It is important to keep in mind that SPF is determined following an FDA protocol which applies sunscreen to the skin at a rate of 2 mg/cm-squared of skin surface. If one puts on enough sunscreen to achieve coverage of 2 mg/cm-squared on skin surface and does so sufficiently in advance to have the sunscreen fully active when the skin is first exposed to the sun, the formula, SPF x unprotected time to sunburn, does in fact estimate the added protective value of the sunscreen.
However, many adults actually do not apply enough sunscreen to receive its full protective dose as indicated by the SPF. This is a problem that does reduce the effectiveness of the sunscreen but it is not a problem of the SPF rating. It’s the old problem of, “operator error.”
The problem of instability of sunscreen on the skin due to exposure to water, rubbing with towels, and the like is a concern. However, recent research indicates that most modern sunscreens that are labeled water-proof or water-resistant are actually very stable on the skin even when the skin is immersed in water, exposed to perspiration, toweled dry, etc. That said, the SPF of sunscreens that are not water-proof or water-resistant do appear to be substantially degraded by exposure to water and rubbing.
Some experts have recently argued that under-application is actually more of a threat to the effectiveness of sunscreen than instability over time. Brian Diffey in the UK published mathematical models that indicate a reapplication within 30 minutes of the first application actually may provide better protection than a reapplication after 2 hours, if one assumes that insufficient sunscreen was applied at the first application and that the amount of sunscreen applied following the second application more closely approximates 2 mg/cm-squared of skin surface.
The argument about sun intensity decreasing the effectiveness of the sunscreen SPF is simply incorrect. The effect of greater intensity UV is to shorten the amount of time it takes to sunburn unprotected skin. Given that the protective value of any sunscreen, regardless of SPF, is to extend the amount of time it takes to sunburn based on the unprotected time to sunburn, all sunscreen will protect for less time when UV rays are more intense. But, this decreased protection is not caused by errors in the SPF and in fact the SPF x unprotected time to sunburn formula works just fine regardless of how intense the UV rays.