be classified into two categories:
personal protective equipment required for specific Occupations, regardless of the utilization of environmental control measures, and
personal protective equipment used to protect workers from hazards which can be efficiently controlled by means of environmental control measures.
The first category would include, for example, hard hats for construction work and heavy industry, safety glasses for work with lathes, grinding wheels and chemical laboratories, protective gloves for handling sharp-edged pieces, face shields and gloves for welding. impervious clothing against chemical splashes and many others, aiming mainly at the control of safety hazards.
The second category would include, for example, respirators to prevent the inhalation of toxic air contaminants which could have been removed by local exhaust ventilation, ear protection for work at a machine which could have been successfully enclosed, etc. The utilization of this type of personal protective equipment should be acceptable only under the following circumstances:
while environmental control measures are being designed and implemented, in which case it should be considered as a temporary solution;
when environmental control measures are technically infeasible, for example painting a bridge or certain types of airport work;
for operations of short duration, for example entering a generator room to make a check;
for sporadic operations such as maintenance and repairs, for example replacing the refractory lining in a furnace, cleaning of a tank, welding in a confined space, etc.;
for operations involving a very small number of workers and which are technically and financially very difficult to control through environmental measures.
In such situations it may be acceptable to isolate the operation and protect the few workers involved through personal protective equipment, limitation of exposure time (e.g. through rotation of workers) and medical supervision.
It should be kept in mind that equipment such as mask-type respirators, ear muffs, impervious clothing. etc.. may be extremely uncomfortable to wear, particularly in hot weather. Therefore, there may be a need for a reduction in the working hours, at least at the operation requiring the personal protection; administrative provisions should make this possible.
For the different types of personal protective equipment see the relevant separate articles. As regards the philosophy of utilization of personal protective equipment in the context of a comprehensive control technology programme, it should always be kept in mind that personal protective equipment has to be:
adequate for the hazards in question. For example, respirators with mechanical filters for dust do not protect against air contaminants in the gaseous state, rubber gloves should not be worn for work with organic solvents, etc. In the case of barrier creams and lotions, special care should be taken in their selection and tests carried out to eliminate the possibility of eventual allergic reactions.
of proven good quality and efficiency. All personal protective equipment should be adequately tested for efficiency. If the equipment does not meet the minimum performance requirements, the workers wearing it will have a false sense of security and will be more likely to overexpose themselves than if they did not wear any protection.
resistant to air contaminants. For example. respirators made of rubber might be attacked by organic solvent vapors, with the resulting formation of cracks and therefore leaks.
checked for validity. For example, chemical cartridges, activated charcoal filters and similar air purifying devices utilized in respirators have a limited validity, which should be monitored; mechanical filters for dust become clogged after a certain time, etc. Such devices should be periodically changed.
fitted to the worker concerned. Mask-type respirators which do not fit the worker's face perfectly permit leakages of contaminated air; loose ear plugs will let the noise pass through, etc.
well maintained and cleaned, as well as routinely inspected. There must be facilities for cleaning and disinfecting personal protective equipment. If the equipment deteriorates (e.g. cracks, missing pieces, etc.). it should be totally or partially replaced.
In addition, workers utilizing personal protective equipment should be adequately trained, educated and motivated.
Limitation of the exposure time
The reduction of the time during which a worker is exposed to a certain hazardous agent may greatly reduce the health hazard involved. This can be achieved through work practices, rotation of workers or administrative controls. One definition of "administrative controls" is "provisions to enable adjustments of the work schedule to reduce exposure". Limitation of the time during which workers wear certain cumbersome types of personal protective equipment (e.g. masks) is also a recommended procedure.
Personal hygiene
Personal hygiene is of particular importance for workers involved with chemical agents and particulate matter. It means cleanliness of both the person and his clothing. Not only should workers be instructed and motivated as to its value but adequate facilities for this purpose should be provided at the workplace. It is useless to require workers to take a shower after work if a sufficient number of showers is not available in the workplace or if, in a cold country, hot water is not provided in winter. And, to take a shower is of limited value if contaminated clothes continue to be worn without adequate laundering.
Adequate locker rooms and, in the case of work with hazardous materials, adequate disposal and laundry facilities for used garments at the workplace are also essential. Clothing contaminated with toxic materials should never be taken home.
Whenever there is the hazard of a skin effect (e.g. cancer, dermatoses) or when dealing with chemicals which can penetrate through intact skin, workers should wash immediately after any contact and not wait until the end of the shift. Depending on the degree of hazard, contaminated clothing should be immediately changed.
In certain situations there may be a need for special soaps and cleaning agents.
Other measures as important for the maintenance and promotion of the workers' health are the pre-employment medical examinations and adequate job placement, periodic medical examinations including biological monitoring and early detection of health impairment, as well as health education - for workers and management - and the application of safety and ergonomic principles. These measures are discussed in detail under the relevant separate articles.
However, control technology and medical control, as well as health education, safety and ergonomic aspects should be integrated into a consolidated programme.
The efficient control of occupational exposure to harmful agents requires a multidisciplinary approach in which environmental and medical sciences complement each other in order to prevent adverse health effects at the workplace.
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Series on evaluation of occupational health hazard control technology published by the National Institute for Occupational Safety and Health, 4676 Columbia Parkway, Cincinnati, Ohio. Among the major industrial sectors and processes covered, see:
Cotton dust control in yarn manufacturing. No. 74-114 (1974). 188 p. Plastics and resins industry. No. 78-159 (1978). 242 p. Control of exposure to metal working fluids. No. 78 165 (1978). 40 p. Manufacture and formulation of pesticides. No. 78-174 (1978), 440 p. Assessment of selected control technology techniques for welding fumes. No. 79 125 (1979). 32 p. Foundry industry. No. 79-114 (1979). 438 p. Dry cleaning. No. 80-136 (1980).
The industrial environment-its evaluation and control. DHEW (NIOSH) publication No. 74-117 (National Institute for Occupational Safety and Health, 4676 Columbia Parkway. Cincinnati) (1974), 719 p.
Workplace control of carcinogens. Proceedings of a topical symposium (Cincinnati, American Conference of Governmental Industrial Hygienists, Oct. 1976), 159 p. Illus. Ref.
Engineering control research recommendations. DHEW (NIOSH) publication No. 76-180 (National Institute for Occupational Safety and Health, 4676 Columbia Parkway. Cincinnati) (1976), 216 p. Illus. 181 ref.
Industrial ventilation-A manual of recommended practice. Committee on Industrial Ventilation (Lansing. American Conference of Governmental Industrial Hygienists, 16th ed.. 1980), 328 p. Illus. 108 ref.
Engineering of industrial ventilation (Engenharia de Ventilação industrial). Mesquita. A.; Guimarães, F., Nefussi, N. (São Paulo, Edgar Blücher, 1st ed., 1977), 442 p. Illus. 39 ref. (In Portuguese)