Standardized laboratory conditions
SPF is determined with sunscreen applied at a thickness of 2 mg/cm².
What Does SPF Mean?
SUN PROTECTION FACTOR
25 30 50 50+
Every sunscreen displays a number on its packaging (for example 25, 30, 50 or 50+) indicating its Sun Protection Factor (SPF).
It is important to remember that SPF refers exclusively to protection against UVB radiation and does not indicate protection against the entire ultraviolet spectrum.
Furthermore, the SPF value provides only an approximate indication of the actual level of protection achieved during everyday use.
Everyday use
Most people typically apply only one-quarter to one-half of the recommended quantity.
As a result, a sunscreen labelled SPF 50 may provide protection closer to SPF 20–30 under normal conditions of use.
A sunscreen should also provide adequate UVA protection, which should be at least one-third of the UVB protection indicated by the SPF value.