Does hyaluronic acid cause sun sensitivity?

Hyaluronic acid does not cause sun sensitivity. The reason you do not need to worry about additional sun avoidance when using hyaluronic acid is that it is not very acidic. The pH of solutions of hyaluronic acid is close to the natural pH of the skin. The form that is most often used in skincare is actually not an acid at all - it is a salt sodium hyaluronate. Either form of hyaluronic acid does not exfoliate the skin and does not make it more vulnerable towards the sun. 

This means that you can safely apply products with hyaluronic acid under your sunscreen during the day. In fact, it is a great way to incorporate hyaluronic acid into your routine: apply a hyaluronic acid serum on damp skin in the morning (go for the high molecular weight one, listed as “hyaluronic acid” in the ingredient list if your skin is prone to sensitivity), and seal it in with the sunscreen. Do not use hyaluronic acid without following up with a moisturizer, especially in a dry climate - otherwise it can “pull” the water from your skin leaving it dryer.  

Photo by  Ameen Fahmy