PREVENTION OR CONTROL OF RISKS
1. Elimination of hazards
e.g. closing down unacceptably unsafe plant/machinery, perhaps even by
a prohibition notice.
Noise injury may be prevented by specification in the design of new
Substitution of hazards
e.g. substitution of benzene by cyclohexane or by toluene, or of asbestos
by man made mineral fibre (MMMF), or substitution or of white-spirit based
paints by water based emulsions.
Do these steps abolish the risk?
The widespread substitution of solvents such as toluene and white spirit
for benzene which can cause leukaemia has given rise to the concept of
'safe' solvents. However these substances are only relatively safe, and
their uncontrolled use may lead to neurological or psychological disease.
Similarly, certain fibrous minerals used as substitutes for asbestos may
themselves have carcinogenic potential, and require careful handling.
2. Enclosure/Segregation so as to reduce exposure
e.g. to reduce exposure to fume or vapour, noise or heat e.g by Segregation
workers from the source of harm. This may be by time or distance, as in
mining where workers retire to a safe distance at the time of blasting,
and when such operations take place on shifts when fewer workers are about.
More commonly, however, it involves enclosure of the process so that there
is a physical barrier between the source of harm and the workers.
Can segregation work for all workers?
Unfortunately not - those involved in maintenance for example can be
at particularly high risk. A case has been described in which an
organic dye process was properly enclosed, but fitters required access
for maintenance and repair. One of these men subsequently developed bladder
cancer. Similarly, in a very clean and well-kept bakery the only man who
developed asthma due to flour allergy was the fitter who had to repair
pipes and silos when they burst. Remember, when looking at a well isolated
piece of machinery, to ask how it is maintained or repaired.
3. Local exhaust ventilation etc
Local exhaust ventilation is used where the above methods are impracticable.
Ideally it is combined with partial enclosure of the process, as in the
familiar fume cabinet. Alternatively, the ventilation is applied very close
to the point of generation of the dust or vapour if appropriate to remove
fume or vapour at source e.g. in soldering. The air velocity required to
draw the substance away is called the capture velocity, and this depends
on the physical characteristics of the substance and the mechanisms of
its release. It is intuitively obvious that lower velocities are needed
to clear evaporating chemicals than particles generated by blasting or
What problems could you think of, in relation to local exhaust ventilation?
This is very effective if properly designed and if the extractor fan
is working. A young female technician developed allergic alveolitis and
asthma due to sensitization to a chemical reagent she was preparing in
a fume cupboard in the laboratory of a university medical school. When
the problem was investigated, it was found the extractor fan was not working.
Three men developed acute mercury poisoning while making repairs in the
boiler of a power station. Some other workers in a distant part of the
plant had spilt some mercury which had vaporised. The extract fan in the
system was running in reverse and the vapour was propelled along pipes
to the boiler in which the other men were working.
Reduction of airborne levels of substances may also be achieved by dilution
ventilation or precipitation. A familiar example is the use of water
and high air flows in mines to reduce dust levels.
Does dilution ventilation abolish the risk?
The substance remains in the air, but in lower concentration. Unforeseen
problems may however occur to make matters worse. In some cases, the supply
of conditioned, humidified air to workplaces has resulted in new problems
e.g. from infection, allergy or other effects, e.g. in operating theatres
it has resulted in humidifier fever among theatre staff.
Electrostatic forces may also be used to reduce levels of dust, commonly
in preventing pollution by smoke stacks.
4. Appropriate education, work organisation and practice
Education as well as good housekeeping remain important principles
of prevention in all circumstances. Workers should be made aware of dangers
by instruction, notices, codes of practice and safety audit. Similarly,
managers should be trained in their responsibilities in these respects.
Good housekeeping, keeping chemicals in safe places, not leaving dangerous
materials (or indeed any materials) lying about, making sure that everything
is clearly labelled, vacuuming floors and benches after use, and other
such measures will reduce risks of illness and accident and will ensure
that everyone is safety-conscious.
Other examples include implementation of policies on safe working practice,
regulated hours of work and careful planning of shiftwork
5. Personal protection
The law generally requires all reasonable efforts be made to engineer the
hazard out or to shield individuals from it. Only then is it permissible
to resort to personal protection. Most such devices are not comfortable
for long-term use and give only partial protection. Included in this category
are respirators, protective clothing and gloves, eye shields and hearing
defenders. Clearly in some circumstances they are mandatory - eye protection
in welding and metal work, ear muffs in caulking (which today means hammering
metal in ships' hulls), and so on. However, they should never be relied
upon as the sole, albeit cheap, method of protecting workers. Where they
are used, care should be taken in their design, in terms of comfort and
wearability, as well as in their efficiency in terms of protection.
A good account is available from CCOHS
protective steel toe capped footwear
What problems in relation to personal protective devices can you think
Sometimes personal protection devices may perform significantly worse
than implied by their 'nominal protection factor'. Face masks are not as
universal in their fitting to the human face as manufacturers and wearers
would hope, and thus often allow leakage. Filters on respirators need changing
regularly or their performance deteriorates. Some disposable respirators
become ineffective after use because of being crumpled in a pocket or soaked
with water. Respirators require to be formally tested to various standards
- in doing this, testing laboratories have found filters that release respirable
fibres, giving higher counts within the masks than outside! Some helmet
type respirators become less efficient if used in moderately windy conditions.
Ear plugs may contribute to infection in the outer ear (otitis externa),
visors may obscure the welding job and thus not be used properly, gloves
may be permeable to chemicals, and so on. The correct personal protection
must be chosen. The following image shows the hands of a worker who had
been provided with high quality thick black rubber gloves as personal protection.
These would probably have been suitable to protect her from caustics. Unfortunately
they were the wrong sort to protect her from the alkylating agent that
she was handling. The blistering on her right hand was the consequence
of the wrong protective measures:
Acknowledgement: Part of this page has been adapted, with permission,
from Practical Occupational Medicine (Copyright)
- Arnold publishers.
A Diploma and a MSc course in Occupational Hygiene are available
at the University of Manchester