An allergic reaction occurs when the immune system reacts against substances like sunscreen or too much sunlight as if these substances were trying to attack the body. This reaction can cause problems that affect the skin, mucous membranes, lungs, digestive organs, and blood vessels.

Symptoms of an allergic reaction may include:

• Rash.
• Swelling of the face.
• Hives. Some people might get hives when they spend too much time in the sun or heat.

Sunscreens can cause skin reactions.

• Chemicals in the sunscreen can cause a skin rash (contact dermatitis).
• Proteins in the skin can react with the sunscreen and cause a rash when exposed to sunlight (phototoxicity).
• A reaction between skin proteins, sunscreen chemicals, and ultraviolet light (UV) exposure can create a substance (antigen) in the bloodstream that causes an allergic skin reaction (photoallergy). The antigen can remain in the body and cause future skin reactions with exposure to light.

A very rare, but more severe, allergic reaction can cause breathing difficulties. Emergency care is needed for any reaction this severe.