The Sun Protection Factor (SPF) isn't calculated with a single, simple equation. Instead, it's determined through a process involving human volunteers in a laboratory setting. The SPF number represents the ratio of how long it takes UV radiation to cause sunburn on protected skin compared to unprotected skin.
Calculating SPF: The Process
The process involves measuring the time it takes for protected and unprotected skin to redden slightly from UV exposure. The formula is:
SPF = (Time to sunburn with sunscreen) / (Time to sunburn without sunscreen)
For example:
- If unprotected skin reddens in 30 seconds, and the same skin protected with sunscreen reddens in 900 seconds, then the SPF is 900/30 = 30.
This means the sunscreen provides 30 times the protection against sunburn compared to no sunscreen. The SPF value is always rounded down to the nearest whole number. You may see an SPF followed by a "+" (e.g., SPF 30+), indicating a slightly higher protection than the stated number.
Factors Affecting SPF
Several factors can influence the effective SPF:
- Application amount: Applying insufficient sunscreen dramatically reduces its effectiveness. The recommended amount is approximately 2 mg/cm², or about 1/3 teaspoon for the face and neck.
- Frequency of reapplication: Sunscreen needs reapplying every two hours, or more frequently if swimming or sweating.
- Type of UV radiation: SPF primarily protects against UVB rays, responsible for sunburn. It offers less protection against UVA rays, which cause skin aging and damage. Broad-spectrum sunscreens protect against both UVA and UVB.
Understanding SPF Ratings
A higher SPF number does not mean proportionally higher protection. The difference in protection between SPF 30 and SPF 50 is relatively small. Choosing an SPF 30 or higher is usually sufficient for most individuals.
