this is strange, but I have the feeling I have to apply more sunscreen to my arms and shoulders. OTOH I feel my legs and torso don’t need as much stuff, neither does my face.
When I sunbath I only wear my swim trunks and I lay on the ground, which means my legs and torso receive the same amount of radiation as my arms and shoulders.
I could understand that my arms and shoulders needed more sunscreen if I walked under the sun, but that’s rarely the case: I lay on the ground.
Does this happen to you?
my arms are thinner than my legs: do I need more sunscreen in the areas where blood vessels are thinner / muscles are leaner?
There are multiple factors that influnce a body part being more at risk of burning. The spots I need to protect the most are my nose, shoulders and ankles. These are all body parts that are constantly visible from above. Ankles are likely more vulnerable because they are usually covered, thus they don’t develop a tan gradually (unless you wear shorts and flip-flops a lot). Noses perspire, and perspiration amplifies burning because skin oils refract sunlight. And shoulders, of course, are probably the most visible body part from above.
Body parts that are in a direct line of sight with the sun for a lesser % of time are less at risk.