Determining a SPF Rating
The SPF of a sunscreen is based on scientific calculations that are rigorously performed in a laboratory.
The calculations of SPF are based on:
- The amount of time it takes for skin to burn when in the sun.
For example, if your skin burns after 10mins in the sun, a SPF 30 would allow 10x 30 mins of safe exposure in the sun without burning, - However, the score is based on the correct amount of sunscreen being applied to all areas and not washed or rubbed off.
- During laboratory tests sunscreen is applied liberally and the SPF rating is based on liberal application.
- In the laboratory sunscreen is applied at 2mg/cm2.
- This is equivalent to approximately 35mls if wearing a swimsuit.
- For a 110ml bottle of sunscreen that is 3 applications!
In reality, people do not apply the same amount of sunscreen on their skin, as used in laboratories during testing!
- Most people apply 0.5-1.5mg/cm2 which is equivalent to 25-75% of the recommended amount.
- Application procedure can be poor at times with
- Missed areas of sunscreen
- Sunscreen being rubbed or washed off
Protection Category | 1998 Standard | 2012 Standard |
|---|---|---|
Low | SPF 4 – 7 | SPF 4, 6, 8, 10 |
Moderate (or medium) | SPF 8 – 14 | SPF 15, 20, 25 |
High | SPF 15 – 29 | SPF 30, 40, 50 |
Very high | SPF 30 or 30+ | SPF 50+ |
Above is a chart on how SPF standards have changed since 1998.