Update 3/2/2021: this has been fixed. Users should be able to restore their purchases now.
Users who purchased version 1.25 are presented with a paywall screen and cannot restore their purchases. I have just released a fix and Apple should be approving it within the next few hours. Once the release is approved, affected users will be able to restore their previous purchase. Apologies for the inconvenience!
There are two different mechanisms involved in production of a tan by UV exposure.
Firstly, UVA radiation creates oxidative stress, which in turn oxidizes existing melanin and leads to rapid darkening of the melanin. UVA may also cause melanin to be redistributed (released from melanocytes where it is already stored), but its total quantity is unchanged. Skin darkening from UVA exposure does not lead to significantly increased production of melanin or protection against sunburn.
In the second process, triggered primarily by UVB, there is an increase in production of melanin (melanogenesis), which is the body’s reaction to direct DNA photodamage from UV radiation. Melanogenesis leads to delayed tanning, and typically becomes visible two or three days after exposure.
The tan that is created by increased melanogenesis typically lasts for a few weeks or months, much longer than the tan that is caused by oxidation of existing melanin, and is also actually protective against UV skin damage and sunburn, rather than simply cosmetic. Typically, it can provide a modest Sun Protection Factor (SPF) of 3, meaning that tanned skin would tolerate up to 3 times the UV exposure as pale skin. However, in order to cause true melanogenesis-tanning by means of UV exposure, some direct DNA photodamage must first be produced, and this requires UVB exposure (as present in natural sunlight, or sunlamps that produce UVB).
Couldn’t recommend the app more. You can see that someone really thought this through with superb execution. Current UV Index, pulled from the Internet, is part of the calculation. Everything else can be adjusted from skin type to base tan, to the sun protection factor of the sunscreen used, and the history of the previous exposures! There is a web page for the app on which the app is explained in detail. This is the best of all the apps I’ve seen, even if it’s only in English.
Original: Kann die App nur weiter Empfehlen. Man sieht, dass sich hier wirklich jemand Gedanken darüber gemacht hat und dieses Super umgesetzt umgesetzt hat. UV Index wird über Internet abgerufen und fließt in die Berechnung der Dauer ein, steht’s aktuell. Es lässt sich alles einstellen von Hauttyp bis Bräune übern Lichtschutzfaktor der verwendeten Sonnencreme, über Historie der genommen Sonnenzeit von den lätzen Tagen alles vorhanden! Es gibt zu der App eine Web-Seite auf der die App Ausführlich erklärt wird. Das ist das Beste was ich bisher von allen Apps die es gibt was ich gesehen habe, auch wenn nur in Englisch.
Some news I wanted to share – started building an Android version of the Sola Sun UV Calculator and Timer. Meanwhile, if you own an iPhone, make sure to download my app if you want to manage your suntan experience and avoid sunburns.