Sun cream: what is the right dose to apply? Let's clarify the SPF protection factor

The effectiveness of sunscreen largely depends on the correct application of the product and most of us use too little of it.

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Sun creams: how much to apply? And what does the protection factor really mean? Do we really use solar products correctly? Let's try to do some clarity

Most of us use sunscreen the wrong way and this could severely damage the skin. Sometimes, in fact, it is not enough to spread it hastily and then expose yourself to the sun: the passage of the sunscreen on our body must follow precise rules and observe certain trappings, on pain of a shivering burn or longer-term trouble. But what are still the things to know about the sun's rays and sun creams?

While sun protection is key to avoiding the harmful and damaging effects of ultraviolet radiation, many of us apply it incorrectly. Where do these errors come from?

According to a new study from King's College London, on average people receive only 40% of the SPF protection offered by a correct dosage of the product. This means that the protection factor is less than half of that indicated on the package.

UVA, UVB and UVC rays

What not everyone knows is the difference between the different types of ultraviolet rays. Characterized from their wavelength, they are divided into:

  • UVC rays (with a wavelength between 100 and 280 nanometers)
  • UVB rays (between 280 and 320 nm)
  • UVA (from 320 to 400 nm)
  • i UVC rays they are filtered by the ozone layer and do not reach the earth's surface
The UVB (about 5% of the total UV radiation), have a strong energy charge and for this reason they are rather aggressive and stimulate the tan.   They are partly retained by the ozone layer, the troposphere and the clouds, but when they hit the organism they do not go beyond the epidermis. Their intensity is influenced by various parameters such as the season, time of day, latitude and altitude.

UVA (about 95% of UV rays) are retained very little by the atmosphere and clouds, so they are more penetrating and, if they do not cause burns and do not really tan, they are able to penetrate to the dermis, accelerating the processes of skin aging. Unlike that of UVB, the intensity of UVAs that reach the earth's surface is constant throughout the year: this is why protective creams should be applied not only in the summer months, but whenever one is exposed to sunlight.

What is it that tans then?

UVAs cause a oxidation of melanin already in our "endowment", so they give the skin a very short color, which in summer appears just a few hours after the first exposure. Only if you continue to expose yourself regularly, UVB, which stimulate the proliferation of melanosomes, give a real tan, organelles responsible for the production of melanin.

According to the National Cancer Institute, the rate of new melanoma cases among American adults has tripled in forty years. Thanks to a more detailed diagnosis, it should not in any case be forgotten that sunscreen must always be used and that the most vulnerable hours are those between 11 and 16.

The SPF, the sun protection factor

The SPF, the Sun Protection Factor, was introduced in the 60s and for decades the SPF 30 protection seemed more than enough to prevent superficial sunburn. Today there are many more "bands" of protection.

Basically, the SPF rating indicates how many times a person's UVB exposure will be reduced once sunscreen is applied. For example, if it takes 15 minutes in the sun for the skin to burn, spreading SPF 15 “stretches” the effect 15 times, which means it takes three hours and 45 minutes for the skin to have the same effect. A sunscreen with an SPF 100 index means that the same person would be protected for more than 24 hours. Here you will find our interesting article or on a study that would demonstrate the effectiveness of 100+ sunscreens.

However, the effectiveness of sunscreen largely depends on the correct application of the product, and most of us use too little of it.

In general, it is necessary apply about 2 mg of sunscreen per square centimeter to enjoy all the protective benefits. That's roughly double what people actually apply to their skin, according to Antony Young, a professor of experimental photobiology at King's College London. In fact, a previous study had already found that the vast majority of people only apply a third of the recommended dosage for sunscreen.

A systematic review

The London scholars led by Young started from the assumption that when we put on sunscreen we do not pay attention to the quantities and, moreover, in many parts of the body the cream does not even arrive. And this is how it has been estimated that a sunscreen with SPF 50 offers at most 40% of the protection it should.

This is why it should use higher protections and to demonstrate this, the researchers asked 16 volunteers to undergo UV rays baths with different degrees of protection (0,75 mg, 1,3 mg and 2 mg per square centimeter), to simulate sun exposure in a only once or in five days. Small biopsies of the participants' tissues revealed that DNA damage was very high even with a low dose of ultraviolet rays. Furthermore, those who protected themselves with doses of 2 mg per square centimeter for five consecutive days suffered significantly less damage than those who only exposed themselves to UV rays once but with a low dose of protection.

Conclusion? The amount of sunscreen applied makes a big difference: The group repeatedly exposed to the rays had significant DNA damage in areas that did not receive sunscreen. This damage, however, was reduced when the sunscreen was applied with a thickness of 0,75 mg per square centimeter and was reduced even more when 2mg per square centimeter was applied, even with higher UV doses. high.

This research demonstrates why choosing an SPF of 30 or higher is so important, explains Nina Goad of the British Association of Dermatologists. While a well-applied SPF 15 cream offers the right cancer protection, because of the way we wear it, it's best to rely on those with 30 or more.

The conclusion, therefore, is simple and valid for everyone (adults and children of any phototype): never forget to use sunscreen and make sure you apply the right amount.

Read also

  • Sun creams: can they still be used after a year?
  • 10 Organic sunscreens with a good Inci
  • Sun creams: what is the right way to spread them? Myths and truths
  • Sun creams: 12 false beliefs and myths to dispel

Germana Carillo

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