How long does it take to develop a tan?

When you expose your skin to the sun, it takes on colour and darkens. This is a protective mechanism to withstand UV rays, potentially accelerating skin laxity and causing hyperpigmentation, sunburns or skin cancers.

However, the time needed to tan varies from person to person and is also influenced by environmental conditions. To prevent redness and skin dryness associated with excessive sun exposure, here are the various factors to consider when calculating your tanning time.

• Phototype.

  The very first factor to consider before sun exposure is the phototype, that is a person’s skin colour and their capacity to tan without burning. The most widely used classification today is the Fitzpatrick scale, which distinguishes six phototypes from the lightest to the darkest. Individuals with phototypes I and II, for example, have difficulty tanning and frequently suffer sunburn. They therefore need to limit their exposure time compared with those with naturally darker skin, otherwise they risk reddening rather than developing a tan.

• Time of day.

  It is when the sun is highest in the sky, at noon, that its rays are most intense. Indeed, the distance travelled by UV radiation through the atmosphere to reach the Earth's surface is shorter in this case. The rays are thus less dispersed and encounter fewer air molecules that can absorb or scatter UV radiation. On average, UV radiation is at its strongest between 11:00 and 16:00.

• The presence or absence of cloud cover.

  It may seem obvious, but the time required to tan will not be the same if the sun is high in the sky or if the weather is overcast. It remains possible to tan when clouds are present, but as they block on average 30 to 60% of the sun’s rays, a longer exposure is required.

• Exhibition site.

  The place where you expose yourself also has an impact on tanning time. Indeed, you will tan more quickly if you are surrounded by snow, sand or are floating on water, as these elements reflect the sun’s rays. Furthermore, at altitude the rays are more intense because you are closer to the sun. This is why it is very important to apply sunscreen on winter holidays in the mountains, due to the strong light reflection by the snow.

• Ambient humidity.

  Even more surprisingly, the humidity level of the environment where you tan can affect the time needed for the skin to tan. In fact, when you are in a humid setting, the epidermis is less prone to dehydration induced by sun exposure. Thus, it experiences less redness and tans more quickly.

Note : The position of the sun in the sky, sunshine, altitude, the presence of reflective surfaces... are all factors taken into account to calculate the UV index, a measure that evaluates the level of the sun’s ultraviolet rays and their potential to cause damage to human skin.

The exposure time required for tanning is generally influenced by two factors:

Tanning does not appear immediately after sun exposure. It is the result of the progressive activation of melanin in the skin and requires several days to become visible. A study conducted in the United States with six volunteers (phototypes II to III.5) has assessed the rate of onset of tanning under different UV exposure regimens. The protocol involved exposing small areas of skin (3 × 3 cm) on the back to solar lamps delivering controlled UV doses, expressed in effective erythemal units (EEU, corresponding to the dose sufficient to induce erythema). Three exposure programmes, outlined below, were compared.

Regardless of the programme, a slight tan appeared after about ten days and progressively intensified. Notably, the pigmentation reached a plateau, that is, increasing the doses did not produce a significantly more intense tan. Indeed, the photographs below clearly show that following programme B or programme C yields similar results. Regarding tolerance, the exposures were generally well tolerated. A few mild erythema episodes were observed, particularly in subjects with fair phototypes (II and II.5), especially in programme C, which employed faster and higher dosing.

This study highlights that tanning requires at least one week to become apparent, but a noticeable tan actually develops after 2 to 3 weeks of regular exposure. It also emphasises that overly frequent or intense exposures do not accelerate tanning but primarily increase the risk of erythema.

It is, however, unfortunate that this study only included volunteers with intermediate phototypes (II to III.5). This limits the generalisability of the results, since the rate of tanning onset and UV tolerance vary greatly with skin colour. Very fair phototypes (I) hardly develop a tan and are particularly at risk of sunburn, whereas darker phototypes (V and VI) require more exposure for a visible tan but benefit from better natural photoprotection . In practice, beyond exposure frequency, it is primarily phototype and the UV index that determine the time needed to tan safely. Here are some indicative guidelines:

Important : If the process of melanogenesis occurs to protect the skin from UV rays, it is not sufficient, which highlights the importance of systematically using a broad-spectrum sunscreen on areas of the body exposed to the sun and to reapply it regularly.

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• POMARES L. M. & al. UV index forecasting under the influence of desert dust: Evaluation against surface and satellite-retrieved data. Atmosphere (2020).

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