Sunscreen Guide: How Do You Choose One, How Much Should You Apply and How Often Should You Reapply?

Why does sunscreen matter?

Ultraviolet (UV) rays from the sun contribute to sunburn, pigmentation, premature ageing of the skin and skin cancer. Dermatology organisations describe sunscreen not as a solution on its own but as a measure used together with shade and clothing. We have dealt with the effect of regular use on skin ageing, and with the situation in the winter months, in our article “Rumour: You don’t need sunscreen in winter or on cloudy days — What does the science say?”; this guide focuses on the details of choosing and using a product.

The measures in this article are taken from the advice to the public of organisations such as the American Academy of Dermatology (AAD), the US Food and Drug Administration (FDA) and the United Kingdom's National Health Service (NHS).

How do you choose a sunscreen? Reading the label

The AAD and NHS recommend products with a sun protection factor of at least 30; the FDA's page for the public gives at least 15 as the lower limit. According to the AAD, a product with a sun protection factor of 30, applied in the right amount, blocks about 97 per cent of UVB rays; higher numbers block slightly more, but no product stops all the rays. A high sun protection factor does not mean that you can stay in the sun longer or that the product can be reapplied less often.

UVA protection is shown in different ways depending on the country: in some countries the term 'broad spectrum' is used, in others a star rating or a UVA symbol in a circle. When you are not sure, you can ask your pharmacist or physician.

According to the FDA, products labelled 'water resistant' must be tested and must state on the label whether the effect lasts 40 minutes or 80 minutes. If you are going to swim or sweat, choosing a product of this kind makes sense; even so, the product needs to be reapplied after you come out of the water and dry yourself.

Mineral filter or chemical filter?

Sunscreens are divided roughly into two groups according to the filters they contain. Products with mineral (physical or inorganic) filters contain zinc oxide, titanium dioxide or both. Products with chemical (organic) filters do not contain these two; they use other active ingredients such as avobenzone and octocrylene. There are also products that contain both groups.

According to the AAD, both groups form a protective layer on the skin that absorbs UV rays; mineral filters also reflect part of the rays. In other words, the difference between them is not as sharp as is often described. Dermatology sources state that products with mineral filters may be less irritating for sensitive skin and young children. Mineral filters can leave a white cast on the skin; products with chemical filters, on the other hand, can cause stinging or irritation in some people.

The question of which group is 'better' has no single answer that holds for everyone. What determines protection is that the product is broad spectrum, is applied in a sufficient amount and is reapplied. The product you can use regularly and that does not bother your skin is the one that suits you.

Tinted sunscreens: visible light and melasma

Sunlight does not consist only of ultraviolet rays; the light we see with our eyes can also affect the skin. According to a dermatology review on the subject, visible light can cause pigmentation in people with dark skin, and although broad-spectrum sunscreens protect against ultraviolet rays they do not provide adequate protection against visible light. The same review states that for a product to be able to protect against visible light it needs to be visible on the skin: mineral filters used as very small particles so as not to leave a white cast do not provide this protection.

Tinted sunscreens also offer protection against visible light, thanks to iron oxides and titanium dioxide in pigment form. According to the review and the AAD, these products may be useful in conditions such as melasma and post-inflammatory pigmentation. There are two points to note here: not every tinted product provides the same level of protection, and there is no standard measure on this, similar to the sun protection factor, that the user can look for on the label. If you are prone to pigmentation, you can ask your physician about this option. The details are in our articles “Dark Spots and Melasma” and “Laser, IPL and Chemical Peels on Olive and Darker Skin”.

How much sunscreen should you apply?

The protection stated on the label is measured by applying the product at a specified thickness in the laboratory. According to the AAD, many people apply only 20 to 50 per cent of the amount needed to reach the protection on the label.

• Whole body: the AAD and FDA state that about 1 ounce (roughly 30 millilitres, a shot glass full) of product is needed to cover an adult's exposed skin. The NHS describes the same amount as about 6 to 8 teaspoons. The larger the body size, the greater the amount needed.
• Face: the AAD recommends at least 1 teaspoon of product for the face; this corresponds roughly to product squeezed along the length of the index and middle fingers.
• Timing: the AAD and FDA recommend applying the product about 15 minutes before going outdoors. The NHS, for its part, recommends applying it twice if you will be in the sun for a long time: 30 minutes before going out and just before going out.
• Lips: the AAD recommends a lip balm with a sun protection factor of at least 30 for the lips.

How often should you reapply?

The AAD, FDA and NHS agree on the same point: outdoors, sunscreen should be reapplied about every two hours, and also after swimming, sweating and towel drying. This applies to water-resistant products too. A product applied once in the morning does not give the same protection in the afternoon on a day spent outdoors.

For someone who spends most of the day indoors, the need to reapply may be different. The organisations' recommendations are written for outdoor exposure; working by a window, going out at lunchtime or having a condition such as melasma can change the plan. You can discuss the routine that suits you with your physician.

Common mistakes

• Applying too little: a common mistake that markedly reduces protection.
• Not reapplying: the AAD states that people who get sunburnt have mostly not reapplied the product, have applied too little or have used a product past its date.
• Skipping some areas: the ears, neck, nape, tops of the feet, hands and scalp where the hair is thin are often forgotten.
• Relying only on the protection in a make-up product: when foundation or moisturiser is not applied in the amount recommended for sunscreen, it may not give the protection on its label.
• Using a product that is past its date or has been left in the heat: the AAD and FDA recommend that, if there is no date on it, a product should not be used three years after purchase, and that a product whose colour or consistency has changed should be thrown away. In some countries the packaging carries a symbol showing the period of use after opening.
• Spraying a spray product into the air and moving on: the AAD recommends spraying until the skin glistens and then spreading it by hand. The FDA states that spray products should not be sprayed directly on to the face and, because they can be flammable, should not be used near an open flame.

Shade, hat and clothing: cream alone is not enough

No sunscreen blocks all the rays. For this reason the organisations recommend the following measures together with sunscreen.

• Staying in the shade during the hours when the sun is overhead. The AAD and FDA give this period as roughly 10 a.m. to 2 p.m.; the NHS states 11 a.m. to 3 p.m. for the United Kingdom. The hours vary with where you are and the season. The AAD's practical measure is this: if your shadow is shorter than you are, seek shade.
• A wide-brimmed hat and long-sleeved, tightly woven clothing. Some garments carry a label showing their level of ultraviolet protection.
• Sunglasses with ultraviolet protection.
• Extra care near surfaces that reflect light, such as water, snow and sand.
• Avoiding sunbeds.

Sunscreen after laser, peels and other procedures

After procedures such as laser, intense light, chemical peels and microneedling the skin is more sensitive to the sun for a while; unprotected sun exposure can increase the risk of pigmentation. The AAD recommends that skin treated with laser be protected from the sun until it has healed, and that tanning also be avoided before the procedure.

However, in the days immediately after the procedure the skin surface may be open or irritated, and not every product is suitable for this period. The physician who performed the procedure decides when to start sunscreen, which type of product to use, and whether to make do with shade and a hat alone in the first days. The written instructions given to you take precedence over the general information in this guide. For general principles see our article “Aftercare Following Medical Aesthetic Procedures”, and if you are considering treatment for pigmentation, our article “Pigment Lasers”.

Products containing retinoids can also make the skin more sensitive to the sun; for those who use these products, daytime sun protection is particularly important. The details are in our article “Retinoid Guide”.

Babies, children and pregnancy

The AAD, FDA and NHS recommend that babies under six months be kept out of direct sunlight and protected with shade and clothing. The FDA does not recommend using sunscreen in this age group and asks that a physician be consulted before use; the AAD, for its part, states that where shade and clothing are not possible a small amount of a product, preferably with mineral filters, can be used. Make the decision for your baby with your paediatrician.

For children over six months, the organisations recommend a broad-spectrum, water-resistant sunscreen in addition to shade and clothing; the AAD states that in children products with mineral filters may be less irritating to sensitive skin.

In pregnancy, sun protection matters particularly with regard to melasma, which is common during this period. If you have any concerns about which product to use in pregnancy, consult the physician following your pregnancy or a dermatologist. For skin care and procedures in pregnancy, see our article “Medical Aesthetics in Pregnancy and Breastfeeding”.

Are sunscreen filters safe?

In recent years, studies showing that some chemical filters are absorbed through the skin and enter the blood have caused public concern. In a randomised study published by FDA researchers in 2020, all six chemical filters were detected in the blood of 48 healthy volunteers above the threshold set by the FDA for requesting additional safety studies. The study was carried out under conditions of intensive use, in which the product was applied to most of the body several times a day.

It is necessary to read correctly what this finding means. A substance entering the blood does not mean that it is harmful; it only shows that its safety needs to be investigated in more detail. The authors of the study stated plainly that the results do not indicate that people should refrain from using sunscreen. As reported by the AAD, the FDA likewise does not say that these substances are unsafe and does not recommend stopping their use; it is asking manufacturers for additional data.

The harm the sun does to the skin is well documented; research on the long-term safety of some filters, by contrast, is continuing. If this uncertainty bothers you, rather than giving up sunscreen you can choose products with mineral filters and place more weight on shade and clothing. If you develop redness, itching or a rash in response to a product, stop using it and consult your physician.