Lasers can cause complications or incidents, depending on whether
the undesirable effect is within or outside the area of treatment. There is no treatment in
medicine which does not expose the patient to some risk of complication. Even though lasers can be
considered particularly safe instruments, their rate of complications is not zero. One can only
hope to reduce their frequency by better training of doctors and improvements in the knowledge of
laser effects on tissue. On the other hand, the rate of incidents
must be, and is for most of the time, nil. These are incidents which are concerned with problems
of safety. Their prevention is achieved through a good knowledge of their causes and the
application of measures which allow them to be avoided.
There are two possible types of incident: those related to the laser beam itself (optical risks), and those due to other causes (non-optical hazards).
Whichever damage mechanism is involved, the brightness or irradiance is always an important risk factor. So the geometry of the beam (direct, diverging or reflected) must be known.
A direct beam, as it emerges from the laser or from an articulated arm, is only
very slightly divergent and the brightness can be considered to be constant within the treatment
area (and beyond, perhaps even through a window). The diverging beam (after transmission through
an optical fibre or focusing by a lens) loses its brightness (and so its danger) in proportion to
the angle of divergence and the square of the distance.
In the case of reflection, the risk depends on the quality of the reflecting surface. A smooth metallic surface results in specular (mirror-like) reflection. A rough, irregular surface produces a diffuse reflection.
The location of any damage will depend on the wavelength of light concerned. The diagram indicates which wavelengths are mainly absorbed in the different parts of the eye: the cornea, the lens, or the retina.
Skin burns are not generally very serious if they are inflicted on a conscious person who can react immediately and move away from the beam or stop its emission. On the other hand, an anaesthetised patient will not react and a burn can potentially be much more serious if nobody is aware that such an event is occurring.
These refer either to individuals (Maximum Permissible Exposure, MPE), to
equipment (in the form of an international laser classification standard and
Accessible Emission Limit or AEL), or to locations (Nominal Ocular Hazard Distances, NOHD).
These refer to the maximum exposure levels to which a person may be exposed without immediate or long term damage. These levels vary according to the type of laser beam, and are given in the British and European Standard BS EN 60825, which includes some fairly complex tables from which the various MPE's can be calculated.
International laser classification
All lasers are classified according to their output, from Class 1, which describes lasers which are of such low power that they represent no hazard at all, to Class 4 lasers which are always hazardous. Most therapeutic lasers are Class 4.
Nominal Ocular Hazard Distance (NOHD)
The NOHD for any particular laser represents the distance within which it is possible to exceed the MPE. A calculated example of the NOHD is shown in the following figure, which also illustrates the benefit of using an "L" shaped room.
Most lasers represent a risk to users on account of their use of high voltages. These risks are often increased by the presence of water which is circulated to cool the tube and sometimes to cool the electric supply itself. However, the design of the equipment does not allow the user access to dangerous areas.
Smoke is produced when tissue is volatilised. It has a very unpleasant smell, it is an irritant and is potentially toxic to the respiratory system. It may contain carcinogenic or other harmful substances. The risk of infectious or viral contamination is a much debated question. There is no doubt that DNA is present in the plume during treatment of viral HPV lesions in the mucous membranes. But it has not been shown that this DNA is infectious, and it appears that it may bind to smoke particles which are absorbed by ordinary surgical masks.
This is a well-known problem in ENT surgery. The intubation tube is very close to
the treatment zone and can ignite if exposed to the laser beam. Accidents can happen due to a
combination of three factors:
These may include the ignition of endoscopes or their components, explosion of bodily gases, gaseous embolisms, toxic gases (notably with excimer lasers),…..
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