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6 of your top sunscreen questions answered

This week we deep dive into some of the most commonly asked questions about sunscreen

Pippa HarmanCo-Founder Renude
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6 of your top sunscreen questions answered
Written byPippa HarmanCo-Founder Renude
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It’s been freakishly sunny in the UK over the last few months (The irony has not been lost on us that the majority of us were essentially trapped indoors on what was apparently the hottest spring in 127 years.) But indoors or outdoors, on sunny days or rainy ones, one thing we know: sunscreen is your skin’s best friend. But how much do you know about this miracle worker? Here we answer 6 common questions about our beloved sunscreen.

1. Do I need sunscreen on a cloudy day, or if I’m not going outside during peak hours?

Yes. Absolutely. Everyday.

80% of UVA & UVB rays can pass through clouds, so any time spent outside will expose the skin to UV rays, even on a cloudy day. UVA rays are the longest wavelength within the ultraviolet spectrum, which allows 75% to actually penetrate through glass, so even if we're inside we are being exposed to UVA rays which can age the skin prematurely, so we should be protecting our skin daily. 

2. What does the term 'SPF' actually mean?

SPF stands for ‘Sun Protection Factor’ and is quite literally the measure of how much longer it will take for the skin to burn on exposure to UVB radiation, vs. not using it. For example, if you would normally sunburn, say, within 1.5 minutes of direct UV exposure, an SPF15 would mean that it will take 22.5 minutes (2.5H) to sunburn based on the same sun exposure. 

During the development and testing of a new sunscreen formulation, human volunteers are used. The product is applied to an area of the back, and a second site is drawn out on the back which is left untreated. Both sites are exposed to a UV lamp for short intervals, and the time it takes for the skin to show erythema (sunburn) is recorded. The number of times longer it takes for the treated area to burn vs. the untreated gives us the final SPF. 

3. Is ‘SPF’ the full story?

SPF is a measure of protection against UVB rays only, as it’s this wavelength of light that leads to sunburn and was originally considered to be the whole picture. As research evolved, we realised that UVA, which has a longer wavelength, can also cause lead to skin damage, and contributes significantly to premature ageing of the skin. 

In Europe, the UVA protection must be at least ⅓ that of the declared SPF to be able to be sold as a sunscreen product. The star rating (originally coined by Boots) is a separate measure for UVA protection, with 1* being the minimum 20-40% protection, and a 5* being 90% UVA: UVB protection, so look out for this indicator or ask brands for this information - it has to be measured during the product development phase (via a ‘critical wavelength’ test), so the data will be available (although not a legal requirement to provide it). 

Newer research also shows lower energy wavelength lights such as infra-red, high energy visible (or blue light) also have a damaging effect on the skin and certain brands (e.g. Heliocare 360 range) are adding ingredients into their formulations to offer extra protection against these wavelengths. 

For full disclosure, the sun also emits UVC rays, which are the highest energy wavelength and would cause a lot of damage to the body, but this wavelength reacts with the ozone layer way above the earth, so the rays don’t reach us on the ground. 

4. What are the different types of sunscreen?

There are two types of sun filters: physical/mineral and chemical/synthetic.


There are just 2 physical filters, zinc oxide and titanium dioxide, which are both white mineral powders and are also referred to as mineral sunscreens. They work by essentially creating a super fine layer over the skin, which reflects UV light, preventing it from penetrating into the skin and causing any cellular damage. This makes it a broad-spectrum filter as it essentially reflects all wavelengths of light.

The downside to it is that it’s not a great option for those with medium-deep skin tones, as they tend to leave a white cast on the skin. Texture-wise, they’re trickier to get right and traditional formulas were thick and greasy, but technology has evolved and there are some pretty light, imperceptible products on the market now. We also wouldn’t advise a mineral sunscreen if you’re planning any flash photography as it will reflect this light too. 


Chemical filters are a bit more complicated, as they need to be used in combination in order to block the full spectrum of wavelengths associated with UVB & UVA. These filters work by absorbing into the skin, and then converting UV energy into heat, dissipating the energy which could otherwise cause harm in the body.

Certain chemical filters are more likely to be irritating for sensitive skin compared with mineral options, so are a less good option for sensitive skin. 

5. How much sunscreen should you use?

Official guidelines are to apply ½ teaspoon to cover the face, neck & ears. 2 tablespoons would cover your whole body (assume you’re in a swimming costume). You should also re-apply every 2 hours, or after swimming, sweating or rubbing the skin, whatever the SPF level. 

Chemical sunscreens are used up in the process of protecting against the skin, so degrade over time. One of the most common UVA filters used in chemical sunscreens, Avobenzone, degrades on exposure to sunlight while protecting the skin, so there won’t be any of this filter left after 2 hours to protect from UVA rays, making it particularly important to re-apply every two hours with chemical sunscreens. 

6. How long does sunscreen last?

Sunscreen is definitely one product you don’t want to use past the PAO (‘period after opening’) - the little open jar you’ll find on the back of the pack which tells you how long the product is stable/compatible in that packaging once opened. As the UV filters in the formulation can degrade over time, particularly at higher temperatures (bathrooms/near windows/on the beach etc.), I would definitely not recommend using a sunscreen past this date. A typical PAO is 12M so if you bought this for a summer holiday last year, it's time to throw it out. 

Which is the best sunscreen for your skin?

Sunscreen can be one of the hardest products in your routine to get right, so if you’re wondering which sunscreen is best for you, you're in the right place. Our experts here at Renude can help you build a skincare routine completely tailored to your unique needs and budget. 

Pippa HarmanCo-Founder Renude
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