Eating, Studying, and Other Social Activities

a problem for extended periods of time. In such cases, the workers need to be protected while

doing the work, using appropriate items of protective clothing such as gloves, respirators,

coveralls, and goggles or full-face respirators, depending upon the level of risk. The levels will

depend upon the contaminants that might exist due to work in the laboratory, and these risks

should be evaluated conservatively to enhance safety of the workers.

Where fume hood exhausts are brought to the roof through individual ducts, the area in which

maintenance is needed may be surrounded by exhaust ducts still in use, since in most cases it

is impractical to shut off operations for an entire building or even a significant portion of one,

because it is too disruptive to the research programs. Therefore, it is probably desirable to have

a standard personnel protective equipment package for the maintenance workers to use,

consisting of half- or full-face respirators providing protection against solvents, particulates, and

inorganic acids, chemically resistant coveralls, and gloves selected to provide a broad spectrum

of protection against chemicals. Requiring personnel to wear these may appear to be excessively

cautious but, as noted earlier, there have been instances where unanticipated severe and longlasting health effects have occurred. Not all employees who work on the roof of a chemistry

building may be employees of the organization. Outside contractors also are used to do a variety

of maintenance duties and, under the hazard communication standard, they must be apprized of

the risks to which they might be exposed. The mix of materials exhausted through ducts is

typically so complex that meeting this requirement is difficult, if not impossible. Recommending

to them to wear equivalent protection should fulfill the spirit of the standard. Unfortunately,

maintenance personnel may scoff at the need to wear protective equipment, or alternatively, be

so fearful of exposure that they may refuse to perform the needed task. It is the responsibility

of the organization to provide sufficient indoctrination and enforcement of their personnel

protection policies that both of these eventualities can be avoided.

Fume hood maintenance is one of the more active areas in which maintenance personnel have

concerns and where both support and laboratory personnel need to assume responsibility for

seeing that the work is properly coordinated. Some simple suggestions that have been found

useful are to ensure that each exhaust duct on the roof is properly labeled with the room location

of the hood itself. Workers have been known to turn off power to motors on hoods in active use.

Where hoods are dedicated to special uses which represent unusual hazards, such as radioactive

materials, perchloric acid, exceptionally toxic gases, or any other especially unusual risk, the duct

should also be labeled with the application involved or a color code employed to identify these

unusual risks. The latter program would alert maintenance personnel to definitely contact the

laboratory from which the duct came before working in the vicinity of the duct. Power to the

motors on the roof should also be provided in such a way as to ensure that the workers on the

roof can completely control the circuits while working to avoid accidental activation of the circuits

from the laboratory. However, should the exhaust motor be turned off by maintenance workers

without prior notification of laboratory personnel, an alarm should sound in the laboratory

warning that the hood is not functional. A tagging and lock-out procedure should also be

employed during the maintenance operation.

Once hoods are removed from service to perform maintenance, they should not be returned

to use until it is verified that they are performing according to required standards. It is easy to

erroneously wire a three-phase motor so that the fan rotates opposite to the desired direction.

Belts may need to be tightened or a pulley size changed to achieve the proper face velocity.

Fume hoods have been used to illustrate some of the problems that can arise from lack of

coordination of maintenance and laboratory personnel, but there are many other possible

problems. Explosions can occur if gas service is turned off without everyone being aware of it

and they leave gas jets open, flooding a facility with gas when service is restored. St ills can

overheat if condenser water supplies are interrupted. Electrical service to an area should be

discontinued and restored only with full prior notification to all persons that might be affected.

Today, with the large amount of computer automation being used, interrupting the power to



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the facility may disrupt the entire operation and bring down the wrath of the scientist on the

worker who caused the problem. Any computer equipment with such a critical function should

be equipped with an uninterruptible power supply with sufficient capacity to allow a safe

managed shutdown procedure. These have become relatively inexpensive. Individual laboratory

technicians or students often modify their facility without informing the groups responsible for

maintenance, thereby raising the possibility of an injury to an unsuspecting service-pers on, or

make a repair which will not be based on an accurate assessment of the conditions which could

affect their work. Also, the as-built drawings for many buildings do not reflect reality due to

change orders during construction which are not added to the drawings. If discrepancies are

known, maintenance personnel should be notified prior to beginning work if the differences could

impact the job.

4. Housekeeping

Another maintenance issue is what reasonably should be expected of custodians. Experience

has shown that there is a tremendous variation in the level of expectations and wishes among

laboratory supervisors. There are those who do not wish custodians to enter their laboratory at

all, while there are those who have no qualms in asking custodians to clean up a hazardous

chemical spill. Most safety and laboratory personnel would agree that the latter is asking too

much, while most would also agree that, if they wish, facility personnel should be allowed to take

care of their own housekeeping, as long as reasonable standards of cleanliness are maintained.

Most laboratory groups, however, fall somewhere between these two extremes.

The salary levels of most custodial positions are usually among the lowest in most

organizations and limit the skill levels one can expect from the persons filling the positions.

Unfortunately, literacy rates are often less than average and, in many cases, it certainly would

be unrealistic to expect a custodial worker to have a significant level of technical training which

would permit an understanding of the problems that they might encounter in a laboratory. As

a result, custodial workers are often quite afraid of the laboratory environment. However,

alternative positions are also usually hard to find for these employees, so they frequently are very

concerned about losing their jobs. Most cannot afford to do so. As a result of these conflicting

pressures, they may attempt to do things they really do not understand and are afraid to ask

about, and may make mistakes in consequence. It is the responsibility of the laboratory

supervisor, working with custodial management, to carefully establish safe constraints on the

areas of responsibility for the custodians in the laboratory.

Among things a custodian can reasonably be expected to do in most laboratories are:

1. Clean and maintain the floor area.

2. Dispose of ordinary trash. However, if other than ordinary solid waste is generated in the

laboratory, it should be placed in distinctively shaped and/or colored containers. If the

custodians are still expected to handle it, then the circumstances and procedures should

be carefully delineated and training given. This latter responsibility is not recommended.

3. Wash windows. If they are expected to wash bench tops or other laboratory furniture,

it should be only when additional supervision is provided by laboratory personnel.

Among items which they should not be expected to do are:

1. Clean up chemical spills. They are not trained to do it according to established regulatory

guides nor to do it in such a way as to ensure that they do not expose themselves to the

potential injury.

2. Dispose of broken glass, syringes, or “empty reagent containers.” These items can be

disposed of by them if they are carefully prepared by the laboratory workers in advance.

For example, broken glass should be disposed of by custodians only if it is placed in a

sturdy kraft board box (or equivalent), sealed, and labeled as “broken glass.” Other items,



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such as syringes, are now considered to be regulated medical wastes in most instances,

as, for example, if they fall under the provisions of the bloodborne pathogen standard.

Syringes are to be placed in a leak- and puncture-proof container specifically intended

for them. The needles are not to be sheared, broken, or removed in any way. The

containers are to be rendered biologically safe and disposed of by techniques which will

be discussed in detail in the section on infectious waste disposal. Custodians are explicitly

not to handle these wastes. Empty reagent bottles should be triple rinsed and then placed

in a box labeled “triple rinsed reagent containers.” All of the restrictions on glassware

s hould be thoroughly explained to the custodial employees. There are specific

requirements under the bloodborne pathogen standard that this be done. If custodians

believe that they are being asked to handle unsafe waste, they should ask their supervisors to intercede for them.

3. Handle special wastes in any way including radioactive materials, chemical wastes, or

contaminated biological materials. All of these require special handling by specialists and

precautions must be taken to ensure that these materials are not accidentally collected

by custodians. The custodians should be given awareness training to ensure that they

have sufficient knowledge to allow them to recognize these special wastes.

4. Clean the work surfaces and equipment in the laboratory, except in special circumstances

and under the direct supervision of a responsible laboratory employee. Even in this case,

a preparatory program should have been carried out in advance by laboratory personnel

to remove or secure items which could be dangerous in the area being cleaned.

Housekeeping also means maintaining the laboratory in a reasonably organized fashion on

a day-to-day basis. This is the responsibility of all laboratory personnel, but individuals will

follow the laboratory manager's own performance as a guide. Reagents not in use should be

returned to proper storage. Secondary containers should be labeled according to the requirements

of the hazard communications standard. Glassware should be cleaned and put away. Trash should

not be allowed to accumulate. Equipment should not be allowed to encroach upon aisles. Cables

and temporary electrical extensions should not become a tripping hazard. Periodically,

refrigerators and other storage units should be gone through and cleaned out. An audit of

materials should be made periodically to dispose of old, degraded, and obsolete materials before

they become a hazard. Chemicals stored inappropriately outside of their hazard class should be

returned to their proper locations. Bottles heavily covered with dust, indicating a lack of use for

an extended period, are likely to remain unused and should be eliminated. No one should expect

a busy laboratory to be spotless, but neither should it be a disaster area. Unless a concerted effort

is made, eventually housekeeping problems tend to slowly accumulate. An effective mechanism

used by the author to combat this erosion of order was to schedule a quarterly “field day” during

which all personnel, including faculty, staff, and students, ceased research and returned

everything to reasonable order. This rarely took more than a few hours and furthered a sense of

cooperation between the various groups of people.

5. Signs and Symbols

Many situations exist in which a person entering an area needs to be made aware that a hazard

exists in the area or needs to know of restrictions placed on persons entering the area. In addition,

there are signs which are intended to provide information to individuals in an emergency. There

are literally hundreds of specialized safety signs and symbols which can be purchased for the

laboratory. Given below is a partial list of some of the more important ones, along with a brief

description of the types of applications for which they would be needed. In many cases, the signs

in this list are mandated by regulatory requirements, while in other cases they represent common

sense safety practices. In most cases, the hazard signs will be prefaced by a risk descriptor,

defining the level of risk represented in the specific instance. The three cautionary words in



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Figure 4.7 Radiation safety symbol.



Figure 4.8 Biological hazard symbol.



normal use, in decreasing order of risk are DANGER, WARNING, and CAUTION.

1. AREA UNSAFE FOR OCCUPANCY - This is used to indicate a contaminated area or an

area otherwise rendered unsafe, temporarily or otherwise, for normal use.

2. AIRBORNE RADIOACTIVITY AREA - Some applications involving radioactive materials

result in the generation of airborne radioactive materials in excess of those permitted by

the standards of the NRC, or of the equivalent state agency in an agreement state. Should

such an operation exist, the boundaries of the room, enclosure, or operating

area where the airborne material may exist must be posted with this sign. The legend will

be accompanied by the standard radiation symbol shown in Figure 4.7.

3. ASBESTOS - Asbestos is still used in a number of products employed in laboratories.

If there is the potential while using these products that asbestos fibers may become

airborne, the area needs to be marked with a sign:

CAUTION

ASBESTOS-CONTAINING MATERIAL PRESENT

Note that a sign such as this does not say that there are asbestos fibers in the air. The

sign is intended to alert people that their actions could result in the generation of

airborne asbestos fibers. If there is a risk that airborne asbestos fibers may be present,

the appropriate department needs to be notified (usually either physical plant or health

and safety) to correct the problem.

4. AUTHORIZED ADMISSION ONLY - This sign may accompany many other signs or it

may stand alone in restricting access to an area to those who have legitimate reasons to

be there, or who are aware of the risks within the area to which they may be exposed.

5. BIOLOGICAL HAZARD - The sign will be accompanied by the standard biological

hazard symbol shown in Figure 4.8, indicating that an agent which may prove infections

to human beings is present within the area.

6. CARCINOGENIC AGENT - The laboratory safety standard requires that areas in which

carcinogenic agents are in use be designated as such. This can be done with a sign such

as:

CANCER-SUSPECT AGENT

AUTHORIZED PERSONNEL ONLY



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Figure 4.9 Eye Protection required Pictograph.



Where the agent might be unusually dangerous, the agent would be specified and any

special protective measures needed would be appended.

7. CHEMICAL SPLASH GOGGLES REQUIRED WHILE WORK IN PROGRESS It is

recommended that this sign (Figure 4.9) be used at the entrances to all active laboratories

where chemicals are employed and are actively being used. In order to enforce the

requirement, care must be taken to select goggles which resist fogging, do not become

oppressively warm while being worn at comfortable room conditions, and



do not exert uncomfortable pressure on the face. They should also accommodate wearing

normal size prescription glasses at the same time. Many goggles which meet the minimum

regulatory standard based on ANSI Z87.1 for impact protection do not meet all of these

practical considerations, but there are several brands that do. When work is not in

progress, or when a person is in an area well separated from the active work area, it may

be permissible, for reasons of comfort, to allow goggles to be removed.*

8. CRYOGENIC LIQUIDS - All containers which contain cryogenic liquids, most commonly

liquid nitrogen (as in the example below) but also other gases maintained at very low

temperatures, should be prominently labeled:

CAUTION

LIQUID NITROGEN

The container of the cryogenic fluid, usually a large flask with walls separated by a

vacuum called a dewar, will also usually be labeled with the cautionary information:

FRAGILE CONTAINER UNDER VACUUM

MAY IMPLODE

9.



10.



EMERGENCY INFORMATION SIGNS - Prominent signs, such as those shown in

Figure 4.10, should be posted near the safety device mentioned to aid in locating them

in an emergency. Symbols can be used in place of or in addition to some of these.

EXPLOSIVES - If explosives are stored in Class 1 magazines, or in outdoor Class 2

magazines, the property must be posted with signs stating,

EXPLOSIVES—KEEP OFF



*



A point needs to be made here, which will not be repeated for reasons of brevity, that signs and rules must

take into account human factors. Otherwise, they are likely to be ignored and weaken compliance with other

rules overall. Unfortunately, the strictures on wearing goggles while performing work in the laboratory seems

to be one less often followed and enforcement is often lax.



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Class 2 magazines must have labels on all sides except the bottom in letters at least 3

inches high,

EXPLOSIVES—KEEP FIRE AWAY

11.



12.



13.

14.



15.



16.



17.

18.



19.



20.



FLAMMABLE MATERIALS - Cabinets containing flammable materials and areas or

rooms where flammable materials are stored or used must be posted with this sign, which

may also be indicated by the symbol shown in Figure 4.10. This sign should always be

accompanied by the NO SMOKING sign, which may be augmented by a standard no

smoking symbol.

HIGH VOLTAGE DANGER - Spaces which contain accessible high voltage panels, such

as switch rooms and electrical closets, should be locked and provided with these signs

to warn persons lacking training and experience in working with high voltage circuits not

to enter. Equipment containing high voltage circuits should also bear the same warning

label.

HYDROGEN -FLAMMABLE GAS, NO SMOKING OR OPEN FLAMES—

This sign must be posted in all areas where hydrogen is used or stored.

INTERLOCKS ON - Equipment with internal hazards, such as X-ray diffraction cameras,

or areas in which the space is rendered unsafe to enter by the presence of a hazard, are

often provided with a fail safe circuit, or interlock, which will turn off the equipment

representing the problem if the circuit is broken. The sign provides a warning that the

interlock is on to prevent access to the hazard.

LASERS - Labeling of lasers should follow 21 CFR 1040, the Federal Laser Product

Performance Standard. The spaces in which lasers are located should also have a similar

warning at the entrance. The label will depend upon the class of laser involved. All of

the labels will include a stylized sunburst symbol, with a tail extending to the left (see

Chapter 5). The signal word CAUTION is to be used with Class II and IIIA laser systems

while the signal word DANGER is to be used for all Class IIIB and Class IV systems.

MACHINE GUARDS IN PLACE - OSHA requires that many machines, such as vacuum

pumps or shop equipment, be provided with guards over the moving parts. Signs should

be posted near these machines to remind employees not to use the equipment if the

guards are not in place.

MICROWAVES - This sign must be posted in any area where it is possible to exceed the

current occupationally legal limit of exposure to microwave electromagnetic radiation.

NO EATING, DRINKING, SMOKING, OR APPLYING COSMETICS -This sign should

be posted wherever toxic materials are used, in the working areas of wet chemistry

laboratories, or in biological laboratories using pathogenic substances.

NO S MOKING - A NO SMOKING sign, as shown in Figure 4.10 must be posted

wherever flammables are in use; where there is a risk of explosion due to the presence

of explosives or from gases, vapors, or dusts; and where toxic materials are in use.

RADIATION AREA - Areas where the radiation exceeds a level established by the NRC

must be posted with this sign (Figure 4.10). If the level exceeds a higher level set by the

NRC, the area must be posted with a HIGH RADIATION AREA s ign. Most of these areas

will be within an area posted with a RESTRICTED AREA— AUTHORIZED ADMISSION

ONLY sign. Specifics on these requirements will be given in Chapter 5. Signs defining

radiation areas should not be used to post areas where radioactive materials are stored

unless radiation levels equal or exceed the stipulated limits. Areas where radioactive

materials are stored should be posted with a CAUTION — RADIOACTIVE MATERIALS

sign.



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21.



22.



23.



24.



25.

26.

27.



RADIOACTIVE WASTE - This is not a sign specifically required by the NRC but is

recommended to denote areas within a laboratory where radioactive waste is temporarily

stored prior to being removed for permanent disposal, in order to help avoid accidental

removal of radioactive waste as part of normal laboratory waste. Much radioactive waste

resembles ordinary trash, such as paper.

REFRIGERATOR (FREEZER) NOT TO BE USED FOR STORAGE OF FLAMMABLES

- All refrigerators or freezers not meeting the standards permitting the storage of

flammable materials (see Chapter 4, Section V.G.) should be marked with this sign.

REFRIGERATORS NOT TO BE USED FOR STORAGE OF FOOD TO BE USED FOR

HUMAN CONSUMPTION - Laboratory refrigeration units used for the storage of

chemicals and biological materials must be posted with this sign to prevent the use of

units to store lunches and other food.

RESPIRATORY PROTECTIVE EQUIPMENT REQUIRED - Wherever airborne

pollutants are present which exceed the PELs established by OSHA, respiratory protection is required (see Figure 4.10). In many cases, AGGIH threshold limit values (TLVs)

are lower than the OSHA PELs and respiratory protection is recommended when the

levels approach these lower limits. It is recommended that in most cases an action level

of half or less of the TLV values be set to accommodate in part the different sensitivity

of individuals to materials.

SAFETY GLASSES REQUIRED - This sign is to be posted wherever there is a risk of

eye injury due primarily to impact.

TOXIC GAS - Areas where toxic gases are used or stored must be posted with this

warning sign.

ULTRAVIOLET LIGHT EYE PROTECTION REQUIRED - This sign should be posted

wherever there is a risk of eye injury due to ultraviolet light emission.



There are many other signs and symbols identifying hazards or denoting specific requirements

to aid in reducing a specific risk. The following generic signs are representative of many of these.

28.



29.



30.



(SPECIFIC ITEM) PERSONAL PROTECTIVE EQUIPMENT REQUIRED - Many other

risks exist which would require specific items of protective equipment. Where these items

are needed, the area should be appropriately posted.

(SPECIFIC) TOXIC OR HAZARDOUS MATERIAL - There are a number of materials

that pose known risks, and the areas in which these materials are used should be posted

with an appropriate sign.

(SPECIFIC) WASTE CHEMICALS ONLY - Disposal of waste chemicals according to

RCRA standards requires that wastes be identifiable, in some cases by class only, but

in most cases it is desirable that wastes not be mixed. Posting of areas or containers with

this sign where several waste streams of different character exist will aid in legal disposal

of waste.



B.



Working Procedures

This section can only touch upon the broad topic of safe laboratory working procedures

because of the immense scope of the subject. The procedure to be followed here is to provide

generic approaches to most of the hazards covered rather than discuss specific instances in

which a given hazard could occur. Some of the more common areas which offer the potential for

mishaps will be covered in some detail, but undoubtedly there will be areas that are considered

comparably important by many that will be touched upon lightly or not at all. Sections will be

devoted to a small number of the more hazardous chemicals to illustrate the precautions that need

to be taken when working with such materials. In addition to physical hazards, such as fire,

electrical hazards, and explosions, health risks will be discussed in some detail, since in many



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cases these are more insidious and less often recognized by many laboratory workers. The

next several sections will be concerned primarily with physical hazards and the latter part of the

chapter will be devoted to short- and long-term aspects of laboratory operations on workers

health.

1. Protection Against Explosions

Unusually careful planning must take place whenever there is any reason to suspect that work

to be undertaken may involve the risk of an explosion. However, not all potentially explosive



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Figure 4.10 Representative group of commercially available emergency, warning, and cautionary signs.



©2000 CRC Press LLC



stations are recognized in advance. Letters from experimenters describing work in which

unexpected explosions occurred can be found in a substantial proportion of issues of Chemical

and Engineering News. Because these incidents were unanticipated, sufficient protective

measures often were not employed; consequently injuries which could have been avoided are

reported in these letters. Explosions may occur under a variety of conditions, the most obvious

being a runaway or exceedingly violent chemical reaction. Other situations could include the

ignition of escaping gases or vapors, ignition of confined vapors with the subsequent rupture

of the containment vessel, rupture of a system due to over-pressure caused by other mechanisms,

or a violent implosion of a large vessel operating below atmospheric pressure. Partial confinement

within a hood can actually enhance the dangerous effects of an explosion; areas in front of the

open face may be damaged more severely than if the explosion were not confined.

Injuries can occur due to the shock wave from a detonation (if the release of energy occurs

at supersonic speeds) or deflagration (if the energy release occurs at subsonic speeds). Most

laboratory reactions belong to the latter class. Hearing loss may result if the shock wave causes

a substantial over-pressure on the eardrums. According to Table C-3.1(a) of NFPA 45, Appendix

C, the equivalent of as little as 1 gram of TNT can rupture the eardrum of a person within 0.75 m

(~2.4 ft), while 10 g is likely to rupture the ears of 50% of persons within 67 cm (2.2 ft) of the

explosion. The shock wave, as a wave, can ‘‘go around" barriers or be reflected and reach areas

that would be shielded from direct line-of-sight interactions. Injuries can occur due to the heat

or flames from the explosion. Fume hood materials should be selected to contain fires occurring

within them. However, if the sash is severely damaged, flames or burning material can escape

through the front opening and the flames may spread to other fuels in the vicinity. Due to this

possibility, flammable materials should not be stored in the open in close proximity to fume

hoods. Respiratory injuries can occur due to inhalation of fumes and reaction products. However,

the most serious hazard is usually flying debris, including fragments of the contain-ment vessel,

other parts of the experimental apparatus, or nearby materials or unreacted chemicals which can

inflict physical injuries. The risk of the latter type of injuries can be reduced by eliminating the

possibility of line-of-sight or single-ricochet paths for missiles from likely sources of an explosion

to workers or to equipment which could be damaged and result in secondary harmful events. The

possibility of extraneous material becoming involved in an explosion is a powerful argument in

favor of not using a hood as a storage area, especially in experimental activities. The reflected

shock wave can act in much the same way as a piece of physical debris in causing damage

external to a fume hood. Overreaction of a worker or involuntary reflexive actions to even minor

explosions can also lead to quite serious secondary injurious incidents.

In addition to immediate injuries, an insufficiently contained explosion can lead to fires or

cause damage sufficient to wipe out expensive apparatus, destroy months or years of research

effort, or even destroy an entire facility. Conservative precautionary measures to reduce the

likelihood of these repercussions are worthwhile from this aspect alone. Ordinary fume hoods

offer a fair amount of protection to the sides and rear of the hood, if they are of good quality with

substantial walls. However, most fume hoods are not intended to provide really significant

explosion protection against a major explosion for a user standing immediately in front of the

hood, although sash materials are usually designed not to contribute to the hazard. This is

accomplished by having the sash material made of either laminated or tempered glass so that the

broken sash will not cut persons standing in front of the hood. The laminated glass may remain

intact but may result in the entire sash being expelled which could represent a hazard. The

tempered glass is more likely to be shattered and contain the explosion less but the small glass



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fragments are normally relatively less dangerous than larger ordinary glass fragments. A hood

with a three-section horizontal sash, where the user stands behind the central section, provides

superior protection to the more common vertical sash hood. If the work to be done involves a

known explosive risk, certainly a hood specifically designed to contain any anticipated explosion,

or to provide safe explosion venting should be employed.

For the majority of laboratories equipped only with ordinary fume hoods, supplementary

measures should be taken to minimize the type of risks described above if a careful analysis of

the planned operation reveals any significant potential for an explosion.

A simple way to reduce the potential risks is to minimize the amount of material involved in

the experiment. The smallest amount sufficient to achieve the desired result should be used. The

trend toward microscale experimentation supports this option. Care should be taken in scaling

up from a preliminary trial run in which minimal quantities were employed. Increasing the amount

of material in use could significantly change the physical parameters so that insuf-ficient energy

removal, inadequate capacity for the reaction products, or excessive pressures could develop

in the scaled-up version of the work and lead to a dangerously unsafe condition. One of the more

violent explosions in the author’s experience was of this last type.

A number of other measures can be taken to enhance the protection of workers against

explosions. Provision of barriers is a straightforward measure. The selection of an appropriat e

barrier will depend upon the circumstances. A variety of factors should be considered.

The strength of the barrier material is clearly an important factor. Tests have been made of

many materials commonly used in laboratory protective barriers and available either in commercial

units or readily amenable for fabrication of custom shielding. Table 4.11 is adapted from a study

by Smith in which each material tested was 0.25 inches or 6.4 millimeters thick. The relative

susceptibility to fracture was measured by either the ASTM D 256 test method or by dropping

balls from various heights. It required 12 to 16 foot pounds of energy to fracture the

polycarbonate material in the ASTM D 256 test. Additional protection can be obtained by

increasing the thickness of the materials used in fabricating the shield, approximately proportional

to the thickness added. An equal thickness of steel would have a relative effectiveness on this

scale of about 40. Resistance to fracture is not the only consideration. Wired glass, for example,

may represent an additional hazard due to the presence of the wire, if shattered. Ordinary

glass should not be used due to the danger of cuts from the flying debris. Methyl methacrylate

is not suitable where high temperatures may occur. However, sheets of methyl methacrylate are

commonly available at moderate cost and can readily be fabricated into custom shields.

Polycarbonate obviously offers considerable strength, but can be damaged by organic solvents.

Steel is resistant to both heat and solvents, but does not offer the desired transparency. However,

there are alternatives to this deficiency such as mirrors, optical devices, or closed-circuit

television. Remotely controlled manipulating devices can be used to control apparatus behind

any shield material.

The simplest types of supplementary protection suitable for moderate risks are commercial

shields which are available from most laboratory supply firms. Shields usually found in catalogs

are of transparent material, most commonly polycarbonate, weighted at the bottom to increase

their stability. Since these are free standing, they often will not remain upright in explosive

incidents and, if the explosion is severe enough, may actually be hurled through the air and cause

injury themselves. Since the scale of an explosion cannot always be accurately estimated, it would

be desirable to secure these shields firmly to the work surface. For small-scale reactions, they

offer a worthwhile degree of added protection. The shields should be located so as to provide

the maximum protection against flying debris, chemicals, or, as noted earlier, external shock wave

interactions. Individuals in the laboratory should be trained to use the shields correctly and not

to move or modify them to improve their convenience in performing tasks, if these changes could

reduce the level of protection.



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