POSTED: 2 May 2023

The Science of Sunscreen: Understanding SPF and How to Protect Your Skin from Damage

No matter where you live, you must protect your skin from the sun. One of the best tools you can use is sunscreen, which helps to protect your skin from harmful radiation. Applying daily sunscreen can not only save you from sun damage but also help to reduce your chances of developing skin cancer. 

In this blog post, we look at the science behind sunburn and skin damage, how SPF works, different sunscreen types and more so you can protect yourself from the harmful rays you’re exposed to daily. 

The science of sunburn and skin damage

Sunburn is a common and painful condition that occurs when the skin is exposed to the sun’s harmful UV rays for an extended period without proper protection. Sunburn can have both immediate and long-term effects on the skin.

UV rays from the sun, particularly UVB rays, can penetrate the skin’s outer layer and damage the DNA in skin cells. When the DNA is damaged, it triggers a response in the skin cells to protect themselves. As a result, the blood vessels in the skin dilate, causing redness and inflammation, which we associate with sunburn. Additionally, the damaged cells release chemicals that stimulate nerve endings in the skin, causing pain and discomfort.

In response to sunburn, the skin may peel and become dry and itchy as the damaged skin sheds cells and tries to repair itself. Over time, repeated sun exposure and sunburns can lead to long-term effects on the skin. 

Sun damage can cause premature ageing of the skin. It can also increase the risk of skin cancer, including common types such as basal cell carcinoma, squamous cell carcinoma and melanoma. 

Exposure to UV radiation is the leading cause of skin cancer, and most skin cancers are directly related to sun exposure. Therefore, sunscreen is essential for preventing skin damage that may lead to cancer. 

Understanding SPF: what it means and how it works

Sunscreen uses chemicals that absorb or scatter the UV rays before they can penetrate the skin — reducing how much UV radiation reaches the skin.

Two types of UV rays can damage the skin: UVA and UVB rays. UVA or ultraviolet B rays can prematurely age the skin and are linked to skin cancer, while ultraviolet B (UVB) rays are the leading cause of sunburn.

SPF stands for sun protection factor, and it measures how well a sunscreen can protect the skin from UVB radiation — the main cause of sunburn. 

Sunscreen products typically indicate SPF as a number, such as SPF 15, SPF 30 or SPF 50, representing the amount of protection the sunscreen can provide. In addition, the SPF number indicates how long a person can stay in the sun without sunburn compared to how long they could stay in the sun without any sunscreen. 

For example, if a person can generally stay in the sun for 10 minutes without getting sunburnt, using a sunscreen with SPF 15 would theoretically allow them to stay in the sun for 150 minutes (10 minutes multiplied by SPF 15) without getting sunburnt, assuming the sunscreen is applied and reapplied correctly.

The active ingredients in the sunscreen, whether organic or inorganic, act as a protective barrier that absorbs or reflects the UVB rays from the skin. The barrier helps to prevent sunburn and reduce the risk of skin cancer caused by exposure.

It’s essential to remember that SPF only measures protection against UVB rays, which cause sunburn. It does not necessarily indicate the level of protection against UVA rays, which can also contribute to skin damage and skin cancer. So, you must choose a broad-spectrum sunscreen that protects against both UVA and UVB rays.

Sunscreen with higher SPF values provides more defence against UVB rays, but the increase in protection is not proportional to the rise in SPF number. For example, SPF 30 filters out about 97% of UVB rays, while SPF 50 filters about 98% of UVB rays. As a general rule, a minimum of SPF 30 is usually recommended. Despite this, you should also protect your skin by staying in the shade, and wearing protective clothing and hats as no sunscreen can provide 100% protection against UV radiation.

Types of sunscreen: chemical vs physical

There are two main types of sunscreens — chemical sunscreens and physical sunscreens.

Chemical sunscreen

Chemical sunscreens contain organic (carbon-based) compounds as their active ingredients. These compounds absorb UV radiation from the sun before it penetrates the skin. 

Chemical sunscreens typically contain a combination of organic compounds. When applied, these compounds are absorbed into the skin, forming a protective layer that absorbs and converts UV rays into heat. The skin then releases the heat.

Physical sunscreen

Physical sunscreens or mineral sunscreens contain inorganic compounds as their active ingredients. They sit on top of the skin and form a physical barrier that reflects and scatters UV rays away from the skin. 

Physical sunscreens do not get absorbed into the skin and provide immediate protection upon application.

Choosing between chemical and physical sunscreens: which is right for you?

The choice between chemical and physical sunscreens depends on personal preferences and needs. Some factors to consider when choosing a sunscreen include skin type, sensitivity and activity level. 

Chemical sunscreens are usually more lightweight and easier to apply, with a thinner consistency that can be suitable for everyday use and under makeup. They may also offer broader coverage against both UVA and UVB rays. 

However, some people with sensitive skin may experience skin irritation or allergic reactions to certain chemical sunscreen ingredients.

Physical sunscreens tend to be thicker and may leave a white cast on the skin due to the reflective nature of their ingredients. However, they are generally better tolerated by those with sensitive skin. They can provide immediate protection upon application, making them a good option for individuals with skin sensitivities or babies and children.

Broad-spectrum and water-resistant sunscreen

Broad-spectrum sunscreen is essential for protecting the skin from UVA and UVB rays. Therefore, choosing a sunscreen that provides broad-spectrum protection to safeguard the skin against the harmful effects of both is crucial.

Water-resistant sunscreen refers to a type of sunscreen formulated to maintain its effectiveness even when exposed to water, sweat, or moisture. The sunscreen will protect for a certain duration, even when the skin is wet or during activities that may cause sweating. 

It is worth remembering that water-resistant sunscreen is not completely waterproof, and reapplication is still necessary after swimming, sweating, or towel-drying to ensure continuous protection.

When choosing a sunscreen that provides both broad-spectrum and water-resistant protection, there are several factors to consider:

  • Look for the term ‘broad spectrum’ on the sunscreen label.
  • Check the sunscreen’s SPF rating. Using sunscreen with SPF 30 or higher is recommended for adequate protection against UV rays.
  • When choosing water-resistant sunscreen, look for the specific duration of water resistance mentioned, such as 40 or 80 minutes.
  • Consider the formulation and consistency of the sunscreen. For example, some sunscreens may be more suitable for water-resistant protection, such as those with a thicker or more water-resistant formulation, like lotions or sticks.
  • Consider your activity level and exposure to water or sweat. Choosing a water-resistant sunscreen can provide longer-lasting protection if you plan to engage in water-related activities or activities that may cause sweating.

How to apply sunscreen effectively

When applying sunscreen, it’s essential to do so correctly to get the full effects. 

Here are some top tips to follow:

  • Apply enough sunscreen to cover all exposed skin — as a general rule, use half to one teaspoon of sunscreen for the face and neck and 6–8 teaspoons for the entire body of an adult.
  • Spread sunscreen generously and evenly — apply it to all areas of exposed skin, including the face, neck, ears, arms, legs and any other sites exposed to the sun.
  • Pay attention to commonly missed areas, such as the back of the neck, tops of ears and the back of the hands.
  • Reapply sunscreen every two hours or immediately after swimming, sweating or towel-drying, as well as after excessive rubbing or wiping of the skin.
  • Use proper techniques for applying sunscreen, such as spreading it evenly and avoiding rubbing or wiping it off.
  • Common mistakes to avoid are using too little sunscreen, neglecting to reapply after sweating or swimming, and not applying it to all exposed skin
  • Choose a broad-spectrum sunscreen with a high SPF of 30 or higher for optimal protection against UVA and UVB rays.

When to reapply sunscreen

You should reapply sunscreen every two hours or immediately after swimming, sweating, or towel-drying. However, other factors can affect the need for sunscreen reapplication as well.

Let’s say you spend extended periods outdoors, especially during peak sun hours (typically between 10am and 4pm). In that case, you may need to reapply sunscreen more frequently as the sun’s intensity can vary throughout the day.

Physical activities such as vigorous exercise can cause sunscreen to rub off or wash away, reducing its effectiveness and requiring more frequent reapplication.

Some sunscreens may be more prone to wearing or rubbing off compared to others, depending on their formulation, you must choose a high-quality sunscreen and follow the manufacturer’s instructions for reapplication.

To effectively reapply sunscreen throughout the day, follow these top tips:

  • Carry a travel-sized sunscreen bottle with you to make it easy to reapply when needed.
  • Apply sunscreen to all exposed skin areas, paying attention to commonly missed areas.
  • Use the same amount of sunscreen as you did during the initial application and apply it generously and evenly.
  • Avoid wiping or rubbing the sunscreen off when towel-drying and reapply immediately after swimming or sweating.
  • Don’t forget to reapply sunscreen to areas covered by clothing, as clothing may not provide adequate protection against UV radiation.

By diligently reapplying sunscreen according to the recommended guidelines, you can ensure that your skin remains protected from the harmful effects of the sun’s UV radiation.

Other forms of sun protection

When it comes to safeguarding your skin from the harmful effects of UV radiation, sunscreen alone may not always be enough. Here are some forms of sun protection.

Protective clothing and accessories 

Clothing and accessories can offer an additional protective layer against UV radiation. Opt for clothing made from tightly woven fabrics that provide more coverage and have a higher SPF rating. 

Darker colours absorb more UV radiation than lighter ones, making them a better choice for sun protection. Additionally, wide-brimmed hats can shade your face, neck and ears, while sunglasses can protect your eyes from UV rays.

Seeking shade and avoiding sun exposure

Seeking shade is another effective way to protect your skin from the sun. When the sun is at its peak, try to stay in the shade as much as possible. If shade is unavailable, consider using umbrellas or creating shade with a beach tent or other portable shade structures. 

The importance of sun protection in different settings

Sun protection is crucial in various settings, not just at the beach or by the pool. UV radiation can still be harmful even in shaded areas, on cloudy days, or during outdoor activities such as hiking, gardening, or playing sports. 

It’s vital to prioritise sun protection regardless of the setting or weather conditions. Applying sunscreen, wearing protective clothing and accessories and seeking shade whenever possible can minimise your skin’s exposure to harmful UV radiation.

Book a consultation with City Skin Clinic

Looking after your skin and protecting it from UV rays is essential. Sun damage can do more than just give you sunburn – melanoma skin cancer affects over 16,700 people annually in the UK, but proper UV protection can help prevent it. 

Remember, sun protection is a year-round effort, and it’s essential to safeguard your skin from the sun’s harmful rays.

If you have any questions or concerns about your skin, book a virtual consultation with City Skin Clinic. Our expert team of aesthetic doctors will be happy to answer any questions and help you access our prescription skincare service to treat skin ageing, hyperpigmentation, melasma and acne


Authored by:

Dr Amel Ibrahim
Aesthetic Doctor & Medical Director
Founder City Skin Clinic
Member of the Royal College of Surgeons of England
Associate Member of British Association of Body Sculpting GMC Registered - 7049611

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