**Question:** A sunscreen ointment has a Sun-Protection-Factor (SPF) of 15. How much of UV radiation it filters out?

**Core Concept:** Sunscreen ointments are products designed to protect the skin from harmful ultraviolet (UV) radiation from the sun. The Sun Protection Factor (SPF) indicates the product's ability to filter out UVB rays, which are the primary cause of skin damage and burns.

**Why the Correct Answer is Right:** Sunscreen ointment with an SPF of 15 means it filters out 93% of UVB rays (UVB = Ultraviolet B). An SPF of 15 indicates that it reduces the risk of sunburn by 15 times compared to the skin without any sunscreen protection. This means that a person with normal skin would get sunburned in 1/15th the time on the sunscreen-protected skin compared to unprotected skin.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because it does not match the correct SPF value mentioned in the question (15). An SPF of 15 represents 93% UVB filtration, so any value other than 15 cannot be the correct answer.

B. This option is incorrect because it does not account for the exact SPF value provided (15). An SPF of 15 indicates a UVB filtration rate of 93%, not 80%.

C. This option is incorrect because it does not match the correct SPF value mentioned in the question (15). An SPF of 15 represents 93% UVB filtration, so any value other than 15 cannot be the correct answer.

D. This option is incorrect because it does not account for the exact SPF value provided (15). An SPF of 15 indicates a UVB filtration rate of 93%, not 75%.

**Clinical Pearls:**

1. The SPF value represents the percentage of UVB radiation that the sunscreen can filter out. A higher SPF value indicates better protection against UVB radiation.
2. Sunscreen is effective against UVB radiation, not UVA rays (UVA = Ultraviolet A). UVA rays are also harmful and contribute to skin aging and skin cancer risk. Use a broad-spectrum sunscreen with an SPF ≥30 for comprehensive protection against both UVA and UVB rays.
3. The SPF is a measure of protection against 9-10% of UVB radiation, as it is based on the time required for sunburn to occur on sunscreen-protected skin compared to unprotected skin. 93% UVB filtration means that 7% UVB radiation reaches the skin.
4. Sunscreen usage is crucial for reducing skin damage, premature skin aging, skin cancer risk, and skin burns due to UVB radiation. Additionally, sunscreens can be used in conjunction with other sun protection methods, such as wearing protective clothing, seeking shade, and using indoor protection measures like wearing sunglasses and a hat.