Resources - Safety Info

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From September 1994 issue of ‘Crucible’ Volume 26.1

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A relatively large fraction of the Canadian population requires corrective lenses to obtain optimal vision. While many of these individuals choose to wear eyeglasses, an increasing number are being fitted with contact lenses. The recent technological advances in the contact lens industry have greatly increased the options available to the contact lens wearer. Futhermore, certain eye conditions make it difficult for some contact lens wearers to switch freely between their contact lenses and spectacles. This raises the question of whether it is permissible, or prudent, to allow students (and teachers!) to wear contact lenses in the science classroom or laboratory.

The intent of this article is to provide the information needed to make an informed decision on whether teachers and their students should be allowed to wear contact lenses in the science laboratory/classroom.

About Contact Lenses

Until the late 1970s, contact lenses were made from two basic materials. The hard contact lens was made of polymethymethacrylate (PMMA), while the soft contact lens was made of a hydrated polymer, hydroxyethylmethacrylate (HEMA), which contained 37.8% water by weight. Although biologically inert and capable of providing excellent optics, both PMMA and HEMA lenses reduced the supply of oxygen to the cornea. This led to adverse changes in corneal physiology and anatomy in some contact lens wearers. Today, PMMA has all but vanished, to be replaced by a variety of rigid plastics which are mostly hydrophobic materials with high oxygen permeability. Lenses made of these polymers are termed Rigid Gas Permeable (RGP). In the soft lens market, HEMA is giving way to polymers which contain as much as 80% water. These soft lens materials (called hydrogels) have a high level of oxygen transmission, and show remarkable structural strength for their degree of hydration. The combination of thinner lenses (as thin as 0.04 mm) and high oxygen transmission has greatly reduced the corneal problems due to hypoxia, but there is still the potential for many other complications. Disposable contact lenses, extended wear schedules (lenses worn continuously for days to months), allergies to preservatives in care solutions, incompatibility between contact lens materials and care solutions, contamination of lenses and solutions, and other factors present many challenges to the contact lens practitioner.

A contact lens must not only provide the wearer with a clearly focused retinal image, but also conform to the shape of the cornea and remain in position in front of the pupil. This mechanical requirement is referred to as the fit of the contact lens. By measuring the curvature of the front surface of the cornea, assessing the tears, and determining the refractive error, a contact lens fitter can select the appropriate material and optical and physical parameters for a contact lens. An improper fit may result in physical and physiological changes in the cornea. If a bad fit is not corrected, the corneal changes may become permanent. The fit of a contact lens and the health of the eye must therefore be assessed at regular intervals; such eye examinations are referred to as "progress checks".

A contact lens does not actually touch the surface of the eye. Instead, it is bathed in the tears which lubricate the eyeball, and floats in front of the cornea on a thin tear film. A rigid lens is smaller than the corneal diameter, and is held in position by capillary action of the tears. The front surface of the contact lens is also covered by a tear layer. In contrast, a soft lens covers the entire cornea and rests on a thin ring of the conjunctiva (the clear membrane covering the white of the eye). The tear film covers the entire lens, and fills in the space between the lens and the eye. For either rigid or soft lenses to fit properly and provide good vision, a good tear film on the eye is essential. The difference between rigid and soft contact lenses are summarized in Table 1.

Tear fluid is essentially a 0.9% NaCl solution (saline) with additional constituents such as albumin, lysozyme, and glucose. It is secreted by the lacrimal glands. Glands at the edges of the eyelids secrete an oily component of the tear film, which helps to retard its evaporation. Specialized cells within the conjunctiva secrete mucin which spreads across the surfaces of the eye and eyelids to increase their wettability. The mucous base of the tear film ensures that the entire eye is evenly coated by the tear film. In an eye examination, the integrity of the tear layer and its components can be assessed by a variety of techniques involving dyes and observations at moderate to high magnification with a biomicroscope.

Every contact lens wearer should also have eyeglasses made to the current prescription as a back-up in case the contact lenses cannot be worn. Ideally, either eyeglasses or contact lenses should give the wearer the same level of vision, but in some ocular conditions (see Table 2 ) only contact lenses can provide optimal visual acuity.1

Contact lenses and hazards in the science laboratory

Surveys of accident reports from industry and emergency facilities show that up to 80% of all eye injuries are caused by small flying particles, dust, and larger objects striking the eye.2,3 About 10% of reported injuries are due to exposure to sources of ultraviolet radiation, mostly electric welding arcs. Other classes of injury tend to be specific to certain occupations or workplace activities. A 1981 survey of accidents in British school science laboratories reported that 22% involved chemicals in the eyes.4

Science experiments do involve some risk of eye injury, particularly in the biology and chemistry classroom. However, hazards may be present in the physics lab as well. In assessing our laboratories for eye safety, and developing safety rules and procedures regarding contact lens wear, we must ask ourselves a number of questions.

• Is there an actual ocular hazard?
• Does wearing a contact lens place the eye at greater risk than wearing glasses?
• Does wearing a contact lens increase the risk to the eye, or increase its susceptibility to injury?
• Is the risk different for various contact lens designs and materials?
• Are there associated risks for a contact lens wearer who removes lenses?
• Do contact lenses negate other safety procedures?

Mechanical Hazards

One of the most common minor eye injuries is the superficial corneal foreign body. The symptoms of pain, excessive tearing, and foreign body sensation are relieved quickly by simple removal of the particle, often by flushing the eye with a stream of physiological saline or water. Such injuries may be caused by high concentrations of airborne dust, or by small particles (under 1 mm size) moving at slow speeds. A missile of suitable size, shape and speed may become embedded in the cornea, conjunctiva or sclera, or in extreme cases, may perforate the eyeball (intraocular foreign body). Contact lenses may provide better eye protection than eyeglasses in some circumstances.

Foreign bodies may become trapped under a rigid contact lens, but this does not occur with soft lenses unless the particle was inserted with the lens. A Swedish study showed that workers wearing soft contact lenses in an industrial setting with high concentrations of airborne metal particles and oil droplets for up to 2 years had no subjective complaints and no evidence of eye damage. This study showed that hydrogel lenses can be worn safely in environments where hard lenses cannot.5

The protection of eyes by contact lenses from flying particles depends on the thickness and rigidity of the lens. There are many documented cases in which a hard or soft contact lens protected the eye from impact of an object which would have caused severe or sight-threatening injuries to an eye not wearing a contact lens. Controlled laboratory studies have confirmed this clinical observation.

Sharp-edged or pointed missiles such as glass fragments are more likely to cause perforating injuries. Rigid contact lenses can protect the eye from these objects better than hydrogel lenses. A case involving a traffic accident has been described in which rigid lenses protected the eyes when the broken windshield glass cut the patient's face across the eyebrow and cheek. It was speculated that spectacles would have increased the risk of eye injury.7

It can be concluded that an individual wearing contact lenses is at no greater risk of mechanical injury due to flying debris in a laboratory accident than a spectacle wearer. Indeed the clinical and experimental evidence suggest that the contact lenses may help to protect the eye from more severe injury. However, contact lenses do not provide adequate protection from mechanical injury. Appropriate protective eyewear (goggles or safety spectacles with solid sideshields) must be worn.

Chemical Hazards

There are many sources of information on the direct toxic effects on the eye of thousands of chemical substances, as well as the secondary effects on the eye and visual system following inhalation, ingestion, or absorption through the skin and mucous membranes. It is often alleged that chemicals are trapped behind hard contact lenses, or absorbed, concentrated and released by hydrogel lenses to further injure an already compromised cornea. Some safety bulletins have claimed that contact lenses cause worse than normal corneal injuries by holding chemicals against the eye, and that the presence of a contact lens on an eye prevents adequate irrigation following a chemical injury. These claims are unsupported by documented evidence, and the descriptions of the accident and resulting injuries to the eye often cannot be reconciled with what is known about the toxicity of the chemical agent.

Noxious gases, vapours, fumes, aerosols and smoke can seep behind inappropriate eye protectors and affect the surface tissues of the eye. The ocular response depends on concentration and physical and chemical properties of the substance. Highly toxic materials which are likely to cause tearing and lid closure upon reaching the eye, will also stimulate the respiratory tract. Systemic absorption of airborne toxins is more likely to occur through the mucosa of the respiratory tract than that of the eye.

Wearers of soft lenses often report that they can peel onions without the usual ocular irritation. A number of studies have shown that hydrogel lenses protect the eye from tear gases. Thus, hydrogel lenses appear to protect the eye from airborne ocular irritants which have low solubility in water.

It is often assumed that water-soluble gases and fumes, and substances that may bind to, or be absorbed into hydrogel materials would cause more severe ocular responses or a chronic effect because of prolonged exposure to the eye. However, this has not been borne out in laboratory studies. For example, Nilsson and Andersson8 found that the uptake of trichloroethylene by hydrogel lenses suspended in a concentration of 640 ppm was up to 90 times the uptake of physiological saline. However, the release into simulated tears was far less than the release into air. They concluded that the 'vacuum cleaner' effect of the soft lens resulted in a lower concentration at the cornea than if the eye were exposed directly to the fumes. This and other studies suggest that when proper eye protection is used, stringent restrictions on contact lens wear are not as important.

The accidental splashing of toxic substances into the eye is one of the most frequent causes of serious eye injury. More serious injuries occur when a large volume of liquid is involved. Most commonly used organic solvents affect only the external eye, causing loss of the corneal and conjunctival epithelium and extreme discomfort. However, the effects disappear with regrowth of the epithelium. Similar responses are found with detergents and surfactants. Wesley9 showed that when solutions of creosote, H2SO4 or NaOH were sprayed onto anesthetized rabbit eyes, those not wearing a PMMA contact lens were 1.8 times more likely to develop total corneal opacity than eyes with the lens, and that an eye was more likely to be damaged if the contact lens were dislodged during the splash or subsequent irrigation. Instead of trapping material behind it, the lens acts as a barrier.

The pH of acid and alkali solutions is an important determinant of the level of damage resulting from a chemical splash. Solutions with extreme pH values cause rapid destruction of the eyelids, anterior eye and surrounding tissues. Weak acid solutions are self-limiting, affecting the superficial cells of the cornea and mucous membranes of the lids and eyeball. Alkali solutions saponify the lipid membrane structure of cells; they readily soften, disrupt and penetrate the anterior eye to reach the internal structures. Thus, alkali splashes have much more severe consequences, even at low concentration.

Nilsson and Andersson8 demonstrated that contact lenses can reduce the eye damage due to concentrated HCl by about 75%, while the presence of a contact lens on the eye in a splash of NaOH neither reduces nor worsens the resulting eye injury. Leaving the lens on the eye for a minute or two after the splash made no significant difference.

These studies indicate that contact lenses may help to protect the eyes from certain chemical agents, but that they should not be regarded as a substitute for protective eyewear. It is important to recognize that a contact lens wearer with appropriate protective equipment is at no greater risk than an individual who does not wear contact lenses. However, even an apparently trivial eye injury involving chemical splash can have disastrous consequences on vision, and emergency procedures including immediate irrigation of the affected eye must be followed. The loss of a contact lens during flushing would be acceptable in these circumstances.

Optical Radiation Hazards

Optical radiation includes electromagnetic radiation with associated wavelengths between approximately 200 nm and 3000 nm. These wavelengths are considered optical because conventional lens material (glass, plastic, and fused silica) can be used to alter the focus and direction of propagation of the radiant energy. Wavelengths between 200 and 400 nm comprise the ultraviolet spectrum, while visible light includes the band from 400 to 780 nm, and the infrared waveband ranges from 780 to 3000 nm. The UV spectrum is divided into 3 wavebands according to their effects on living cells. These are UVC between 200 and 280 nm, the UVB from 280 to 315 nm, and the UVA from 315 to 400 nm.

There are many sources of optical radiation that both students and teachers may encounter in the school science laboratory. These include a wide variety of spectral lamps, black light (UV) sources, lasers, infrared (IR) lamps, and various sources of visible light. The exposure factors which must be considered include wavelength(s) and intensity of exposure, the duration and frequency of exposure, and whether contact (or spectacle) lenses can protect the eye from the radiant energy. The transmission of optical radiation by the tissues of the eye determines whether a given tissue may be damaged by the radiation; to have an effect, the photon must reach the target tissue and be absorbed by it.

The effects of optical radiation on the tissues of the eye are well known (see Table 3 ). They are characterised by exposure thresholds which are expressed as the total energy per unit area of irradiated tissue needed to produce a just detectable level of damage. Exposure thresholds for the UV are on the order of mJ.cm-2, while IR thresholds are at the kJ.cm-2 level; these relate to the photochemical and physical mechanisms leading to tissue damage.

Short wavelength optical radiation triggers a complex series of oxidative chemical reactions at membranes structures within cells involving the generation of highly oxidative species such as H2O2, OH-, and other free radicals. Damage occurs when the intracellular defence mechanisms which neutralize free radicals in the cytoplasm are overwhelmed. The severity of the photic injury depends upon the location of the damaged membrane. UVC exposure produces lethal damage to nuclear membranes in cells; it is therefore used to sterilize items which are readily damaged by other sterilization techniques. UVB exposure affects the membranes of cytoplasmic organelles and may disrupt metabolic processes in the exposed cell. Whether this results in temporary or permanent impairment of the cell, or cell death, depends upon the total dose of absorbed energy. Similar oxidative photochemical injuries can result from high levels of exposure to blue and green light (400 to approximately 550 nm). A characteristic of photo-oxidative injuries is the latent period of 6 to 12 hours between exposure of the tissue and the onset of clinical signs and symptoms. During this time, the damaged tissue continues to function normally.

Unlike the skin, the tissues of the eye do not develop a tolerance to UV radiation with repeated exposure. Tanning of the superficial layers of the skin is a defence mechanism which increases the threshold exposure for sunburn. No such defence exists in the eye.

Damage resulting from exposure to long wavelength visible light (> 600 nm) and IR radiation is due to conversion of optical radiation energy into heat by passive absorption. Thermal injury occurs when the intracellular temperature rises more than 20o C above ambient. Cell impairment or death occurs as cytoplasmic proteins are coagulated. Unlike photochemical injuries, thermal injuries have no latent period before the onset of clinical signs and symptoms.

Exposures to optical radiation may be described as acute or chronic. In an acute exposure, the radiant energy is delivered to the target tissue within a period of seconds to hours. Chronic exposure occurs over periods ranging from days to decades. The frequency of repeated exposures is also a factor. For example, repeated subthreshold exposures of the cornea to UVB during a day can accumulate to a suprathreshold total exposure, resulting in photokeratitis.

The principal concern in contact lens wear is the exposure of the cornea and conjunctiva to UVB radiation. Light sources which are rich in UVB include mercury lamps, xenon flashlamps, and special sources. Suprathreshold exposure (greater than 0.025 J.cm-2 ) to UVB results in inflammation of the cornea and conjunctiva (photokeratoconjunctivitis), commonly known as welder's flash or snow blindness. The symptoms of extreme pain, sensitivity to light, profuse tearing and greatly reduced visual acuity begin several hours after exposure. In contrast, the physical signs are much less dramatic, involving sloughing of the superficial layers of the corneal epithelium, and swelling and clouding of the cornea. In extreme exposures, the entire thickness of the epithelium may be lost. If the corneal endothelium is damaged, the swelling and loss of transparency of the cornea is even greater.11 The cornea returns to normal within days. Welders experience many episodes of photokeratitis throughout their working lives, apparently without permanent effects on the cornea. However, it has been shown that there are changes to the corneal endothelium with long-term repeated exposure to UVB from welding arcs.12 Exposures of more than 0.75 J.cm-2 of UVB can produce anterior subcapsular cataracts.13

Chronic exposure to environmental UV may be a factor in the development of droplet keratopathy and earlier development of some age-related changes in the cornea. The prevalence of degenerative changes to the conjunctiva (pterygium and pinguecula) appears to correspond to exposure to solar UV and certain occupations. Chronic exposure to UVB and possibly UVA in the environment has also been implicated in the development of age-related cataract.14

The need for ocular protection from environmental and occupational exposures to UV radiation is generally accepted. Many spectacle and contact lens materials provide little protection from UV. Recently, several RGP and hydrogel contact lens materials have been introduced that offer various levels of protection from UV.15 Several studies have shown that areas of cornea that are covered by UV-blocking contact lenses are protected from photokeratitis.16-19 It has also been demonstrated that an eye wearing a UV transmitting contact lens is at no greater risk of UV-induced injury than an eye without a contact lens.18

Visible light levels which are high enough to produce retinal damage, are uncomfortable and trigger aversion responses and the blink reflex to protect the eyes. When individuals look at light sources such as the sun (especially during eclipses) and lasers, a temporary or permanent loss of function of the exposed retina may result. Visible light damage may be either photochemical (due to UVA or short wavelength visible light), thermal (due to long wavelength visible light and/or IR), or a combination of both. The blue light threshold for retinal damage is several orders of magnitude below that for red light or IR. The primary site of injury is the photoreceptors; it is more usual for the damaged cells to recover, and there is normally only a temporary loss of function of the affected area of retina. Thermal injuries occur at the retinal pigmented epithelium and normally result in a permanent loss of function in the affected area.

Contact lens wearers often complain of photophobia (severe discomfort from light) when wearing their lenses. Although 8% more light reaches the cornea through a contact lens compared to a spectacle lens, it is difficult to understand how a relatively modest increase in retinal irradiance can result in visual discomfort. It has been suggested that fluorescence of the crystalline lens of the eye in the presence of high levels of UVA in the environment may be a factor.20

Should contact lenses be worn in the school science laboratory?

The question of whether contact lenses cause a greater risk of ocular injury in an accident has been raised ever since they were first introduced. Much misinformation and many misconceptions appear and reappear in the popular press as well as in industrial safety articles. There are many anecdotal reports as well as documented cases and laboratory studies which show that contact lenses provided protection or at least reduced the severity of injury. In many instances, contact lenses are far safer than spectacles. There is no evidence that contact lenses increase the risk of accidental injury to the eyes.

Contact lenses are popular among adolescents. When students wearing contact lenses enter the school science laboratory, they should be informed of the hazards that my be encountered as they participate in classroom activities and instructed in the proper use of protective eyewear. It is important that the teacher has conducted an appropriate hazard analysis, and ascertained whether contact lens wear during a particular experiment is unduly hazardous. An individual who must wear contact lenses during that session (see Table 2 ) may be adequately protected by wearing closed eyecup goggles or unvented cover goggles. In the event that a contact lens wearing individual is exposed to volatile materials, smoke, fumes or airborne irritants, it may be advisable to clean and disinfect the contact lenses after the class.

Stones and Spencer21 have concluded that a universal ban on contact lenses in the workplace cannot be justified. When proper eye safety practices are followed and contact lens wearers are adequately trained in the proper use of their lenses, there should be no greater risk of eye injury in the industrial workplace. Each case must be decided on its own merits. The same can probably be said of the school science laboratory.

First Aid Procedures

Regardless of how carefully experiments are preformed or whether adequate safety precautions are taken, accidents do happen. Contact lens wearer should carry a card or Medic Alert bracelet identifying the type of lenses worn. They should also have contact lens solutions and spectacles on hand in case the lenses have to be removed.

In the event of a chemical splash in the eye:

• Remove the contact lenses immediately, if possible.
• Irrigate the eye immediately with copious amounts of fresh water for at least 30 minutes.
• If the lenses cannot be removed, irrigate the eyes with lenses in place and let the stream of water dislodge the lenses.
• Transport the victim to emergency care facilities as soon as possible, continuing to irrigate the eyes.

In the event of mechanical trauma or foreign body injury:

• Cover the injured eye with a hard protective shell. DO NOT APPLY PRESSURE DIRECTLY TO THE EYE.
• Cover both eyes lightly with gauze bandage to prevent eye movements which may cause further injury.
• Transport immediately to emergency care facility.
  

TABLE 1
Differences between Rigid and Soft Contact Lenses

Table 2
Reasons Why a Contact Lens May Not Be
Removed in the Science Laboratory (Adopted from Cullen1 )

Table 3
Optical Radiation Effects on the Eye

References

1. Cullen AP. The Environment in Bennett ES, Weissman BA (eds). Clinical Contact Lens Practice. Philadelphia, J.B. Lippincott, 1992.
2. Chartrand PE. Analysis of 1989 accident causal statistics for eye injuries in the Ontario construction industry. Toronto, Construction Safety Association of Ontario, 1990.
3. Duke-Elder S, MacFaul PA. System of Ophthalmology, Vol. 14, Injuries, Part I. Mechanical Injuries. St. Louis, C.V. Mosby Company, 1972. pp 34-48.
4. Everett K, Jenkins EW. A Safety Handbook for Science Teachers, 4th Ed. London, John Murray (Publishers) Ltd., 1991.
5. Nillson SEG, Lindh SHS, Andersson L. Contact lens wear in an environment contaminated with metal particles. Acta Ophthalmol (Copenh) 1983; 61: 882-888.
6. Ritzmann KE, Chou BR, Cullen AP. Contact lenses as eye protectors against mechanical trauma.Internat Contact Lens Clinic 1992; 19: 162-166.
7. Dickinson F. Contact lenses: The safety factor. Optician 1969; 157(4070): 355.
8. Nilsson SEG, Andersson L. The use of contact lenses in environments with organic solvents, acids or alkalis. Acta Ophthalmol 1982; 60(4): 599-608.
9. Wesley NK. Chemical injury and contact lenses. Contacto 1966; 10(3): 15-20.
10. Cullen AP, Chou BR, Egan DJ. Industrial non-ionizing radiation and contact lenses. Can J Pub Health 1982;73:251-254.
11. Cullen AP, Chou BR, Hall MG, Jany SE. Ultraviolet-B damages corneal endothelium. Am J Optom Physiol Optics 1984; 61: 473-478.
12. Karai I, Matsumara S, Takise S, Horiguchi S. Morphological changes in the corneal endothelium due to ultraviolet radiation in welders. Br J Ophthalmol 1984; 544-548.
13. Pitts DG, Cullen AP, Hacker PD. Ocular effects of ultraviolet radiation from 295 to 365 nm. Invest Ophthalmol 1977; 16(10): 932-939.
14. Taylor HR, West SK, Rosenthal FS et al. Effect of ultraviolet radiation on cataract formation. N Engl J Med 1988; 319(22): 1429-1433.
15. Chou BR, Cullen AP, Dumbleton KA. Protection factors of ultraviolet-blocking contact lenses. Internat Contact Lens Clin 1988; 15: 244-250
16. Pitts DG, Lattimore MR. Protection against UVR using the Vistakon UV-Block soft contact lens. Internat Contact Lens Clin 1987; 14: 22-29.
17. Bergmanson LPG, Pitts DG, Chu LWF. The efficacy of a UV-blocking soft contact lens in protecting cornea against UV radiation. Acta Ophthalmol (Copenh) 1987; 65: 279-286.
18. Cullen AP, Dumbleton KA, Chou BR. Contact lenses and acute exposure to ultraviolet radiation. Optom Vis Sci 1989; 30(6): 407-411.
19. Ahmedbhai N, Cullen AP. The influence of contact lens wear on the corneal response to ultraviolet radiation. Ophthal Physiol Opt 1988; 8: 183-189.
20. Dumbleton KA. The effect of UV-A induced lenticular fluorescence on visual finction. M.Sc. Thesis, University of Waterloo, 1988.
21. Stones I, Spencer G. Contact lenses in the workplace - the pros and cons. Publication P85-1E. Hamilton, Canadian Centre for Occupational Health and Safety, 1985.