It appears that this may be the first study to date to use the concept of Skin UV Index to evaluate the potential efficacy of a sunscreen. Based on the WHO’s sun policy guidelines ³² that have been adopted by Australia and New Zealand ¹⁰, one may consider that all sunscreens with SPF ≥ 4 should generally offer effective protection against sunburn and melanoma. This is because the estimated SUI values (Figure 1) are all below 3 when sunscreens are applied in full compliance with the product instruction. In other words, incidences of sunburn or solar radiation-initiated melanoma after sunscreen use may not be used to evaluate their intrinsic sunscreen activities because these incidences may only largely reflect relative degrees of non-compliance with the application instruction under certain givenconditions. Therefore, the present work may provide a rationale of why an SPF 8 sunscreen may be effective against melanoma in the recent Australian study (15; more discussion later). Also, it appears that our consumers have been routinely warned to avoid sun exposure during the peak sun hours from 10 am to 2 pm without considering the UV Index for that day. For example, if the solar UV Index is only 3 or 5, rather than 10 as used in the present simulation, an SPF 4 sunscreen would yield a SUI of only 0.75 and 1.25, respectively, which probably should be considered as generally very safe. Also, in different places or on different days the peak solar UV Indexes could be less than 2 or 3 which generally may not cause a long-term harm to our skin ^{2, 8}.

It is well recognized that in the real world, full compliance with product instruction is probably very rare. Regardless of sunscreen strength or sunscreen brand, the most serious problem is probably the missing application, especially in the back of the trunk, on the legs, around the ears and on the eyelid area ^{18, 19, 25, 35, 36, 37}. It is very likely that sunburn would occur on these missed areas when exposed to intense burning sunlight ^{19, 20, 25, 26, 35, 36, 37}; the sunburn is a known major risk factor for melanoma incidence. This is consistent with observations that melanoma occurred most often in the back of trunk in men where it is usually covered with clothes except during sunbathing ³⁵. According to a telephone survey of 100 British adults, less than half of the people questioned would always apply sunscreen to all uncovered areas ¹⁹. In an elegant Danish study ²⁵, it was found that the sunscreen was applied to only 80% of the total available exposed skin; in other words, a total of 20% of the exposed area did not have any sunscreen applied. Therefore, it is not surprising to find a report showing that after one week of a sunbathing vacation at a subtropical resort all Danish participants (N = 25) suffered unintended sunburn ³⁸; in this study a broad-spectrum SPF 30 sunscreen was applied and a term “sunburn holiday” was used to describe such a holiday ³⁸. Also, in one study involving 1,360 children from Sweden ³⁹ sunscreen use was determined to be an independent risk factor for causing sunburn. In a study from Austria ⁴⁰, sunscreen use was found to triple melanoma risk. Interestingly, sunscreen use was found to increase in Norwegian women between 1997 and 2007 but this increase was not accompanied by a decrease in sunburn ^{13, 41}, as well as by a reduction in melanoma incidences in later years ⁴².

Recently in the US, results from two randomized, double-blind, split-face studies ^{29, 30} seemed to show superiority of SPF 100+ sunscreen over SPF 50+ sunscreen in preventing sunburn. For example, in a one-day study 5% of participants using SPF 100+ developed sunburn while 55% of participants using SPF 50+ developed sunburn ²⁹. Most likely, the outcome might be quite different if the studies were conducted at a beach for sunbathing for one or several days. In another study ³⁰ using an SPF 100 sunscreen on 7 body sites (back of the trunk not included) for sunbathing at a beach 25% of the participants developed sunburn. Likewise, the sunburn rate might be much higher if more days and the back of the trunk were included in the study. In a recent large-scale comprehensive novel study ⁴³, sun-sensitive and non-sun-sensitive sunscreen users without employing other sun protection measures were associated with 64% and 26.2% of sunburn incidences, respectively. For sun-sensitive users use of sunscreen with all other protective measures such as seeking shade, wearing long sleeves and hats still yielded a 26.2% probability of getting sunburned ⁴³.

The above limited review of the literature may suggest that using sunscreen to totally prevent sunburn in the real world may be virtually impossible and the possibility of incurring unintended sunburn especially after repeated sunbathing at a beach with high solar heat intensity would be very high or almost a certainty. It is not surprising that sunburn did indeed happen to his patients in the US who had applied the sunscreen diligently after being advised by their dermatologist ²⁸.

Since in the US and many Northern European countries, sunscreens have been mainly used intermittently for intentional purposes ^{12, 13} such as at beaches with high or very high UV Indexes, the chances of suffering from unintended sunburn and hence unintended melanoma might be quite high. Therefore, one may hypothesize that sunscreen use may be considered as a major contributing risk factor for the exponential increase of melanoma incidences observed globally in the last few decades ⁴². The above hypothesis seems consistent with the known steady increase of global sunscreen sale in the last few decades apparently due to repeated advice from health professions and aggressive marketing campaign by the sunscreen industry. This view is also consistent with results from an extensive meta-analysis showing that sunscreen use was positively associated with melanoma incidences in countries of higher latitudes such as the US and Norway ¹⁶. The reported efficacy of using SPF 15 or greater for reducing melanoma incidences in Norway ¹³ may thus appear to be inconsistent with the present reasoning. It is noted in that study ¹³ sunscreen users reported significantly more sunburns than nonusers which is consistent with the present work. On the other hand, regular, daily, non-intentional use of sunscreen should be protective against sunburn, melanoma and other sunlight-related skin damage in lower-latitude, subtropical countries such as Australia, where the whole population is exposed to high ambient radiation ^{10, 15}. It appears that the benefits from daily non-intentional use of sunscreens should also be attainable for those people residing in temperate climates.

The above simulations assume that an SUI of 3 may cause sunburn especially for sun-sensitive Caucasians or the white population in the US. Most of residents in the US may probably be able to tolerate a higher SUI for the following reasons. First, there does not seem to have had noticeable adverse reports related to the new Australian sun policy in the last several years ^{10, 44} since its implementation. Second, it seems well accepted that daily exposure to mild or moderate (UV Index 3 to 5?) sunlight may be protective against sunburn and melanoma ^{4, 8}. Third, contrary to classical concepts that photoaging contributes up to 80% or 90% of skin aging, it was recently reported that generally skin aging may be mainly caused by the intrinsic nutritional factor ^{2, 8}. Fourth, regular weekend exposure to sunlight without sunburn was found to be protective against melanoma in a British study ⁴⁵. Although the solar UV Indexes in England were not reported in the study ⁴⁵, they were probably expected to be about 4 to 6 in summer months ⁴⁶. For non-white populations in the US their skin may be expected to tolerate even much stronger sunlight with the SUI probably approaching about 6 to 8 for one hour without incurring sunburn (8; more studies needed). In this regard the melanoma rate for blacks was reported to be about 20 to 30 times less than for whites ⁴⁷. Moreover, there was no evidence to support the association of solar UV exposure and melanoma incidence in black or Hispanic population ⁴⁷ and the major locations of their melanomas were often in relatively non-sun-exposed areas, such as the soles of feet, the palms of hands, toenails and oral gums ^{47, 48, 49}. It seems that there is a great need for research to be conducted relative to the use of sunscreens in non-white populations because of its rapidly increasing share of the total population in the US.

When only 50% or 25% of the SPF 8 sunscreen is applied as this might occur in the real world, it is expected to yield a SUI of 3.6 or 5.9 (Figure 2 and Figure 3). However, when the decrease in solar UV intensity with a reduction of applied amount is estimated by a linear method ^{50, 51}, the SUI would decrease to 3.1 or 5.0. In view of the discussion in the above section, it is likely that most residents in Australia can partially or fully benefit from the low SPF sunscreen especially when the applied amount is about 50% or more. Also, the SUI would decrease considerably if skin exposure to peak sun light is avoided, that may in turn enhance considerably the efficacy of the sunscreen.

In view of the above simulations and discussion it is proposed that an SPF 8 sunscreen may be adequate for sun-sensitive, light-skinned white populations. For non-sun-sensitive, darker-skinned, non-white populations in the US, sunscreens with very low SPF numbers such as 2 to 6 may be adequate. Support of this suggestion may be evident from the following example. If a person who can tolerates a UV Index of 7 without incurring a sunburn is regularly exposed to a sunlight with a UV Index of 8 while wearing an SPF 2 sunscreen, then the estimated Skin UV Indexes will be 4, 5.7 and 6.7 when full, 50% and 25% compliance in the amount are applied, respectively. Sin these Skin UV Indexes are all below 7, therefore, that person may not be expected to develop sunburn and is then safe to be exposed to that sunlight (footnote). Potential variability in sun sensitivity among different ethnical groups in non-white populations in the US may remain to be explored. Among some two dozen active sunscreen ingredients approved today for marketing, only two natural minerals, namely, zinc oxide and titanium dioxide, are regarded by the US Food and Drug Administration as safe and effective ¹. For various reasons such as their extremely high chemical stability, non-absorption into the body ^{1, 52} and apparently proven safety track record in the last several decades of worldwide use, they should probably be considered as preferred active ingredients (detailed discussion being beyond the scope of this work).

In addition to providing a new perspective on skin aging and skin anti-aging as briefly mentioned above, the recent  nutrition-based aging/anti-aging  theory of Chiou ² has also been successfully applied to develop a very simple, safe, quick method for regeneration of diverse tissues such as hairs, nails, skin and gums in humans by topical application of a high concentration of propylene glycol, a nutrient, apparently through rejuvenation of a tissue’s stem/progenitor cells ^{53, 54}. This theory ² could also provide a rationale of why a dramatic age-reversing, potentially age-prolonging, systemic effect could be observed after a simple rejuvenation of hearts in old rats ^{8, 55}. It is also of interest to note that high concentrations of propylene glycol can also serve as an extremely effective, safe, universal topical microbicide ^{56, 57} with highly desirable moisturizing and absorption-enhancing properties.

**  It may be useful to report an accidental  observation below: A long-acting (lasting about 12 hours) moisturizer containing about 50% of glycerin ⁵⁸ and determined to have an SPF of about 2 was used by one Taiwanese female   before hiking in the summer while her sister did not wear any cosmetic or sunscreen. At the end of hiking the sister without applying the moisturizer got sunburned while the other sister did not get sunburned. The sun protection effect of this moisturizer is probably due to the moisturizing property of glycerin and water as well as the firming property ⁵⁸ of glycerin through apparent rejuvenation of stem/progenitor cells in the skin ⁵³.