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UVB and UVA: how different are they and why they represent a threat?

UVB (B as in Burn): They are very energetic and but are stopped by the epidermis. They are responsible for sunburn and, on a longer term, can trigger skin cancers. UVA (A as in Age or Allergy): They are poorly energetic but, deeply penetrate the skin and can reach the dermis. UVA rays are dangerous as there is no warning sign to them. They accelerate skin aging and can induce cutaneous reactions as well as hyperpigmentations as well. Protection factors:

SPF (Sun Protection Factor), protection factor against erythema (sunburn) inducedby both UVB and UVA rays (mostly UVB), determined in vivo. IPD (Immediate Pigment Darkening) and PPD (Persistent Pigment Darkening), UVA protection factors, measured in vivo.