Final answer:
The scale used to measure sun tolerance of the skin is referred to as the sun protection factor (SPF), which specifically measures the product's protection from UVB rays and helps in guarding against immediate skin damage and long-term risks like skin cancer.
Step-by-step explanation:
The scale used to determine sun tolerance of the skin is known as the sun protection factor (SPF). SPF measures a sunscreen's effectiveness in protecting against UVB rays, which are solar wavelengths harmful to the skin, particularly within the range of 280-320 nm. The higher the SPF value, the greater the protection offered against UVB radiation. For instance, a sunscreen with an SPF of 30 can filter out 97% of UVB rays. Although moving to an SPF of 50 might seem like an improvement, it only filters out approximately 1 more percentage of UVB rays, which indicates that increasing SPF beyond 30 has a minimal additional effect.
It's important to note that while SPF is a critical measure for sun tolerance, it does not quantify protection from UVA radiation, which is the longer wavelength solar radiation between 315-400 nm. Thus, for comprehensive sun protection, a sunscreen should be selected that protects against both UVB and UVA radiation.