The Sun in Phuket

 The Sun in Phuket, very hot ! 

Each morning, this star that makes us live, slowly rises into the sky to offer us a new day...
The sunrise around 6am is undoubtedly the best moment of the day.

Coming to spend a holiday in Thailand is the wish of many people. The weather is warm, the sea is beautiful, the beaches are superb and the people are welcoming.

The sun which is our friend can also become an enemy for lack of precautions. Some explanations about the position and stronger radiation in this part of the world . of this sun to be wary of.

 The solar system, reminder...

 Staying in the tropics is great!

The Tropics: it's been a dream word for a long time: used for advertising purposes, for movie or novel titles such as: "the drink of the tropics", "Radio tropics", "Flavors of the tropics", "Christmas in the tropics", "Thunder in the tropics", "Fantomas in the tropics" etc.. ... and then, let's not forget either, much more real and sometimes much more violent... the "tropical storms", "tropical rains", "tropical heat" which are quite frequent in this region, sometimes devastating.

 Staying in the tropics, What is it?

It is to be in this zone located between the two fictitious lines that are the Tropics of Cancer and Capricorn, which are located at equal distance from the equator (cancer north, Capricorn south of the equator) zone tinged with blue on the sketch opposite.

Everywhere in this tropical zone you will have the sun at its zenith, i.e. right above you, vertically: this is one of the particularities of this zone. In Europe for example, and all regions outside this tropical zone, the sun never reaches its zenith.

In Phuket, the sun is at its zenith on September 3 and April 10. From September 3 it crosses the vertical and advances southward... it is the approach of winter in the northern hemisphere and summer in the southern hemisphere. 
From April 10, it is the opposite, the sun crosses the vertical and advances towards the north, the summer approaches in the northern hemisphere.

North of the equator at a latitude of 23°26' N lies the Tropic of Cancer.

The Tropic of Cancer is one of the five main parallels shown on land maps. It is the parallel of 23° 26' 14" N, the northernmost latitude on which the Sun can be seen directly at the zenith on the June solstice.

The Tropic of Cancer is named after the sun entered the constellation of Cancer at the June Solstice about 2,000 years ago.
Because of the precession of the equinoxes, (the slow change in direction of the Earth's axis of rotation) the Sun is now in the constellation Gemini at this solstice.

South of the equator at a latitude of 23°26' S is the Tropic of Capricorn :

The Tropic of Capricorn is one of the five main parallels shown on Earth's maps. It is the parallel of 23° 26' 14" S, the southernmost latitude on which the Sun can be seen directly at the zenith on the December solstice.

The Tropic of Capricorn is named after the sun entered the constellation of Capricorn at the December Solstice about 2000 years ago.

Because of the precession of the equinoxes, , (the slow change in direction of the Earth's axis of rotation) the Sun is now in the constellation of Sagittarius at this solstice.

 In the tropics: an abundance of fruit!

The rambutans, the durian, the jack food, the pineapple, the banana, the papaya, the mangosteen... a delicious and sometimes quite special, even strange fruit tapestry (Durian).

There's no shortage of fruit in the tropics... 

 The evolution of the sun in the sky

Everybody knows that the sun's radiation in the tropics is much more violent than in higher latitudes such as France for example. If the risks of sunburn and sunstroke are limited in France to the summer season, from June to September, in Phuket, these risks extend throughout the year.

It is obvious that the geographical position of Thailand and of Phuket in particular in relation to the equator, is for the most part there.

Far from me wanting to do a geophysics course, (although it is sometimes nice to go back to a CM1 or CM2 level) I just want to point out the difference of the position of the sun compared to what we are used to know in "our Nordic countries" and thus to put the finger on the importance of the position of the sun in Phuket.

 Back to school...

Many, no doubt, have forgotten what the summer or winter solstice is or the equinox of March and September. It's good from time to time to go back to our younger years, when (at least for me) our teachers still had the means and the time to teach us interesting and useful things. 

I wonder if nowadays primary school students are still being taught these essential things that are part of our lives. The sun, the moon, the tides, the seasons... but that's another debate. Let's get back to it! 

On the two animated sketches above, you can see the different positions of the sun depending on the time of year. The two black curves represent the summer and winter solstices. The moving orange curve represents the variations in the position of the sun from sunrise to sunset, depending on the time of year.

 Positions of the sun on the equator from one solstice to another

Equator: 0° 0' 0.000" N 98° 47' 5.062" E 

The latitude of the equator is 0°. Its length is 40,076 km. It is about 10,002 kilometres from the two Earth's poles.

The exact position of the equator is not fixed. The equatorial plane is defined as perpendicular to the Earth's axis of rotation. Although this axis is relatively stable, its position changes over time, changing the position of the equator at the same time. However, this effect is very small.

The Earth's equator is modelled in two standard systems as a circle of radius with an integer number of metres. In 1976, the International Astronomical Union standardized this radius to 6 378 140 m, a value that was later refined to 6 378 137 m by the International Union of Geodesy and Geophysics and adopted for the WGS 84 geodetic system. In these models, the length of the equator is equal to 2π times the length of this radius, i.e. 40 075 035 m in the first case and 40 075 016 m in the second.

During the course of a year, the Sun passes directly to the zenith of the equator twice, at the equinoxes of March and September. Places on the equator experience the fastest sunrises and sunsets in the world. These are also the places on the globe where the Sun moves directly from the zenith to the nadir, and vice versa. In theory, they experience a constant 12-hour day and night duration all year round. However, due to atmospheric refraction and the size of the Sun, these durations vary by a few minutes.

Sunrise and sunset times at the equator do not follow the variations encountered over more temperate latitudes: they are earlier and earlier from mid-February to mid-March, become later between mid-March and mid-July before becoming earlier and earlier again until early November; they then occur later and later until mid-February. These variations do not exceed half an hour, however.

At night, all the stars cross the sky from east to west, tracing a semicircle centred at the southernmost or northernmost point on the horizon.

At tourist attractions on the line of the Earth's equator, it is demonstrated that in a sink, water empties without being deflected by the Coriolis force and that the direction of rotation changes if you move a few metres1. 1 This is a trick because the Coriolis force due to the rotation of the globe is too small to have time to influence the direction of rotation of the water flow in an emptying sink.

Inclinaison de la terre lors des équinoxes
Illumination of the Earth by the Sun on the June solstice.
Illumination of the Earth by the Sun on the December solstice.

 The Positions of the Sun in Paris, from one solstice to another

Paris: 48° 51' 23.810" N     2° 21' 7.999" E 

On this view we can observe that the sun will never rise above 65° at the time of summer.

In winter, the sun is much lazier and finds it difficult to reach a height of 18°, which often results in very sad days at the time of the solstice in December.

Here in Paris on December 21st during the solstice, the sun remains very low even in the middle of the day...

Here in Paris on June 21 at the solstice, the sun reaches an angle of up to 62°.

 The Positions of the sun in Phuket from one solstice to another

Phuket: 7° 52' 49.612" N     98° 23' 32.101" E 

The sun in Phuket goes from north to south and vice versa in April and September.

It is never below 74° in June or 59° in December.

The hottest period in Thailand, especially in Phuket, is March, April and most of May. The sun is at this time between 80° and 90°, so almost vertical.

We can also notice that if in France the north facade of a house is never flooded with sunshine, that it accumulates humidity and cold... in Thailand this notion of north is very different since the sun illuminates and heats the northern parts (from April to September)... this can be taken into consideration when building a house.

In Phuket, it is on September 3rd and April 10th that the sun will be at its zenith, and that it will go from south to north or from north to south...

Here in Phuket, during the December solstice, the sun is in the south and will light up the southern facades of the houses.
Here in Phuket, during the June solstice, the sun is in the north and will illuminate the northern facades of the houses.

All information concerning the positions of the sun can be consulted on this site:

 Equinox, Solstice and Tide (video)

 Duration of days in Thailand

All times shown here are local (Phuket time UTC +7).

Given the proximity of the equator (878 km) there is little difference between sunrise and sunset from one end of the year to the other. Basically, we can say that it is daylight from 6am to 6pm.

The length of the day in Phuket: On this graph we can see that the longest days are between April and September.
The difference is about one hour of daylight between the two periods .

Phuket's climate is tropical and humid, mainly influenced by monsoon winds that change direction according to the seasons.
From March to September, with the sun's rays coming directly over the equator, the Asian continent is warmer than the Indian Ocean. This causes a mass of warm and humid air from the ocean over Phuket and southern Thailand, bringing rain and monsoon from the southwest.

The winds blowing in mid-April come mainly from the southwest, making the air heavy and humid.

The climate in Phuket is pleasant: temperatures throughout the year are fairly regular. It's good all year round, unlike regions further north such as Chiang Mai or Chiang Rai where the temperature differences are much greater.

Traditionally, the Thai New Year (April 13, 14, 15) announces the beginning of the rainy season.
Conversely, also traditionally, it is said that Loy Kratong (mid-November) announces the end of the rainy season.

 Long live Phuket vacation

Sunshine... means danger, sunburn, sunstroke... and in Phuket, much more than in Europe, you have to be extremely wary. The UV radiation index in Phuket is mostly at 11, which is extreme and can be dangerous, especially for young children with white skin.

Moreover, you will quickly notice that the sun hits much harder than it does at home... you just have to take a few steps on the beach or elsewhere to feel cooked. 

Thai people never go in the sun, unless they are forced to (construction workers for example or farmers) ... and it is impossible for them to do otherwise. People who were born and raised in these tropical countries know how to protect themselves very well. Umbrellas are not only used to shelter from the rain, as the name suggests, but also to avoid the sun. These utensils are therefore used all year round. It is not uncommon to see Thai people dressed in a jacket, or with their faces covered by a hood... it is not to protect themselves from the cold, but from the sun.

Thai people don't like to tan. For city people, being tanned is a sign that you come from the fields, basically, that you are a peasant, a worker. On the other hand, the whiter you are, the more you like it, so to speak.
Sometimes, without wanting to get a tan, you can get quite a sunburn... so there...

 Sunburn, watch out, it burns!

A sunburn can happen fast and it can be very painful. You must protect your skin and not skimp on the quality of a protective cream.

However, it can happen that you get caught and you can get a bad sunburn, painful not to be able to sleep...

The skin is red and painful and will come off (the skin "peels"). Small blisters may develop. Sunburn is due to overexposure to UV rays from the sun or sunbeds.

Children under the age of 15 are more sensitive than adults to the sun's harmful rays.

Children under the age of 4 are particularly sensitive to the sun's harmful rays.

Too much sun is certainly not good for a child's skin and especially for a baby's skin.

Children's skin is different from that of adults. Natural protection against UV rays is not fully developed in children. The risk of developing skin cancer later on is greater in children who have suffered sunburn. The speed at which you burn depends on your skin type.

There are 6 different types:

  • Skin types 1 and 2: fair skin, freckles, red or blond hair, light eyes: burn quickly, always become red.
  • Skin types 3 and 4: lightly tinted to matt skin, dark blond to black hair, black eyes: rarely burn, sometimes become red.
  • Skin types 5 and 6: dark skin, dark to black hair, black eyes: rarely burn, does not become red

 UV index of the sun in Phuket

The UV Index is a scale that measures the intensity of the sun's UV radiation and the risk it poses to health.

The UV Index is divided into five categories, corresponding to a level of risk:

Note that in Phuket the index is most of the time at 11!

 The Health Effects of Ultraviolet Radiation

Ultraviolet (UV) radiation has certain advantages:

  • they can kill germs
  • they can treat a variety of skin conditions
  • They help the body synthesize vitamin D.

However, as with any form of radiation, overexposure to UV radiation (rays) leads to adverse health effects, such as :

  • sunburn
  • premature aging of the skin
  • skin cancer
  • eye problems
  • weakening of the immune system

Limit your exposure to UVA and UVB rays as much as possible, as these present many more risks than benefits.

 Health Risks

Damage to the skin

A tan is visible proof that the skin has been damaged by UV rays. It is the skin's reaction to overexposure to UV rays. Sunburn is also caused by too much UV radiation: the skin reddens, gives a burning sensation and may even swell or blister.

Overexposure to UV rays is also responsible for premature aging of the skin:

  • appearance of wrinkles, hardening of the skin
  • skin discoloration
  • loss of elasticity
  • appearance of brown spots ("age spots")
  • precancerous lesions (called actinic keratoses)

Skin damage is cumulative, which means that the daily damage adds up over the long term. UV rays damage the DNA of skin cells. Affected cells can die or repair themselves; if the damage is too extensive to repair, it can promote the development of skin cancer.

Did you know that?

During the summer months, extreme heat can pose health risks, especially if your skin has been sunburned. This is because sunburned skin sweats less efficiently, which reduces the body's ability to regulate its temperature.

 Eye damage

Overexposure to UV radiation can cause temporary painful eye injuries (photokeratitis and photoconjunctivitis). Reflection of light off water, sand, snow, concrete and glass can irritate the cornea and conjunctiva within minutes. It is the sensitive tissues of the eye and eyelid that become irritated. Snow blindness" is a term used by skiers and snowboarders to describe such irritation when they are not wearing protective eyewear.

Whatever the season, remember to wear sunglasses that will protect you from UVA and UVB rays. Exposure to the sun's UV rays can cause premature aging of the eye, lens opacification (cataracts), non-carcinogenic growth of tissue (pterygium), loss of vision (macular degeneration), and eyelid cancer.

 Immunosuppression and infectious diseases

Exposure to the sun can also weaken the immune system (the body's natural defence against disease). UV rays can decrease the body's resistance to bacteria and increase the risk of infections. UV rays can also awaken the chickenpox virus and reactivate the herpes simplex viruses I and II (which cause cold sores).

Did you know that UV rays can also reactivate herpes simplex virus I and II (which causes cold sores).

Tanning has not always been popular. Having the lightest skin possible was once popular in some countries, because being tanned meant working in the fields. Since only people who were comfortable could pay people to do these tasks for them, being pale meant being rich. ---> ( It's still going on in Thailand )

 Sun Safety Tips

Tanning is always risky. Protect yourself by following these tips:

  • Cover up. Wear a light-coloured, long-sleeved shirt, pants and a wide-brimmed hat made of fabrics that allow air circulation. Make sure that the sunglasses you buy offer protection against UVA and UVB rays.
  • Limit the amount of time you spend in the sun. Stay out of the sun and heat between 11 a.m. and 4 p.m. The sun is at its strongest when your shadow is smaller than you. Look for places with lots of shade, such as a park with large trees, partial roofs, awnings, umbrellas or gazebo-style sun shelters. Always bring an umbrella to the beach.
  • Check the UV ratings. Listen to your local radio or television station, or check online for UV readings in your area. When the UV index is 3 or higher, wear protective clothing, sunglasses and apply sunscreen.
  • Apply sunscreen. Apply sunscreen when the UV index is 3 or higher.
  • Drink plenty of cool fluids, especially water, before you feel thirsty. If sunny days also bring heat and humidity, stay cool and hydrated to avoid heat-related illnesses. Dehydration (not enough fluid in the body) is dangerous, and thirst is not always a good indicator of how often you should drink.
  • Avoid using tanning beds. If you do use them, you need to understand their risks and how to protect yourself.

Ask your doctor or pharmacist about the potential risks of interaction between the medicine you are taking and UV rays.

 Health benefits

Production of vitamin D

Vitamin D helps the body absorb the calcium and phosphorus it needs for bone development.

Vitamin D is synthesized by exposure to the sun's UVB rays. However, certain factors (such as season, time of day, cloud cover, smog, skin pigmentation and sunscreen application) affect the amount of UV radiation absorbed by the skin. You will also find vitamin D in fatty fish, eggs and fortified foods (such as milk and margarine). Eating Well with Canada's Food Guide will help you meet your vitamin D needs.

Few Canadians are deficient in vitamin D. However, if you have darker skin, are over the age of 50, don't drink milk or avoid the sun, you are more at risk. Talk to your health care provider who may advise you to take a vitamin D supplement.

The treatment of certain skin diseases

UV rays are used to treat many diseases such as rickets, psoriasis, eczema, infant jaundice, lupus and vitiligo.

 Risk factors for damage caused by ultraviolet rays

Did you know that?

Light-skinned people are more likely to experience UV damage, but dark-skinned people can also be affected.
UV rays can cause damage to anyone, but some people are more sensitive than others.

Several factors can increase your sensitivity to UVR.

You should be especially vigilant if :

  • you have a pale complexion (skin color)
  • your hair is blond, red or brown...
  • you have more than 20 moles, and several of them are larger than 5 mm or if you had at least one large mole already at birth
  • You have freckles and a tendency to blush rather than tan.
  • you had severe sunburns as a child and in early adulthood.
  • you have a hereditary disease such as xeroderma pigmentosum, Gorlin's syndrome or lupus.
  • you've already been treated for skin cancer.
  • cases of skin cancer have been reported in your family...
  • you have been treated with radiation therapy (for psoriasis or another skin problem).

You're also more at risk if:

  • you spend a lot of time outdoors (farmers, gardeners, lifeguards, construction workers)
  • you tan in the sun or with tanning equipment
  • you live or take holidays in a region where the sun is intense (in altitude or in tropical regions)
  • you have had a transplant and are taking medicines that weaken your immune system.
  • you are taking medicines that may increase your sensitivity to UV light (such as birth control pills, antibiotics, anti-inflammatory drugs, tranquilizers, antinausea, antidepressants, medicines for high blood pressure, heart problems, or diabetes).
  • you are in contact with certain chemicals such as tar, soot, coal tar, asphalt, creosote, paraffin, petroleum products and arsenic.

 Ultraviolet or UV rays


Ultraviolet rays, commonly known as UV, are electromagnetic radiation of the same nature as visible light but with shorter wavelengths and therefore not perceptible to the eye.

Their short wavelengths give them a higher energy than light, to the point of causing burns to the human body.

The UV spectrum is subdivided into three bands called UVA, UVB and UVC. The 200 - 320 nm band, used in photoengraving, is called actinic UV.

The sun's rays

On the ground, we receive radiation from the sun consisting of 5% UV, 39% visible and 56% infrared. The UV has undergone a significant attenuation by the atmosphere before arriving on earth.

UVC is filtered by the ozone layer and does not reach the earth.

UVA and UVB, which are attenuated by the atmosphere and suspended particles, nevertheless strike our planet in a proportion of
of 98% and 2% respectively. In space, the intensity of UV rays is such that astronauts protect themselves from them by using visors covered with a thin gold film.

UV radiation increases by 10% every 700 metres. At an altitude of 1500 m, the radiation is 20% more intense than at sea level. 
High clouds stop only 5 to 10% of UV. Those of average altitude stop 30 to 70%. Only the large dark clouds, very low, stop all UV; generally in this case, there is not enough sunlight to encourage undressing.

Harmful effects of UV rays on the body

UVCs are the most dangerous for the organism, fortunately they don't reach the earth.
although some specialized equipment does produce them. Reducing the thickness of the layer

of ozone directly threatens our health.

UVAs are the least energetic but they are more numerous and more penetrating. They are present and dangerous throughout the day. They produce an immediate pigmentation that does not last long: it appears in a few minutes and disappears in a few hours.
Their deep penetration of the tissues down to the dermis destroys elastic fibres and collagen, leading to slackening of the tissues, loss of skin firmness and the appearance of wrinkles. They can lead to genetic changes within the cells.
causing skin cancer (carcinomas, melanomas).

Their dangerousness is aggravated by the fact that they do not cause any discomfort or burning sensation such as sunburn (1000 times less than UVB rays) unless the duration of exposure is very long. UVA rays can sometimes trigger photosensitization reactions (increased UV effects) when taking certain medications.

WARNING, windows do not stop UVA rays.

UVB rays are very dangerous, especially between 12 noon and 4 p.m., because the sun is at its zenith and its rays are hardly refracted by the atmosphere. They are stopped by the stratum corneum of the epidermis (only 40% reaching the dermis) but their energy causes, in case of overexposure, an inflammatory reaction leading to burns that can go up to the 3rd degree. The 1st degree burn is called "sunburn". The redness is due to the dilation of the blood vessels. Its disappearance is accompanied by desquamation and depigmentation.
Prolonged exposure to UVB causes thinning of the epidermis and spots on exposed areas such as hands, arms and face. 
The use of suitable protective glasses can protect the eyes because they can cause lesions leading to photokeratitis (inflammation of the cornea), acute solar retinopathies, cataracts...  They are stopped by glass and clouds. 

Radiation is 100 times more intense in July than in December.

Under their action, the skin develops a tan that disappears in a few weeks.
It is not enough to protect oneself from direct rays, as one must be wary of rays reflected by sand, snow, water or grass, which have a high reflective power, respectively 15, 85, 20 and 2%. This is why you tan more on the beach than in the grass in your garden.
Also, UV rays penetrate the water and at a depth of 50 cm, 60% of UVB and 85% of UVA rays are still present.


At the beginning of reasonable exposure, UVA reacts with melanin precursors and triggers immediate skin pigmentation.
In the days that follow, UVB stimulates the melanocytes, located at the epidermis/dermis interface, which synthesize melanin: this is delayed pigmentation. The melanin produced will then stain the neighbouring cells of the epidermis.
The light tan that sets in is a natural mechanism for protecting the skin against sunburn, which in no way dispenses with photoprotection. This production of melanin takes an average of 70 hours to be effective.
The tan gradually disappears when the epidermis renews itself, it will have completely disappeared in 25 days.
After the age of 30, the number of melanocytes decreases by 8 to 20% every 10 years, making it more difficult to tan.
With each exposure to the sun, the skin becomes less elastic,
the cells do not renew themselves as much, the coloured pigments agglomerate and form skin spots.


- These are products based on a water-resistant dye DHA (dihydroxyacetone). There are different categories depending on the concentration of DHA, intended for fair skin (2.5% to 3%) or darker skin (5%).
- Self-tanners in capsule form are often based on beta-carotene, a colouring agent named after the carrot inside which there is a large quantity. It is a natural pigment that turns into vitamin A once absorbed. It does not increase melanin production, but nevertheless provides partial protection from the sun, thanks to the ability of vitamin A to trap free radicals generated by UV rays.

WARNING, these products are not intended to protect the body from the sun.
Protection factors

The UV Index

The WMO, the World Meteorological Organization and the WHO, the World Health Organization, have jointly set up a solar risk scale, called the UV Index. 

In summer, the UV index reaches 7 - 8 at the seaside. It increases by 10% per 1000 meters of altitude and also in cases where there is a lot of reverberation: snow, sand and water at the beach.

In order to help you protect yourself as well as possible, the solar weather forecast, with the UV index, is now given every day during the summer by the media (newspapers, radio, TV, internet...).

The sun protection index

The sun protection index, or SPF, or sometimes SPI, quantifies the photoprotective efficacy of solar products with respect to ultraviolet rays. An index of protection N multiplies by N the time that one can be exposed before getting sunburnt, it therefore characterizes an anti-UVB protection. If without protection, you catch a sunburn after 15 minutes of exposure, with a protection factor of 8, you will get it in 2 hours (8 x 15 min).
Attention, UV filtering is not proportional to the index. Treatments with an index lower than 10 are not recommended and above 25, the extra protection is very small but nevertheless effective.

The protection is specific to a given person, it depends on the sensitivity of his skin classified among 6 phototypes numbered from I to VI.

Darker skins have more melanin, which naturally filters out UV rays. Albinos cannot tan due to a total lack of pigmentation.

In practice, the amount of product applied to the skin is less than half the amount tested in the laboratory (2 mg/cm2 of skin). In addition, it is necessary to reapply the product every hour on average because of sand, swimming, rubbing of towels, etc...

We consider that the body surface to protect is about 1.5 m2 so 4 hours of beach for 4 people should use about 480 g or a bottle of 350 ml, what a budget !...

Always favour treatments that cover the entire UVA and UVB spectrum. As there is no standard UVA protection class standard yet, do not compare the indices, which remain at the discretion of the various laboratories.

Information on the packaging and explanations

  • - The term "sunscreen" is prohibited, as there is no 100% UV protection.
  • - Waterproof means that the product retains 70% of its filtration capacity after four baths of about 20 minutes.
  • - Water resistant is similar to waterproof except that the number of baths is only two.
  • - Broad spectrum means that the product filters both UVB and UVA.
  • - SPF or Sun Protection Factor is the same as the IP rating.
  • - Organic filters: Filtering is provided by molecules produced by chemical reaction, generally derived from petroleum. They attenuate solar radiation.
  • - mineral filters: These products are generally in the form of a white cream based on titanium oxide, talc, mica, zinc oxide or kaolin. They are very effective for the whole spectrum, they act by reflection of radiation.
  • - IPD (Immediate Pigment Darkening) and PPD (Persistent Pigment Darkening): These are anti-UVA protection indices, they characterize immediate pigmentation and persistent pigmentation.

UVA-induced pigmentation appears rapidly after exposure but this coloration disappears progressively as soon as exposure stops. It is already visible after 2 hours and then slowly fades over the course of 24 hours.

  • The IPD protection factor is calculated as the ratio between the doses required to produce the response respectively with and without sunscreen applied to the skin after two hours of exposure.
  • The wavelength of maximum effectiveness for PDI induction is at approximately 340 nm.
  • The calculation of the PPD protection factor is done in the same way as for PPI but 2 hours after the end of exposure when the pigmentation has stabilized.

The usefulness of UV for health

UVB also enables vitamin D3 to be synthesized by epidermal cells from a cholesterol derivative; it is then transformed into its active form by the liver and kidneys. Vitamin D facilitates the absorption of calcium and phosphorus into the body. A deficiency leads in children to rickets and in adults to osteomalacia (softening of the bones). Exposure of the face for a few minutes a day is sufficient, there is no need to expose oneself naked for several hours.

  • UVCs are used in sterilization, they are called germicides because of their short wavelength. They can be found in hospitals and laboratories, in dentists and hairdressers to sterilize instruments as well as in the food and water treatment industry. The wavelength used is 253.7 nm.
  • UV light is used in the treatment of diseases such as psoriasis (a skin disease characterized by red patches of scaly, itchy skin).

So in Thailand and especially in Phuket, protect yourself during the day, sunbathe very gradually but don't overdo it... and always be attentive.

And then, nothing beats a beautiful sunset like this one, taken at Naiharn Beach.