No sunscreen can provide 100% UV protection

First published June 2017; updated June 2021

The role of sunscreen

Alongside other protective measures, such as clothing and shade, sunscreens play an important preventative role against the acute and long-term effects of overexposure to ultraviolet (UV) radiation from the sun, including: sunburn, skin damage, premature ageing of the skin and skin cancer.

Solar UV radiation

UV radiation is a component of sunlight and is divided into three categories, classified according to wavelength: UVA, UVB and UVC. The ozone layer blocks all UVC, so we only need to protect ourselves against UVA and UVB.


Longer wavelength UVA rays penetrate more deeply through the layers of the skin than shorter UVB wavelengths. UVA is mostly involved in skin ageing while UVB is mainly responsible for sunburn. However, both types of UV are linked to the development of skin cancer.

Learn more about UV here. 

Sunscreens: how they work

Sunscreens help to protect our skin by filtering out UV radiation through the use of chemical (organic*) and physical (inorganic) active ingredients. Chemical filters absorb UV radiation, while physical filters (e.g. titanium dioxide or zinc oxide) reflect UV radiation.

*In this context, the term organic does not mean ‘natural’ or ‘additive-free’ but is a word used by scientists to describe molecules containing carbon atoms.

Sunscreens act like mirrors or sponges

A handy way to think about it is that physical sunscreens act like a mirror and reflect UV radiation away from the skin, while chemical sunscreens act like a sponge and absorb UV radiation.

Understanding sunscreen labelling

SPF stands for Sun Protection Factor and shows the degree of protection the sunscreen provides against UVB only. It is rated on a scale from 2-50+. The higher the factor, the greater the level of protection against UVB.



UVA Star ratings range from 0-5, and indicate the level of protection the sunscreen provides against UVA, compared with the level of protection it provides against UVB (i.e. the ratio between the level of UVA and UVB protection offered by the product). The higher the number of stars, the greater the level of protection against UVA.

Sunscreens with a low SPF can still have a high number of stars, not because they are offering high UVA protection, but because the ratio between UVA and UVB protection is the same as offered in sunscreens with higher SPF. This is why it is very important to choose a sunscreen with a high SPF as well as high UVA protection (ideally 4 or 5 stars).

The EU recommends that the UVA protection offered in a sunscreen should be at least one third of the SPF. Sunscreen products meeting this requirement are eligible to display a UVA logo, with the letters UVA enclosed within a circle.

Open jar / period-after-opening symbol

An ‘open jar’ symbol located on the sunscreen container indicates the period of time after the product is first opened, that it is safe to use (i.e. there is no decrease in the effectiveness of the sunscreen).

This ‘period after opening’ is usually specified in months (often abbreviated to M, e.g. 12 M) or years, and located within or just beside the open jar symbol. The manufacturer’s storage instructions should also be followed.

Choice of sunscreen

Choose broad-spectrum sunscreens (offering protection against UVA and UVB), with an SPF of at least 30+ for adults and 50+ for children, with high UVA protection, and water resistant, used in addition to protective clothing and shade.

‘More is better’

While individual manufacturers provide instructions specific to their products, an average sized adult requires at least 36 grams of sunscreen (about six full teaspoons) to cover their body and achieve the protection indicated on the label – this is the amount used when products are tested to measure their SPF, calculated on the basis of applying 2mg of sunscreen per square centimetre of skin.

Many people don’t realise that applying less than the recommended amount may reduce the protection which a sunscreen provides to a higher degree than you would think is proportionate, e.g. only applying half the required amount can reduce the protection by a shocking two-thirds!

When to apply and reapply

Sunscreen should be applied liberally and evenly 15-30 minutes before sun exposure to allow it time to dry, and again shortly after going outdoors to ensure that all areas are covered. Reapply frequently, at least every two hours and after perspiring, sport, swimming, or friction (such as towel drying).


A higher price doesn’t necessarily indicate better protection.

What to look for

Look for ‘high protection’ broad spectrum sunscreens, with an SPF of at least 30 + for adults /50+ for children, and a UVA circle logo and/or 4 or 5 UVA stars.

The Health Products Regulatory Authority advises that sunscreens should only be bought from a reputable source. In this way, the product can be traced back to the supplier. It also recommends checking the label of the sunscreen for a European address – if a product has been imported from outside the EU, it may not meet European safety assessment requirements.

Know your skin type

A person’s natural skin colour influences their sensitivity to UV radiation and skin cancer risk, so sun-safe recommendations (protective clothing, shade and sunscreen) should be used alongside information on skin type, providing a better sense of the care you need to take in the sun. Learn more about skin type here. 

The Vitamin D debate

Vitamin D is important to keep teeth and bones healthy. Dietary sources of vitamin D include oily fish and fortified foods. Your body also produces vitamin D during short periods of incidental sun exposure. Vitamin D produced in the skin is ‘biologically regulated’ – long periods of sun exposure can actually break it down, consequently reducing benefit and increasing risk of cancer.

So the advice from dermatologists is to get some sunshine as you go about your day to day activities without burning or deliberately tanning. Vitamin D supplements are sometimes required by those at risk of low levels, for example: pregnant and breastfeeding women, young children, older people, and those with darker skin types e.g. the Health Service Executive provides guidance on supplementation for different age groups.

Protecting your skin

UV cannot be seen or felt, so you need to defend yourself against overexposure.

Know the UV index

The UV index measures the UV level at the surface of the Earth, and gives an indication of the potential for skin damage. It is calculated in a way that indicates the risk of developing sunburn, which is mainly caused by UVB. The UV index ranges from zero upwards – the higher the number the greater the risk. Learn more about the UV index here.  

An Irish context

In Ireland, make sun protection part of your daily routine particularly from April – September, when the intensity of sunburn producing UV is greatest, even when it is cloudy! Stay safe by limiting time in the midday sun when UV is strongest, typically between the hours of 11:00am-3:00pm.

Remember the 5 ‘Ss’ of sun safety: Slip, Slop, slap, Seek, Slide

  • Slip on clothing: Cover skin as much as possible e.g. wear long sleeves, collared t-shirts, clothes made from close-woven material that does not allow sunlight through.
  • Slop on broad-spectrum (UVA/UVB) sunscreen with an SPF of at least 30+ for adults and 50+ for children, with high UVA protection, and water resistant. Reapply regularly.
  • Slap on a hat with a wide brim: Protect your face, ears and neck.
  • Seek shade: Sit in cover of trees to avoid direct sunlight and use a sunshade on your buggy or pram. Keep babies and children out of direct sunlight.
  • Slide on sunglasses with UV protection: Guard your eyes from harm.

Next article: Protecting your family while out in the sunshine

For more information on how to Protect & Inspect™ against Melanoma Skin Cancer visit our information and resources page.