Carta de Proteção Solar - Guia do Índice UV e Tipo de Pele

The UV Index is a standardised scale, defined by the World Health Organization and calculated by national weather agencies, that describes the strength of sunburn-causing ultraviolet radiation at ground level. It is not a temperature or a brightness measure, which is why a cool, overcast spring day can carry a higher UV Index than a hot, hazy afternoon in late summer.

A fórmula

UV Index = (integrated spectral irradiance weighted by the erythema action spectrum) / 25 mW per square metre

• The erythema action spectrum weights each wavelength by how effectively it reddens human skin
• Dividing by 25 mW/m2 scales the result so that each whole number step is a meaningful change in burn risk

Defined by the World Health Organization, WMO, UNEP and ICNIRP as the Global Solar UV Index. The scale is open-ended: values above 11 occur at high altitude and in the tropics.

UV Index bands and what they mean

The bands below are the WHO exposure categories used by weather services worldwide. The advice attached to each band assumes unprotected skin that is not already tanned; someone who burns easily should treat their own threshold as one band lower.

• UV rises roughly 10 percent for every 1,000 metres of altitude, which is why mountain and ski burns are common in cold weather.
• Snow reflects up to 80 percent of UV, sand around 15 percent and water around 10 percent, so reflected exposure adds meaningfully to what comes directly from the sky.
• Cloud cover reduces UV far less than it reduces visible light and heat. A thin overcast can still transmit most of the UV.

What sunscreen labels actually promise

SPF is measured against UVB, the wavelengths that cause sunburn. The number describes how much longer skin takes to redden compared with no sunscreen, under laboratory application of 2 milligrams per square centimetre. That application rate matters enormously, because most people apply between a quarter and a half of it, which reduces the protection achieved far below the labelled figure.

The relationship between SPF and protection is not linear, and the practical differences at the top of the range are small. SPF 15 filters about 93 percent of UVB, SPF 30 about 97 percent, and SPF 50 about 98 percent. No sunscreen blocks everything, which is why "sunblock" is no longer permitted on US labels.

Only products labelled broad spectrum are tested for UVA protection as well, and only broad spectrum products of SPF 15 or higher may claim to reduce skin cancer and early skin ageing risk. Water resistance claims must state either 40 or 80 minutes; "waterproof" and "sweatproof" are prohibited.

Applying enough, and reapplying

A full body application for an average adult takes roughly one ounce, about a shot glass, and a face and neck application takes about half a teaspoon. Applying half as much does not give half the SPF; the relationship is steeper than linear, so under-application can drop an SPF 30 product to an effective SPF in single digits.

Reapplication every two hours, and immediately after swimming, towel drying or heavy sweating, is standard guidance. Shade, clothing and timing do more than any sunscreen: a long-sleeved shirt provides continuous protection that does not wash off, wear off, or depend on how much was applied.

• Sunscreen applied 15 minutes before going outside allows the film to form properly.
• Most people miss the ears, the back of the neck, the tops of the feet and the hairline.

Limitações

• The UV Index is a forecast for a location and time, not a measurement of your exposure. Shade, clothing, reflection and how long you are outdoors all change the dose you actually receive.
• SPF is tested at an application rate most people do not achieve. Real-world protection is typically well below the labelled number.
• SPF describes UVB protection only. UVA protection depends on the broad spectrum designation, which is a separate test.
• Skin type changes the threshold substantially. The guidance in the bands assumes an average response; people with very fair skin burn at lower indices and shorter exposures.
• Some medications, including certain antibiotics, diuretics and retinoids, cause photosensitivity, and the usual bands do not apply while taking them.
• No level of sun exposure is entirely without risk, and this chart is general guidance rather than personal advice.

Perguntas frequentes

What UV Index is dangerous?

Protection is advised from a UV Index of 3 upwards. At 6 to 7 unprotected skin burns readily, at 8 to 10 it burns quickly, and at 11 or above it can burn within minutes. People who burn easily should treat their own threshold as one band lower.

Is SPF 50 twice as good as SPF 25?

No. SPF 30 filters about 97 percent of UVB and SPF 50 about 98 percent, so the practical difference at the top of the range is small. Applying enough and reapplying matters far more than the number on the bottle.

How much sunscreen should I use?

About one ounce, roughly a shot glass, for full body coverage, and about half a teaspoon for the face and neck. Most people apply a quarter to a half of the tested amount, which reduces the protection achieved well below the labelled SPF.

Can I burn on a cloudy day?

Yes. Cloud reduces visible light and heat much more than it reduces ultraviolet, so a thin overcast can still transmit most of the UV. Checking the UV Index rather than the brightness is the reliable approach.

What does broad spectrum mean?

It means the product has passed a test for UVA protection in addition to its SPF rating for UVB. Only broad spectrum products of SPF 15 or higher may claim to reduce the risk of skin cancer and early skin ageing.

Referências

1. World Health Organization. Radiation: The ultraviolet (UV) index.
2. US Environmental Protection Agency. UV Index Scale.
3. US Food and Drug Administration. Sunscreen: How to Help Protect Your Skin from the Sun.
4. American Academy of Dermatology. Sunscreen FAQs.