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?
Even though you say you are laying on the floor, I do believe those areas receive more sunlight. Shoulders in particular can be hit by sunrays whether you are lying on your back or on your stomach. Additionally, it might not be much, but any time you spend standing makes the shoulders receive much of the light directly.