Photoallergy is a type of skin reaction that occurs when certain chemicals applied to the skin interact with ultraviolet (UV) radiation from sunlight. It is an allergic response that develops over time and involves the body's immune system. When the skin is exposed to a combination of certain substances (such as certain medications, skin care products, fragrances, or sunscreen ingredients) and UV light, it can lead to a photoallergic reaction.

UV radiation is capable of altering the chemical structure of these substances, turning them into something the immune system recognizes as foreign or potentially harmful, also known as an allergen. This process is known as photoactivation. Once these photoactivated substances are formed, they can bind to skin proteins, prompting the immune system to react. This response can lead to symptoms like redness, itching, and inflammation on areas of the skin exposed to the sun. Importantly, these symptoms may not show up until one to two days after the exposure and can spread to areas not directly exposed to the sun. Because of the immune system involvement, once a photoallergy develops to a certain substance, future exposure to that substance (in the presence of UV light) can trigger the same allergic reaction.

The symptoms of sun sensitivity occur in areas of sun exposure and are:

• redness
• hives
• blisters
• tanning
• hyperpigmentation

If these are caused by skin care products, you will see these symptom only in areas that ahve been exposed to both:

• sun
• skin care product

Lorem ipsum dolor sit amet consectetur adipisicing elit. Illum neque eaque, autem sit soluta, voluptatum libero magnam tempore ullam at harum vel, ad reprehenderit, nemo veniam quas in voluptas hic. Lorem ipsum dolor, sit amet consectetur adipisicing elit. Natus id officia omnis suscipit aut architecto repellat a quia eaque reiciendis blanditiis perferendis hic, nihil, mollitia. Iste velit aperiam, numquam dolorem.

A phototoxic reaction, similar to a photoallergic reaction, is a type of skin reaction that occurs when certain substances on the skin interact with ultraviolet (UV) radiation. However, the mechanisms and symptoms of phototoxic reactions are different from those of photoallergic reactions because they are not caused by the part of the immune system that medicates allergy such as IgE antibodies.

In a phototoxic reaction, certain chemicals applied to the skin, when exposed to UV light, become activated and can directly damage surrounding skin cells. 

Phototoxic reactions can occur in anyone exposed to the reactive substance and sufficient UV light, regardless of previous exposure, immune status, or history of allergies.

The symptoms of a phototoxic reaction usually develop within hours of UV exposure and closely resemble an extreme sunburn. This may include redness, pain, swelling, and blistering on the areas of skin exposed to the sun. The symptoms of a phototoxic reaction are typically confined to the areas directly exposed to the sunlight and don't spread to shaded areas.

There is a myth that retinoids make you very susn sensitvie.  You can read more about retinol, tretinoin and other topical retinols and how they react in the sun in the blog.

1. Goldenberg A, Jacobs S. in Ch. 16 Contact dermatitis Type 4 Sensitive Skin in Baumann's Cosmetic Dermatology (McGraw Hill 2022)
2. Monteiro, A. F., Rato, M., & Martins, C. (2016). Drug-induced photosensitivity: Photoallergic and phototoxic reactions. Clinics in dermatology, 34(5), 571-581.
3. Di Bartolomeo, L., Irrera, N., Campo, G. M., Borgia, F., Motolese, A., Vaccaro, F., ... & Vaccaro, M. (2022). Drug-induced photosensitivity: clinical types of phototoxicity and photoallergy and pathogenetic mechanisms. Frontiers in Allergy, 3, 876695.
4. Montgomery, S., & Worswick, S. (2022). Photosensitizing drug reactions. Clinics in Dermatology, 40(1), 57-63.
5. Guan, L. L., Lim, H. W., & Mohammad, T. F. (2022). Recognizing photoallergy, phototoxicity, and immune-mediated photodermatoses. Journal of Allergy and Clinical Immunology, 149(4), 1206-1209.
6. Gackowski, M., Osmałek, T., Froelich, A., Otto, F., Schneider, R., & Lulek, J. (2023). Phototoxic or Photoprotective?—Advances and Limitations of Titanium (IV) Oxide in Dermal Formulations—A Review. International Journal of Molecular Sciences, 24(9), 8159.