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What Does the SPF Number Mean?

When preparing for a summer vacation, people usually take sunscreen with the highest SPF number. That makes them feel comfortable and safe, but are they really happy about their decision at the end of the hot summer day? How much does the SPF number matter? Let's find out!

What Does the SPF Do?

When our skin is exposed to the sun, the pleasant warm feeling we experience cunningly hides plenty of risks to our health. By absorbing the sun’s UV rays, unprotected skin can develop not only a nice-looking tan, but also sunburns, cell DNA damage, photoaging, pigmentation, and even skin cancer in the long run.

SPF is a feature of sunscreens that protects our skin from the sun. In particular, from all or most of the harmful effects mentioned above. Just so you know, SPF is short for Sun Protection Factor. The alternative term is UPF – Ultraviolet Protection Factor.

Unfortunately, some people apply sunscreens with SPF only on the beach, thinking that in the city the sunlight is not bright enough to worry about protection. This is a fundamentally wrong approach.

SPF must be applied every day, even in cloudy weather. While most UV rays are only dangerous from spring to fall and are blocked by clouds, certain types of UV rays are active all year round and all day long. Our skin can suffer due to their ability to seep through our attire, particularly clothing made of thin fabric such as linen shirts, as well as clouds, and glass.

A Quick Look at UVB and UVA Rays

From physics classes, we remember that UVB radiation has short wavelengths and cannot penetrate deep into our skin. Therefore, UVB rays are mainly responsible for sunburns and photoaging, but can be effectively blocked by sunscreens. UVB rays also get stopped by clouds or even thin clothing.

However, there are also longer wavelength UVA rays that cause fewer visible changes but increase the likelihood of skin cancer. Unlike UVB, they pass through clouds and glass and are active all year round. Wearing linen garments won't shield you from UVA rays.

The SPF Number and How It Is Calculated

Different SPF numbers indicate a degree of protection from UVB radiation. Basically, they tell how much more effective a particular sunscreen is compared to no sunscreen at all.

Here is how cosmetic laboratories determine a product’s SPF number:

  1. First, they apply sunscreen on a volunteer’s skin, expose it to UV lamps, and calculate the time required for the skin to develop first signs of damage (usually, slight irritation, or redness from burns).

  2. Then they run the same test on a new skin area, but without any sunscreen, i.e., the skin is unprotected.

  3. Finally, they divide the amount of time it took the skin to develop redness with the SPF by the amount of time to redness without the SPF. The ratio they get is what we know as SPF 15, 30, and so on.

For example, let’s say it took the skin with an SPF on to become slightly irritated in 12,000 seconds (3 hours and 20 minutes), and the same effect was achieved in 800 seconds (approx. 13 minutes) without an SPF. 12,000 divided by 800 equals 15, meaning that this particular sunscreen offers 15 times more UVB protection than bare skin.

What about the UVA radiation, you may ask? Is it determined in the same way? Let’s find out together.

Akt Therapy Super Gloss with SPF 50

Akt Therapy Super Gloss with SPF 50

UVA Radiation and PA Value of a Sunscreen

For effective protection against the less intense but equally harmful UVA rays, sunscreen needs to be labeled as "broad-spectrum."

In Asia, in particular, in Japan, broad-spectrum sunscreen products come labeled as PA, where PA stands for “Protection Grade of UVA”. The PA rating system goes as follows:

  • PA+: some protection.
  • PA++: moderate protection.
  • PA+++: high protection.
  • PA++++: extremely high protection.

However, unlike the UVB protection, the PA system is not quantifiable. Rather, it gives a general idea of how effective a particular sunscreen is at shielding human skin from the long-wave UVA rays.

Therefore, Asian products often come with both markings: an SPF rating indicated in numbers and a PA rating accompanied by pluses. The more pluses you see next to the PA sign, the higher the protective power of this product against UVA radiation.

Is a Higher SPF Number Better?

Generally, the higher the number – the better the protection from the aggressive UVB rays. However, the relevant protective power diminishes with the increase in SPF value.

This is how it works:

SPF Level

UVB Protection %


Offers 90% protection from UVB rays


Offers 93% protection from UVB rays


Offers 97% protection from UVB rays


Offers 98% protection from UVB rays


Offers 99% protection from UVB rays

The closer you approach 100, the lower the return you receive on our investment, as sunscreens with the highest SPF values cost significantly more money. However, even SPF 100 does not guarantee complete protection.

“What spf number is best for face?”, people often ask. Your face gets more regular exposure to the sun; therefore, facial skin is usually less sensitive to UV radiation. So, go for an SPF with anything from 10 to 30 for the face.

As seen from the table above, even the SPF 30 value is pretty effective against the UVB rays on all skin areas, including the face. The increase in protection is not linear, though, and it diminishes the closer it gets to 100. People often get a false sense of protection with the SPF 50+, as they overly rely on what the label says and neglect simple, yet still very important application routines.

Akt Therapy D-Luxe Daily Bronzer with SPF 30

Some Application Recommendations

A comprehensive protection formula encompassing both the SPF number and its application routine states that only 50% of protection effectiveness comes from the sunscreen's SPF value, while the other 50% is hidden in the application routine.

Use The Right Amount of Sunscreen

Always mind the amount of sunscreen you apply. For the maximum degree of protection with SPF, apply about 1.25 ml of cream to the entire face and neck area.

As for the other parts of the body, the simplest way to estimate the sufficient amount of cream is to use the famous two-finger method. It says that a strip of sunscreen should be properly applied along the entire length of the index and middle fingers. This quantity should be enough for all the body areas exposed to the sun to be properly covered with sunscreen.

Cosmetologists have also researched an alternative formula: two milligrams of sunscreen per square centimeter of skin. To make it easier, you can measure it in teaspoons. An adult will need up to five teaspoons of sunscreen (30 grams) to be applied over the entire body to protect them from the scorching sun on the beach.

Always Check the Ultraviolet Index

When choosing protection, focus not so much on the air temperature, but on the ultraviolet index. It's available in any weather app for Android or iOS.

If the index is greater than 2, choose an SPF with a minimum of 30 for daily activities and 50 for increased sun exposure. If the index is below 2, SPF 30 will be enough for exposure to the sun, and 15 for staying in the shade.

Keep in mind that these are average numbers. If you have a white, pale skin type, you’d need a higher SPF number. On the contrary, dark skin needs less SPF.

Reapply Sunscreen Regularly

Do not forget that sunscreen, irrespective of the SPF value, needs to be updated. By default, during prolonged exposure to the sun, sunscreen must be updated every 2 hours. In city conditions, when you spend most of your time in the office or move from the door of your house to your car, a single application in the morning and another one in the evening (just when you are ready to leave your office) will be enough.

Reapplication is also essential when you spend much of the time outdoors, especially when swimming. Water and sweetness literally wash out the thin sunscreen film from your skin. Even movements, such that cause your clothes to intensively rub against your skin, greatly diminish your sun protection.

The Key Takeaway

The main takeaway from this guide is that the spf number meaning is always relevant. A sunscreen labeled SPF 30 does not ensure 97% protection against the sun’s radiation. Rather, it only gives a general idea of this particular product’s effectiveness when used correctly. The latter stipulates applying a sufficient amount of sunscreen for each body area, depending on your skin type, weather conditions, and ultraviolet index. It also means reapplying it every two hours, and each time after you go swimming.

These all lead us to an inevitable and rather shocking conclusion – in the right hands, a quality product labeled SPF 15% can turn out to be more effective than an SPF 50+ used carelessly. A broad-spectrum sunscreen plus a diligent application routine will do wonders for your skin and your health.

AKT Therapy