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Chapter 2.6 Safety in Laser Cleaning Edited by Martin Cooper The Conservation Centre, National Museums Contents 2.6.2
Hazards of Laser Cleaning Systems 2.6.2.2
Debris Generated During the Cleaning Process 2.6.2.4
Electric and Fire Hazards 2.6.4.1
Administrative Controls 2.6.4.3
Personal Protective Equipment 2.6.6
Summary of Safe Working Practice 2.6.7.1
Laser Safety Standards 2.6.7.2
Institutions and Organisations 2.6.7.3
Laser Safety on the Web 2.6.7.4
Laser Safety Publications 2.6.7.5
Laser Safety Consultancy and Training Laser
radiation is a unique form of light that can be utilised to clean
delicate and valuable artworks in an extremely selective and
controllable manner. The properties of laser radiation that make laser
cleaning such a valuable technique for the conservator also give rise
to hazards that must be properly controlled to ensure a safe working
environment. Laser safety within the European Union is covered by a
common standard, EN 60825-1 (Safety of laser products part 1; equipment
classification, requirements and user’s guide). This
chapter provides an introduction to those working with lasers in
conservation as to how EN 60825-1 can be implemented so that safe
working practices are followed. It is important to note here the new
user guidelines IEC TR 60825-14 (Safety of laser products part 14: a
user’s guide), which give advice to users on laser hazards,
risk assessment and protective control measures. It is intended to
replace the user guidelines in EN 60825-1, leaving the latter purely as
a product compliance standard. Although all reasonable effort has been
made to ensure the accuracy of the information contained in this
chapter, it should not replace the requirements of
international and national standards, national laws and regulations and
formal advice received from safety consultants.
2.6.2
Hazards of laser cleaning systems The unique properties of a laser beam give rise
to certain hazards, which must be controlled to ensure safe working.
Laser radiation entering the eye can cause permanent damage and the low
divergence of a laser beam means that energy can be delivered over
relatively large distances with very little change to its properties. A
laser beam can therefore remain hazardous a long way from its source.
In addition to hazards from the laser beam itself, hazards associated
with use of the laser beam for cleaning also exist: ejection of
particulates and vapours from the surface and noise generated
by equipment and the cleaning process. The hazards associated with the
use of laser cleaning systems in conservation are outlined here. Ocular hazards The
eye is an extremely sensitive and important part of the body. The
ability of the eye to collect and focus light means that it is
vulnerable to laser-induced damage. Damage can occur at all
wavelengths, although the area of tissue affected is
wavelength-dependent (Tab. 2.6.1). Laser radiation emitted in
the visible (400 – 700 nm) and near infrared (700 –
1400 nm) spectral regions poses the greatest risk as light is
transmitted effectively through the liquid in the eye and focused to an
extremely small spot on the retina at the back of the eye. This
wavelength range is known as the retinal hazard region. This high concentration of energy can be
sufficient to damage the retina permanently. This damage can be
especially serious if it occurs to the central region (fovea) of the
retina, which contains a particularly high concentration of light
sensors. Damage here can seriously affect the sight in that eye.
Unfortunately, most accidents occur with the viewer looking in the
direction from which the laser beam has travelled and damage to the eye
is, therefore, most likely to occur in this critical region. Laser radiation at short wavelengths
(<300 nm) in the ultraviolet and in the far infrared
(>3000 nm) regions is absorbed at the front of the eye, which
can cause damage to the cornea. Laser radiation in the middle infrared
(1400 – 3000 nm) and near ultraviolet (300 – 400
nm) regions is absorbed mainly by the lens, which can lead to formation
of a cataract. Even
short exposures to a high intensity laser beam can cause damage. This
is partly due to the fact that the duration of laser pulses used in
cleaning applications (typically 10 ns) is extremely short compared to
the time needed for protection afforded by the blink reflex
(approximately 0.1 s). This means that injury can be caused by a single
pulse. Tab. 2.6.1: Wavelength-dependence of
laser damage to the eye.
Skin hazards Laser radiation can also cause injury to skin,
though in general skin hazards are considered to be less serious than
eye hazards. The part of the skin affected will depend on the
wavelength of the laser beam. Visible and near infrared (400
– 1400 nm) radiation is able to penetrate into the
epidermis (50 – 150 µm from surface) and dermis (1 – 4
mm), whereas far ultraviolet and far infrared radiation is absorbed
strongly in the stratum corneum (8 – 20 µm). Potential effects of irradiation of skin by
a laser beam are summarised in Tab. 2.6.2. Tab. 2.6.2: Summary of the effects of a laser
beam on skin.
2.6.2.2
Debris generated during the cleaning process Laser
cleaning may involve evaporation, smoke-generating pyrolysis, melting,
and/or spalling of the layer to be removed. These waste products are
composed mainly of material removed from the substrate. In addition,
small amounts of compounds that were not part of the original material,
gases generated by thermal cracking of organic substances and their
eventual combustion, can be created. These laser-generated air
contaminants are known to create objectionable odours and visible
smoke. Removal of some organic materials may release potentially
harmful bacteria, fungae (hyphae and spores) into the vicinity. The
human response to chemicals depends on several factors, including the
type of chemical, its concentration and form, and the individual (age,
sex, genetics, health, etc.). Some chemicals are irritants, some cause
immune responses for allergic persons, and some can be the cause of
disease after many years of exposure. Debris
resulting from the removal of sulphation crusts by laser cleaning can
include a significant proportion of particles with diameters of 3
µm or less. Such small particles can penetrate the lungs down
to the alveoles, and are potentially harmful by solubilization of their
alkaline and metallic salt compounds. Such small particles are also
able to enter the body through the skin.
There
are three sources of noise at laser cleaning workplaces: —
the laser system itself, —
the fume extraction device, and —
the laser cleaning process. The rapid ejection of material from a surface
during laser cleaning leads to the generation of sound waves, which
propagate away from the surface and are heard as a
‘snapping’ sound. The long term effects of exposure
to all three sources of noise together have not yet been investigated.
2.6.2.4
Electric and fire hazards High
voltage electrical equipment is often contained inside a laser. This
does not normally pose a hazard unless the protective covers of the
laser are removed. Extreme caution should be exercised during
maintenance and servicing when the covers of the laser cleaning system
may be removed. Maintenance and servicing of lasers should only be
carried out by people who have received appropriate training.
Most laser cleaning systems use water in the cooling system, which
would create a serious hazard if allowed to come into contact with the
electrical equipment inside a laser. Under normal conditions, this will
not happen but, of course, great care should be taken during
maintenance and servicing. Fire hazards may be the consequence
of electrical faults, or may be due to ignition of flammable materials
by the laser beam, as reported in some medical applications.
A
laser cleaning system, as with all devices incorporating a laser, is
assigned to a product class according to the potential of its
accessible laser emission for causing injury. These product classes are
detailed in the European standard IEC 60825-1. The product classes are
outlined in Tab. 2.6.3. The classification of a laser product depends
on the accessible laser emission. A product may be
assigned to class 1 if its output power is too low to be considered
hazardous, or if the engineering design of the product is such that it
is not possible, under reasonably foreseeable conditions of use, to
access the laser radiation. A class 4 laser may be embedded in a
product, which is assigned to class 1, if the design of the product
meets the safety requirements detailed in IEC 60825-1
(Fig. 2.6.1). Most laser cleaning systems used in conservation
are class 4, i.e. their use requires extreme care. The type of laser
radiation used to clean the surface of an artwork is unsafe and the
nature of the work usually requires that the laser beam be delivered
through a flexible handheld optical fibre or articulated arm
(Fig. 2.6.2). It is important to note that modifications to a
system may lead to a change to its classification and hence to the
safety controls appropriate to its use. Maintenance work may also
necessitate the implementation of additional safety controls. Table 2.6.3: Summary of laser classification
(for a full description see IEC 60825-1 or the equivalent national
standard). Most laser cleaning systems are class 4.
In
order to establish the precautions necessary for safe working with
lasers it is necessary to identify the hazards and to establish the
level of risk (a combination of the likelihood of harm occurring and
the severity of injury that would result). This is achieved by way of a
risk assessment (health and safety at work legislation requires that a
risk assessment is carried out for all work-related hazards). The risk
assessment will need to identify all hazards, establish the conditions
under which these hazards may exist and identify the level of risk to
which people may be exposed. Appropriate safety controls can then be
put in place to reduce the level of risk to an acceptable level. Both
the risk assessment and safety controls should be documented. There
must be a formal written framework within which laser safety is managed
by the organisation. A
Laser Safety Officer (LSO) should be appointed to take administrative
responsibility of laser safety on behalf of the employer and to ensure
compliance with safe working procedures. The role of the LSO includes
ensuring appropriate control measures are in place, regular monitoring
of laser hazards and the effectiveness of the control measures
implemented and maintenance of records of such monitoring. The role of
the LSO should also include the specific approval of control measures.
This is especially important with outdoor cleaning where the safety
installation will be temporary and conditions may be far from ideal. The safe use of laser cleaning systems in
conservation requires adoption of a number of safety controls, governed
largely by the classification of the system. These are divided into
three main areas: administrative, engineering and personal protective
equipment. The controls outlined here should be implemented for a class
4 product (most laser cleaning systems currently in use in conservation
are class 4).
2.6.4.1
Administrative controls Administrative controls are concerned with
controlling who uses the laser cleaning system and how it is used. The
procedural aspects of the administrative controls, i.e. the
‘local rules’, are extremely important as these
establish the organisational framework within which the laser will be
used. Use of a laser cleaning system should, therefore, be restricted
to personnel who have received training in safe use of lasers. Access
to the laser system should be controlled through a key or password
entered via the control panel. Keys should be stored in a separate area
from the cleaning system.
Engineering controls are concerned with
maintaining the laser beam within a ‘controlled’
area, making access to the ‘controlled’ area safe
and use of equipment to remove by-products of the cleaning process from
the work-area. Most laser cleaning systems produce a diverging laser
beam. As the laser beam travels away from the system, eventually it
will spread out to such a large extent that it can be considered safe,
i.e. it has become too weak to damage the eye. This distance is
referred to as the Nominal Ocular Hazard Distance (NOHD). The NOHD is
dependent on the characteristics of the laser cleaning system being
used, including wavelength, pulse length, pulse energy and beam
divergence. The NOHD for laser cleaning systems in conservation may
range from tens of metres to hundreds of metres. Practically, this
means that laser cleaning should be carried out within a controlled
area, within which the laser beam is confined. Appropriate warning
signs and a warning light (activated when the laser is in operation)
should be clearly visible. Interlocks should be installed at
points of access to the controlled area so that the laser system is
turned off when someone enters the area. When setting up a controlled
area (either within a large studio or on-site), the path of a laser
beam escaping from the area (if a roof has not been included) should
always be considered: Specularly reflecting surfaces (e.g. shiny metal
pipes) may exist above the work area or windows in the upper floors of
a neighbouring building may be directly in the line of sight.
In such cases, the possibility exists of people outside the controlled
area inadvertently being exposed to hazardous radiation and so a roof
should be included. Special care should be taken on scaffolding to
ensure gaps between scaffold boards are covered or filled, if work is
taking place either above or below the laser cleaning area.
Particulates and fumes generated by the cleaning process should be
extracted at source.
2.6.4.3
Personal protective equipment The use of personal protective equipment (PPE) is necessary where protection cannot be guaranteed by the implementation of appropriate administration and engineering controls, which is almost always the case in conservation. Since the direct laser beam and both specular and diffuse reflections generated from the surface of an artwork during laser cleaning are hazardous to the operator, PPE in the form of protective eyewear is essential. Such eyewear is available from specialist manufacturers in the form of spectacles or goggles (Fig. 2.6.3). Eyewear must provide adequate protection at the wavelength being used (EN 207 covers laser protective eyewear); this means that changing the wavelength of the emitted beam will usually necessitate a change in protective eyewear. The protection level required is determined by the characteristics of the laser system being used: wavelength, pulse length, maximum pulse energy, maximum repetition rate and beam divergence. It is recommended to wear long sleeves and gloves to provide additional protection from fine particulates entering the body through the skin (Fig. 2.6.3). Working at ultraviolet wavelengths may necessitate skin protection from the associated beam hazard.
In
addition to the controls outlined in section 4 attention should be paid
to other factors, such as avoiding exposure of skin, quality of
illumination, presence of shiny surfaces, covering windows within a
controlled area, number of people within the controlled area and
general hygiene. Laser
radiation can pose a hazard to the skin. It is, therefore, important to
avoid exposure of skin to the laser beam. At infrared wavelengths
simple avoidance by careful aiming of the laser beam at the target is
usually sufficient. Prolonged cleaning at ultraviolet wavelengths,
however, may require protection of the skin from diffuse reflections. The
workplace should be well-illuminated. This allows the conservator to
see the work properly (especially when wearing tinted eye protection),
to move around the work area freely and see other hazards such as
trailing cables. Ideally, the walls surrounding the workplace should be
light coloured and diffusely reflecting. The work area should ideally
be well-ventilated. Specularly
reflecting surfaces should be removed from the workplace to reduce the
chance of stray reflections. Conservation tools, e.g. scalpels and
paint brushes often have shiny surfaces; these should be covered with
non-reflecting tape. Care should be taken when wearing jewellery during
laser cleaning operations. If the boundaries of the controlled area
contain windows then these should be covered using a material, which is
opaque to the laser radiation being used (glass transmits most
of the wavelengths used in laser cleaning very efficiently). Attention
should also be paid to the number of people present in the controlled
area. Ideally, only one laser should be used at one time and no other
work should be undertaken while the laser is in use. If it is necessary
to use more than one laser cleaning system simultaneously, then a
controlled area should be created for each laser. The
beam from a class 4 laser can in certain situations set fire to
flammable materials. It is, therefore, important to keep all
such materials including cleaning solvents away from the beam. It would
be prudent to install a suitable fire extinguisher in the work area. As
with other conservation procedures, general health and safety practice
should be followed, i.e. no eating, drinking or smoking in the
workplace; wash hands and face following laser cleaning work. Immediate
work areas should be cleaned (using HEPA class filtration industrial
vacuum cleaners) following laser cleaning to prevent accumulation of
fine particulate matter. The whole of the laser cleaning area should be
cleaned at regular intervals. Laser
cleaning for excessive lengths of time can lead to eye strain, in much
the same way as prolonged working at a computer screen. Tolerance
levels vary from person to person but intense concentration on a small
area for an extended period of time can become tiring. Regular breaks
(which may simply involve stopping for a few seconds to allow eye
muscles time to relax) should be taken. Fig. 2.6.4 shows the laser cleaning
set-up in the sculpture department of a national museum in the
Figs. 2.6.5
– 2.6.7 show laser cleaning on scaffolded sites, where either
tin hoarding or opaque plastic sheeting has been used to create a
controlled area within which laser cleaning is able to proceed safely.
As with working in a studio environment, safe working procedures must
be set up prior to work commencing. All conservators who will be
working on site must fully understand and comply with the procedures.
For site work it is often necessary to have several conservators
working at the same time. In such situations it is recommended that the
laser work area and non-laser work area are separated by an opaque
screen and warning signs are clearly visible. It is important that
people cannot accidentally walk into a laser work area. Care must also
be taken to ensure that conservators working on a level above or below
that where cleaning is being undertaken are not put at risk through
gaps between the different levels. All contractors should be made aware
of the laser work being carried out and the safety requirements of the
laser cleaning contractor should be established at the earliest
opportunity to avoid delays and additional costs at a later stage. It is important to stress that the LSO should be responsible for approving outdoor installations before work commences.
2.6.6
Summary of safe working practice This
is intended to provide a brief summary of good safe working practice
for conservators undertaking laser cleaning activities with a
class 4 laser cleaning system, emitting visible or near infrared
radiation.
—
Written procedures based on the findings of a risk assessment should be
in place and followed. —
Potential users of laser systems must receive appropriate training
before being allowed to start laser cleaning work. —
Servicing and maintenance of laser cleaning systems must only be
carried out by people who have received appropriate training and have
relevant expertise. —
Store keys to laser cleaning systems away from the systems. —
Appoint Laser Safety Officer. —
Regularly check safety controls and ensure compliance among staff. —
Take regular breaks during laser cleaning activities to avoid
eye/muscle strain. —
Only one person should be working within the controlled area at any
time (anyone else should only be observing). —
Consult with Laser Protection Adviser (possibly expert from outside
organisation) if extra advice required.
—
Laser cleaning should be carried out in a controlled area within which
the laser beam is confined. —
If there is no roof to the controlled area, ensure that the laser beam
cannot pose a hazard to people within the nominal ocular hazard area. —
Ensure windows covered by material opaque to the wavelength(s) being
used. —
Ensure laser warning signs and warning light are clearly visible. —
Ensure access to controlled area is restricted; use interlocks if
possible. Inadvertent entry into the controlled area should not be
possible. —
The work area should be well-illuminated. —
Remove shiny objects, e.g. scalpels, from the vicinity of the object
being cleaned. —
Only one laser cleaning system should be used within a single
controlled area. —
Do not put water containers on top of the laser cleaning system. —
Use localised extraction to remove debris at source. Ensure equipment
used correctly and filters changed when necessary. —
Ensure all power cables, leads etc. do not present tripping hazard. —
Clean the work area regularly to avoid accumulation of debris over time.
—
Wear appropriate laser safety eyewear. —
Ensure eyewear fits correctly. —
Clean eyewear only with appropriate fluid and soft tissue/cloth. —
Store eyewear carefully to avoid damage. —
Replace eyewear if damaged. —
If changing wavelength, ensure correct eyewear in place before starting
work. —
Ensure all people inside controlled area are wearing safety eyewear at
all times while laser is operating. —
Avoid exposure of skin to laser radiation. —
Wear an appropriate face mask during cleaning.
2.6.7.1
Laser safety standards EN
60825-1: Safety of laser products Part 1: Equipment classification,
requirements and user’s guide. This is the main European safety standard for
laser products. IEC TR 60825-14: Safety of laser products part
14 (user’s guide). Advice on laser hazards, risk assessments and
control measures. Intended to replace user’s guidelines in EN
60825-1. Published in the EN 207: Personal eye protection –
filters and eye protectors against laser radiation (laser eye
protectors) European standard defining performance and
testing requirements for laser protective eyewear. CLC/TR 50448 (European document due to be
published) Guide
to levels of competence required in laser safety.
2.6.7.2
Institutions and organisations British Standards Institute (BSI): www.bsi-global.com European Agency for Safety and Health at Work
(EASHW): http://agency.osha.eu.int International Committee on Non-Ionising
Radiation Protection (ICNIRP): www.icnirp.de
International
Electrotechnical Commission (IEC): www.iec.ch International
Standardization Organization (ISO): www.iso.ch Health
Protection Agency: www.hpa.org
2.6.7.3
Laser safety on the web A
Brief Guide to Laser Safety: http://www.bioptica.co.uk/laser_safety.htm General
rules and precautions with references for further reading. Elements
of Laser Safety: http://gary.myers.net/elements.htm Mainly
discusses how the human body, especially the eye, can be affected by
various frequencies of laser radiation. Handbook
of Industrial Laser Safety: http://info.tuwien.ac.at/islt/safety Information
on laser radiation, laser hazards, gas and fume emission, risk
assessment, training and education, safety standards. Laser
Accidents: http://www.adm.uwaterloo.ca/infohs/lasermanual/documents/section11.html Laser Hazards: http://www.adm.uwaterloo.ca/infohs/lasermanual/documents/section6.html Eye
and skin hazards from laser light. Laser
Institute of Lasers
in Health Care: http://www.ccohs.com/oshanswers/phys_agents/lasers.html Questions and answers on the use of lasers in
health care. Non-Beam Laser Hazards: http://www.adm.uwaterloo.ca/infohs/lasermanual/documents/section7.html Various
electrical, radiation, fire, explosion and chemical hazards. Rockwell
Laser Industries: http://www.rli.com/ Laser safety consulting, training and a full line of safety products including eyewear, barriers, signs, hazard analysis software and instructional materials. Site includes much safety related information
Laser
Safety, by Roy Henderson and Karl Schulmeister, Handbook
of Industrial Laser Safety (CD), by Karl Schulmeister, Thomas Puester,
Mike Green and Roy Henderson, Austrian Research Center Seibersdorf: http://info.tuwien.ac.at/islt/safety Industrial
Laser Safety, edited by Mike Green (Prolaser Consultants), Summary of Cleaning
safely with a laser in artwork conservation COST Action G7, Booklet on safety in laser
cleaning for conservators, 2006.
2.6.7.5
Laser safety consultancy and training Association
of Industrial Laser Users: www.ailu.org.uk Austrian
Research Centre Seibersdorf: www.healthphysics.at/laser Bioptica: www.bioptica.co.uk Irepa Laser: www.irepalaser.fr Prolaser Consultants: www.prolaser.co.uk
EU
COST Action G7: 'Artwork Conservation by Laser': http://alpha1.infim.ro/cost/
This
chapter has been taken from ‘Cleaning safely with a laser in
artwork conservation’, a booklet produced for conservators as
part of COST Action G7 ‘Artwork Conservation by
Laser’. The editor is grateful to the COST G7 members for
their contribution to this booklet, particularly Veronique
Vergès-Belmin, Margarida Pires, Renzo Salimbeni, Johann
Nimmrichter, Alain Diard and Martin Labouré. Photographs
have been reproduced with kind permission of National Museums
The
Conservation Centre National
Museums Whitechapel |
