Xeroderma pigmentosum presents many challenges for those who have the disease as well as those who care for them. There are a number of variations (i.e., complementation groups) of the disease itself, as well as a range of symptom manifestations among individuals within any group. Given the rarity of XP, it is understandable that much is not yet known about how to best provide protection. Of necessity, we resort to generalizations such as "avoid all sources of UV radiation."
We are well aware that this is easier said than done. What to avoid? How to protect against that which we cannot avoid? Simple questions. The answers are not so simple. We will attempt to answer here, but rest assured that as we learn more, the answers will improve.
What to avoid? Ultraviolet (UV) radiation is first on the list. Also, XP patients exhibit photosensitivity, so must avoid bright visible lighting. In addition, XP patients are susceptible to other agents (carcinogens) that may lead to DNA damage, and are advised to avoid these substances.
For this discussion we consider only sunlight and any other light source capable of emitting UV. UV has the greatest potential to do harm. XP patients are advised to avoid all light sources that are likely to be high in UV content. The sun is the most prevalent and most significant source of ultraviolet radiation (UV) that causes the DNA damage leading to development of cancers.
Ultraviolet is that portion of the electromagnetic spectrum with wavelengths ranging from just beyond (shorter than) violet light (thus ultraviolet) down to the wavelengths known as x-rays. The shorter the wavelength, the greater the energy level, and the greater the effect upon DNA. For XP patients it is this DNA damage that is important. The XP patient lacks the capability to repair this damage. This repair deficiency varies, depending upon complementation group.
The longer UV wavelengths (UVA) are closer to visible light and represent the greatest proportion of the UV reaching the surface of the earth from the sun. The amount of UVB from the sun is much less than UVA, but the effects upon DNA are much greater. UVC from the sun does not reach the surface of the earth due to the protection of the ozone layer. That this protection is changing is of much concern for everybody. Still, the XP patient is at risk from very low levels of UV (UVB being the most concern, but we cannot discount the effect of UVA). How low? We don't know. That is the most important point in determining the XP Society position with respect to protection.
Since no authority can say, with any degree of certainty, what amount of UV is safe, the XP Society must resort to less than precise descriptive terms. We began by stating that UV must be "zero." This had its basis in the fact that a "zero" reading could be achieved, using meters available at the time, in any home by covering the windows (tinting plus blinds or drapes) and using only low wattage incandescent lighting. Later, as we acquired other, more sensitive, instruments we could provide a little more direction. Using meters capable of measuring UVA and UVB we find that this environment represents UVA+B levels of approximately one microwatt per square centimeter or less. These same meters would indicate 5,000 or greater microwatts per square centimeter in direct noon sunlight in summer. Please understand that these figures, while real, only apply if the same type and brand of meter is used in a similar environment. We also have anecdotal evidence that indicates that very short exposure to low levels of sunlight (full shade in one case, sunlight through an open window at dawn in another) caused immediate severe burning for one XP patient.
The lack of DNA repair capability is one of the defining characteristics of XP. The effect of exposure to UV is cumulative. Thus dose (in non-technical terms: amount of UV multiplied by time) is very important. If we accept that the "living room" environment described above is an achievable practical standard for long-term exposure, can we accept greater exposure for shorter periods? Here necessity enters the equation. A greater exposure to UV for a short period of time, combined with the "living room" exposure for the balance of the day, will always yield a larger dose for the full day. No matter how brief the "short" exposure.
Thus, all decisions that result in greater exposure must be made in qualitative terms. How important is the need to travel or be in a building where the lighting is not as safe as the (protected) home? How effective is the combination of sunscreen and protective clothing? We expect that children with XP can and will go to school, travel to medical appointments, and participate in other important activities. Travel routes, time of day, use of protective sunscreen, covering the full body with protective clothing, and face/eye protection are all important factors. Even the best fabrics or eye coverings have a finite, measurable degree of protection from visible and UV light. Remember the measurements we got using our UVA+B meters? There was, approximately, a 5,000 to one ratio of full sunlight to the "safe living room." That means that UV transmission of clothing or face/eye protection must be 0.02% or less. Put in another way, they must block 99.98% of the UV. There also must be visible light reduction sufficient to eliminate effects of photosensitivity. We have no data for photosensitivity.
When we take into consideration the irreversible damage to DNA, we must be concerned with the likelihood of accidental exposure when the patient is outdoors. Can a glove, a face shield, a hood be easily pulled off in normal play or other activity? What about fabric tearing? Or the effects of fabric stretching? What about the cumulative extra dosage of UV if outdoors for extended periods? These are among the reasons we have cautioned against relying on protective garments for routine use or for extended outdoor activity or for unsupervised play. When concerns were brought to our attention by parents of children who had such garments, we brought this to the attention of everyone through our print and Web media.
Real concerns have been raised regarding the effectiveness of certain "UV-safe" garments.* While you could pay to have a photobiology laboratory test a garment for sun protection, there remains the problem of test interpretation: what criteria should be the basis for conclusions? All published standards relate to sun protection for the general population, not for the special case of the person with XP. Still, we do recommend that people concerned about the effectiveness of any product contact the distributor or manufacturer, in advance of purchase, requesting test results relating to UV protection. This will certainly aid in making informed choices.
We conclude that the purpose of the XP Society to educate and support is better served if we provide all of this background, as well as anything we learn from research, to families of patients so that they can make individual, intelligent choices. We advocate maximum protection from all sources of UV. Protective garments are but one component in the effort to provide the best possible protection. These garments should fit all the requirements we discuss here, to the extent possible. A well-designed protective garment used for important transportation or other excursions is prudent. The use of such a garment to allow a child to play in the full sunlight is an unreasonable risk in our opinion.