Misunderstanding SPF – Sun Protection Factor

Many of us apply sunscreen that mention the SPF (Sun Protection Factor) on the label every day as we wish to protect our skin from the harmful rays of the sun. There are a large number of over-the-counter sunscreen products that are available in 15, 30 and higher SPF values. People have a perception that the higher the value of SPF, the greater the protection will be against the harmful sunrays, which are a major cause of skin damage and can result in skin cancers, premature ageing and eye damage. But the meaning and value of SPF is generally misunderstood by people. To make things easier for you to understand, here is a detailed explanation of what SPF is, what its value means and how useful it is for your skin. This will help you to clear up many ambiguities regarding SPF sun protection and its use.

What sunscreen products are there?

The major factor in causing sunburn is UVB rays, while it is the UVA rays that are the cause of wrinkling, leathering, skin sagging and other effects related to ageing. UVA rays also accelerate the effects of UVB rays, which are increasingly becoming a cause of skin cancer. The ability of a sunscreen to protect your skin varies in terms of its UVA and UVB protection.

What is meant by SPF?

You must have seen sunscreens that mention SPF values of 15 or higher. The SPF measures a sunscreen’s capability to protect the skin from UVB rays damage. For instance, if it takes 20 minutes for your unprotected skin to turn red in the sun, it will take 15 times longer if you use 15 SPF sunscreen, which is about 5 hours.

You can also use percentages to measure the values. 15 SPF filters almost 93% of the incoming UVB rays, while SPF 30 prevents 97% and SPF 50% prevents 98% rays from reaching the skin. It may seem that these differences are negligible, but if you have a sensitivity towards light then these extra percentages will make all the difference. It is also important to mention here that there is no sunscreen that can provide protection against all UV rays.

However, there are some issues with this SPF model that need to be highlighted:

  • Firstly, there is no sunscreen that is effective for more than 3-4 hours without reapplying, irrespective of its strength.
  • Secondly, the ‘skin reddening’ is only the skin’s reaction to UVB rays, and not UVA rays. It also does not tell you how much UVA damage the sun has caused your skin. There can be considerable damage inside, without there being any indication of sunburn on your skin.[1]

People usually choose a sunscreen based on the SPF value, believing that by preventing sunburn the product will also protect their skin against UV damage, but this is not really the case.

There are many sunscreen products that do not provide sufficient protection from UVA sunrays. Although UVB radiations with higher energy are a major cause of sunburns and cancer-causing DNA mutations, the UVA rays with lower energy levels also cause damage to the skin layers. These rays penetrate through the deep skin tissues and are mostly responsible for creating free radicals, stimulate premature skin ageing and may also cause skin cancer. [2]

Types of UV Rays and their effects on the skin

Sunlight mainly comprises of two types of harmful rays, including:

  • UVA (having a longer wavelength, also known as bronzing rays)
  • UVB (having shorter wavelength, also known as building rays)

Refer to the table below to find out the differences between UVA and UVB sunrays[3]:

Constantly present in the atmosphere, irrespective of season or weather Do not have the same strength all the year around; more prevalent in the summer
Considered to be relatively safer More dangerous, compared to UVA
Cause premature ageing, leathery skin, pigmentation, damage skin cells beneath skin layers Responsible for sunburn and skin cancers.  Recent studies also reveal UVA to be a risk factor in melanoma as well as UVB
Present with equal intensity throughout the day Mostly prevalent at midday
Penetrate through deep skin layers Cannot penetrate through superficial skin layer

How the system of SPF works?

The SPF is a measurement of the amount of protection time the sunscreen will provide from sunburn, skin reddening or burning from UVB radiations, compared to how long it would take for your skin to become red or sunburned without sunscreen protection.

SPF Guide

Please refer to the table given below to see the SPF values, and how useful they are in providing sun protection.

SPF Values Percentage of UV light blocked (mostly UVB)
2 50%
4 75%
10 90%
30 96.67%
50 98%
70 98.57%
100 99%


SPFs should be used wisely and not be part of your mandatory skin regimen as they contain heavy metals (such as titanium and zinc oxide) and many other ingredients that do not serve any other purpose than to preserve and deliver the ingredients that block the sun’s rays. If an SPF sunscreen is used at 8am and then you drive to the office, it will have little benefit by lunch time because most of the effects of SPFs last for only about 4 hours at room temperature, and far less if you are outside in a hot and humid temperature.

Research suggests SPF sunscreens should be applied with a thickness of 2mg/cm square on the skin to get the optimum benefits, but most people only apply a thickness of 0.5 mg/ cm square. As a result, the actual SPF protection they receive is almost a third of the actual SPF value on the label.

Potential side effects of SPF sunscreens

Despite the benefits of SPF sunscreens, there are potential side effects of using high levels of them. These are:

  • Above SPF 50 sunscreens provide a minimal increase in UVB protection (98 % of UVB protection)
  • Although it is important to have UVA protection (as it contributes to skin ageing and even skin cancers), the SPFs primarily measure the protection provided against UVB
  • The individuals who use the high SPF value sunscreen may not get sunburn (as UVB is a major cause of sunburn) but without the UVA screening agents, they will still receive dangerous doses of skin-damaging radiation
  • The use of higher SPF products may cause individuals to neglect other kinds of sun protective behaviours like wearing a hat, long-sleeved clothing, using sunglasses, seeking shady places etc.

This creates a false sense of protection among people. As skin damage can happen without skin reddening, the use of SPF sunscreen should be considered as only as one important part of a broad sun protection routine.  Wearing off of the SPF due to sweating should also be calculated in hot weather.

Bottom Line

Considering all of these facts about SPF sunscreens, it is of crucial importance to include both UVB and UVA protection in your skin care regimen in order to protect your skin from harmful sunrays. The appropriate use of SPF value should be between 15-30 but this may depend also on your skin type. In addition to having the SPF, your sunscreen should also have some of the UVA blocking agents, such as zinc oxide, titanium dioxide, oxybenzone, ecamsule etc. The sunscreens that offer both UVB and UVA protection are usually labelled as broad spectrum, UVA/UVB protection or multiple spectrum.

[1] http://www.skincancer.org/prevention/sun-protection/sunscreen/sunscreens-explained

[2] https://www.ewg.org/sunscreen/report/do-sunscreens-prevent-skin-damage/

[3] http://www.skincancer.org/prevention/uva-and-uvb


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