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 20 th 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 D 3 supplements or the vitamin D 3 blood level [J Allergy Clinical Immunology
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 dose 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 spend 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
Type II (light skin, freckles often, blond or brown hair)—20 minutes.
Type III (the Spainard 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
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 19 th century American author
Henry James, “have always been the two most beautiful words in the English language.”