Earlier this month, a physician whom I’ve known since our high school days called me to ask my thoughts about a chronic bumpy rash on sun-exposed areas of one of his co-workers. Besides quoting the phrase ‘a picture is worth a thousand words’ (which, by the way, is attributed to an early 20th century American advertising manager), I told him that some rashes on sun-exposed areas are primary skin conditions without any role for an allergen. I mentioned a few such conditions and advised him to tell his co-worker to see a specialist.
As we engage—to varying degrees– in our annual worship of the sun, I thought it would be helpful to understand how sun-related rashes or photodermatitis are organized in medical thinking. Being “allergic to the sun” is a bit complex to fully understand. The history is still important in determining the diagnosis. But, visualizing the location(s) and the three-dimensional characteristics of the skin condition does take priority.
The most straightforward explanation of a sun-related skin reaction is when there is no prior skin condition and a sunscreen had just been applied to the area before sun exposure. Blisters, as well as red swollen itchy skin, can occur. Oxybenzone (benzophenone-3) is the most widely used UV absorber and consequently is the most common cause of sunscreen allergy. However, an inactive ingredient—such as a fragrance, lanolin, a preservative or a formaldehyde releaser—can also be the explanation.
Patch testing with suspected specific chemicals over four days under a specialist’s care can be done to try to find the cause. If you want to see if you or your child will develop an allergic reaction to a sunscreen, you could test it by putting a small amount over a less obvious area of the skin that would be sun-exposed (such as the forearm) for 15-20 minutes a day over a few days and see if there is such a reaction. Products containing either titanium dioxide or zinc oxide and having the fewest added inactive ingredients should be used when there is sunscreen allergy without a specific chemical that has been identified.
Sunlight reactions can occur from other substances—natural or synthetic. Plants containing furocoumarins can cause a phototoxic response. This type of chemical is present in certain fruits (fig, lemon, lime) and vegetables (celery, fennel and parsley). Medications that can cause phototoxic reactions include Doxycycline, Bactrim and fluoroquinolones such as Cipro. A typical skin change is linear itchy skin on the sun-exposed area that can progress to blisters and hyperpigmented skin. Phototoxic dermatitis usually starts from the free radicals that form within minutes of exposure to sunlight.
Meanwhile, photoallergic dermatitis takes from hours to days to begin. Photoallergic reactions require T cell (the immune cell mentioned in developing immunity from the COVID-19 vaccines) activation. Among medications that are creams or ointments, those that contain nonsteroidal anti-inflammatory drugs (NSAIDs) such as piroxicam (Feldene) and ketoprofen (Orudis) are the most common photoallergic cause. Oral medications that can lead to this type of reaction include a medication for certain heart arrythmias called quinidine and sulfanilamide medications such as Lasix.
The other major category of sun-related dermatitis contains those that are unrelated to external substances. The most common of these is what I mentioned to my friend from high school and college and is called polymorphous light eruption. This primary skin condition occurs especially in people under 40 years old and is more common in women. The condition begins each year in the spring– hours to days after the first major sun exposure. Many cases improve over the course of the spring and summer.
Among other non-allergic conditions of photodermatitis, more common ones include (a) solar urticaria, which appears as hives instead of as patchy redness or blisters within minutes of sun exposure, and (b) chronic actinic dermatitis, which tends to occur in people older than age 50 who have had a prior allergic contact dermatitis. Solar urticaria has responded to omalizumab (Xolair)—the biologic that has been used for allergic asthma and recently approved to treat nasal polyps. Chronic actinic dermatitis may require treatment with immunosuppressive medications.
If we don’t develop any of the above conditions, we still prefer to avoid melanoma, sunburn and wrinkles. We should also want to take advantage of the outdoors because sunlight has actually been shown to be beneficial for our immune systems. A 2019 report showed that UV light exposure can be associated with reduced allergic inflammation in children, regardless of the use of oral vitamin D3 supplements or the vitamin D3 blood level [J Allergy Clinical Immunology 2019, 143:929-31].
An attempt to answer the question of how many minutes are needed for the body to make the required daily dose of vitamin D was addressed in a 2017 study from Spain—a northern mid-latitude country [Science of the Total Environment 1 January 2017, 744-750]. For those with the most common skin type in Spain—light or light brown skin with rare freckles—the amount of time needed to reach the daily amount of natural vitamin D is the following:
- January: 130 minutes with 10% of the body exposed at noon.
- April & July: 10 minutes with 25% of the body exposed in the middle of the day.
- October: 30 minutes.
These numbers would change for those who are fair-skinned or dark-skinned. As a reminder of the amount of time that can be spent outdoors before sunburn and the risks of skin cancer and wrinkling all increase, the thresholds for each skin type are as follows:
- Type I (very light skin, freckles very often, reddish or strawberry blond hair)—10 minutes.
- Type II (light skin, freckles often, blond or brown hair)—20 minutes.
- Type III (the Spaniard complexion mentioned above)—30 minutes.
- Type IV (light brown or olive-colored skin, no freckles, dark brown hair)—50 minutes.
- Type V (dark brown skin, dark brown or black hair)—more than 60 minutes.
- Type VI (dark brown or black skin, black hair)—more than 60 minutes.
Despite these time durations, we should not tempt fate. The following advice from the American Academy of Dermatology website is still the best way to avoid problems from the sun:
- Apply one ounce of a sunscreen with a SPF of 30 or higher that protects from both UVA and UVB rays so as to cover the sun-exposed areas, and reapply it either every two hours or after swimming or sweating.
- Wear sun-protective clothing, including a wide-brimmed hat and large frame/wraparound sunglasses, whenever possible.
- Try to avoid being outdoors during the hours of 10am to 2pm, when the UV rays are the most intense.
These recommendations are ‘nothing new under the sun’ (Ecclesiastes 1:9). But, they will allow us to enjoy ‘summer afternoon’ which, in the words of the 19th century American author Henry James, “have always been the two most beautiful words in the English language.”