August is Cataract Awareness month, Children’s Eye Health and Safety Month, and for North Americans, the first total solar eclipse in 38 years. All of that in the same month makes for a good opportunity to talk about eye safety as it relates to ultraviolet phototherapy.
Generally we think of three general kinds of eye hazard: short wave UV-effects on the outer parts of the eye, PUVA related cataract risk, and visible/Infrared injuries to the back of the eye. Essentially, there is a progression of damage; the longer the wavelength, the deeper the damage, although a greater number of photons is required to cause damage in the deepest part of the eye.
Direct UV Effects: The most harmful UV photons that can pass through air are in the UVC range, from 200 to 280nm, with peak damage potential at approximately 270nm. Germicidal lamps used for sterilization purposes (254nm) and Broadband UVB phototherapy lamps (280-340nm) are both close enough to this action spectrum to be quite harmful. As little as 10 seconds of direct exposure to germicidal UV room sterilization equipment can cause very painful ultraviolet keratitis. Narrowband UVB sources present less of a hazard, with approximately 30 seconds of exposure needed to trigger damage. Fortunately, all UV photons are very easy to block with protective goggles.
Outside of inadvertent exposure to sterilization equipment, other common sources for direct UV corneal damage include welding arcs, and prolonged exposure to sunlight on reflective surfaces with “snow –blindness” being a well-known example. Less common direct UV exposure associated eye diseases include Climatic Droplet Keratopathy, Pinguecula , and Pterygium .
Oral PUVA Delayed Effects: In some animal models, oral PUVA has been conclusively linked to cataract formation, and psoralen photoproducts can be detected in the human eye following PUVA therapy without adequate protective, post-treatment, eyewear use. Thankfully, retrospective studies appear to indicate that a reasonable attempt at 24 hour post-treatment UVA protection of the eyes mitigates this risk entirely. This need for post exposure protection from low level environmental UVA is a disadvantage of oral PUVA compared to topical PUVA or Narrowband UVB therapy.
Visible Light/IR: Blue light, whether it be from phototherapy sources such as photodynamic therapy lamps, from general lighting lamps or sunlight has the potential to cause photochemically-induced retinal injury, and is potentially linked to age related macular degeneration. Strong IR sources, such as heat lamps, common medical lasers (Nd-YAG, Co2, etc.), and filtered sunlight are particularly dangerous to the retina since they do not appear bright to the human eye and therefore do not trigger the instinct to look away. To protect against visible and IR sources, eyewear has to be matched exactly to the source in question. For example, laser goggles used with YAG lasers will not be adequate for use with high intensity blue LED sources for PDT. In all cases, the manufacturers of the equipment in question should be able to provide guidance on acceptable eyewear.
(As a final comment, although looking into the sun involves UV risk, solar eclipses are also potential sources of very high levels of visible and infrared radiation, which can cause extensive thermal injury to the eye. No phototherapy eyewear is adequate to protect against the combination of UV, Visible, and IR exposure involved in looking directly at the sun, and only specifically made Mylar eclipse filters and #14 (or darker) welding glass should be used.)
Depry et al. UVB-protective properties of contact lenses with intended use in photoresponsive eyelid dermatoses. Photodermatol Photoimmunol Photomed. 2013 Oct;29(5):253-60. doi: 10.1111/phpp.12064.
Yam and Kwok. Ultraviolet light and ocular diseases. Int Ophthalmol. 2014 Apr;34(2):383-400. doi: 10.1007/s10792-013-9791-x. Epub 2013 May 31.
Zhou, et al. The role of ultraviolet radiation in the pathogenesis of pterygia (Review). Mol Med Rep. 2016 Jul;14(1):3-15. doi: 10.3892/mmr.2016.5223. Epub 2016 May 9.
Van Deenen and Lamers. PUVA therapy and the lens reconsidered. Doc Ophthalmol. 1988 Oct-Nov;70(2-3):179-83.