Understand sunscreen basics

Why sunscreen is important for a healthy skin

The sunlight which reaches earth surface is a combination of UVB (290–320 nm) and UVA (320–400 nm) radiation.  Both UVA and UVB radiation are deleterious causing a wide variety of damaging biological effects. Repeated sun exposure induces adverse effects to the skin which includes sunburn, pigmentation, photo-allergies, photo-aging, photodermatoses causing skin rashes and skin cancers.

What UVB and UVA rays mean?

The ‘B’ in UVB rays stands for burning. UVB radiation is responsible for causing sun burn & tanning while UVAs are responsible for long term skin damage. The ‘A’ in UVA rays stands for ageing. Although it doesn’t cause sunburns, it actually reaches deeper into our skin and damages its dermis layer, which leads to skin aging and wrinkles over time.

Difference between UVB & UVA rays

Importance of SPF number and PA factor in sunscreen

What SPF stands for?

SPF (Sun Protection Factor) refers to blockage of UVB rays only. It measures only UVB protection & may not provide UVA protection. Higher the SPF, the smaller the increased benefit. SPF 50 isn’t twice as strong as SPF 25. While SPF 25 filters out 94% of UVB, SPF 50 filters out 98% & SPF 75 filters out 99% which means a slight benefit.

SPF is a measure of how much UVB radiation is required to produce sunburn on (with sunscreen) protected skin relative to the amount of solar energy required to produce sunburn on (no sunscreen )unprotected skin.

Suppose your unprotected skin gets burnt within 5 minutes, applying an SPF 30 means you’ll get the same sunburn within 5 x 30 = 150 minutes. This means sunscreen with SPF 30 should be reapplied after every 150 min to prevent sun burn in direct sunlight.


This SPF calculation is not that simple & there are other factors like sunscreen quantity used, type of weather, geographic locations etc which matters in calculating the time for which the skin will remain protected. In actual, SPF is not directly related to time of solar exposure but to amount of solar exposure. Solar intensity is generally greater on clear days than cloudy days. Eg: Exposure of 1 hour at 8 a.m = 15 min at 1 p.m.

This means that SPF does not inform consumers about the time that can be spent in the sun without getting sunburn. Instead, it allows consumers to compare the level of sunburn protection provided by different sunscreens. For example, consumers know that SPF 25 sunscreens provide more sunburn protection than SPF 10 sunscreens.

On what factors SPF in sunscreen depends on?

What PA stands for ?

PA rating is developed by Japan which indicates the level of protection against UVA rays. It indicates how much UVA protection the sunscreen offers. You actually can’t feel the pain given by UVA rays as there is no direct harm but they silently damage your skin. This is why PA rating really matters in a sunscreen. It ranges from PA+ to PA++++ which goes from slight UVA protection to highest UVA protection. This means PA++++ gives you the maximum benefit against UVA radiation.

PA rating is measured on the process called Persistent Pigment Darkening (PPD) which calculates the amount of time in which sun’s UVA rays cause the skin to become brown. It varies from person to person & not everyone’s skin turn brown in the absence of UVB rays & presence of only UVA rays.

Unlike SPF, PA rating doesn’t decide how long it will protect you from UVA rays. This is why some agencies in USA and Europe use the SPF ratings to include both UVA and UVB protection. This is where Broad Spectrum term comes into picture which confirms that the sunscreen is tested to provide the protection against both UVB and UVA rays.

Types of UV filters

The most important factor which matters in the sunscreen formulation is the quality of UV filters used in it. A customer should not only check the SPF value & PA factor on sunscreen label but also the UV filters used in ingredient list.

UV filters can be either organic or inorganic and act by absorption, reflection or diffusion of UV rays depending on their chemical nature and physical properties.

Organic (Chemical) UV Filters

Organic filters are active ingredients in a sunscreen which absorbs UV radiation & converts it into unnoticeable heat or fluorescence. This process leads to the concern of photo-stability of UV filters:

Stable Chemical UV filters: When organic UV filters at ground state transforms to a higher excited energy state after absorbing sun rays, they quickly release the excited state energy to return to their ground state where they are stable and ready to absorb additional UV energy.

Un-Stable Chemical UV filters: Sometimes UV filter unable to transform back to its ground state after absorbing sun rays leading to its degradation and the filter losses its absorption capacity. The filter is then said to be photo-unstable. These unstable UV filters generate photo-products that are suspected to cause damage to skin cells. This concept leads to two conclusions:

  • Chemical Sunscreens must be reapplied while you are exposed to direct sunlight.
  • Chemical sunscreens are more suitable to wear indoors or on non-sunny days as they do not protect much from sunburn in direct sunlight.

However research on the photo chemistry filters has led to the development of new UVA filters that have photo-stable structures. Stable filters are combined with the unstable UV filters to form a stabilized sunscreen formulation.

Photo-Stable broad spectrum UV filters

Inorganic (Physical) UV Filters

Inorganic UV filters are a group of mineral oxides which are titanium dioxide (TiO2) and zinc oxide (ZnO) works by absorbing, reflecting & diffusing UV rays depending upon their particle size. Out of 2 physical filters, ZnO is the most stable & unique among sunscreen ingredients which is truly a broad-spectrum blocker, protecting from both UVA & UVB rays. TiO2 protects from UVB rays very well but it does not protect from UVA much.

Non – Nano Physical UV filters: These are titanium and zinc powders with medium particle size (>100nm). They reflect light leaving a white appearance on the skin. They sit on the surface of the skin and block, scatter & absorb UV rays while providing better protection from UVB & UVA rays as compared to chemical filters.

Nano Physical UV filters: The issue of white cast in large particle size of TiO2 & ZnO lead to formation of their nano-sized particle(<100nm) variants. The nano sized filters act primarily as UV-absorbing materials, and not as UV-scattering or UV-reflecting materials. Due to their minimal visual appearance they are used often by cosmetic brands in moisturizers & foundations with SPF.

Photo-stability of physical UV filters

Non-nano Zinc Oxide is the most photo-stable UV filter because it sits on the skin surface & act as a barrier. It absorbs less & reflects more UV rays reaching the skin. Thus doesn’t break down in sun (unless u swim or sweat) & provide protection for long hours without reapplication. Cricketers prefer to use sunblocks containing only ZnO.

Titanium dioxide carries some photo-toxic traits & that’s why it is always used along with other UV filters & not as a standalone product in sunscreens.

Nano physical filters absorb most of the UV rays and become photo-active. This lead to generation of free radicals by them which can damage surrounding cells on skin. To reduce the side-effects, the nano particles are coated with other non-active ingredients like Al(OH)3 etc to mix them more effectively & make them less photo-reactive. This coating is more mandatory while using nano-TiO2 as they are suspected to cause more problems at smaller particle size.

The concern with nano UV filters is that their quality is not well-controlled & tested by brands. Thus making them insufficient to inhibit all their photo-activity. This is the reason, they do not provide sufficient sun protection when used standalone. They are preferred to be used alongside chemical filters in a synergistic way to create very high-SPF, non-irritating sunscreens in light, breathable formulations.

How particle size effect ZnO/TiO2 effectiveness

As the particle size of zinc decreases, so does its absorbance peak (it absorbs UVA less well), and scattering becomes a less significant part of its UV attenuation. Large particles of zinc oxide scatter more radiation and better attenuate longer frequencies of radiation. The science of nano and non-nano physical filters is not that easy and you can check more on it here.

Why multiple chemical UV filters are used in sunscreens

Some chemical UV filters are good in neutralizing UVB rays as well as short UVA-2 rays (320-340 nm) but they does not protect from long UVA-1 rays (340-390 nm). Eg: Oxybenzone, Uvinul T 150, TiO2 are not that effective in preventing complete wavelength of UVA rays. That’s why I have categorized them under UVB filters only.

UVB filters in sunscreen

These filters when combined with other UVA filters which are effective in preventing complete UVA rays, forms a broad spectrum formula. For example: Avobenzone is a photo-unstable filter but it provides protection against complete wavelength of UVA rays. When avobenzone is combined with stable OC filter or more, it forms a broad spectrum stable formula.

Classification of sunscreens based on UV filters

Chemical sunscreens

Sunscreens which contain active UV filters which are easily penetrate into skin & absorb UV rays to produce heat. They are normally light weight, non sticky and doesn’t leave a white cast. Unlike mineral sunscreens, they penetrate more deeply into the skin and play a significant role in accelerating premature ageing.

Since chemical filters are not photo stable, multiple filters are used to ensure the most effective broad-spectrum protection from both UVA and UVB rays. They are likely to cause allergic reaction leading to skin irritation but these days chemical sunscreens are formulated with anti-oxidants to save your skin’s health.

Physical Sunscreens

Sunscreens which contains standalone mineral UV filters like ZnO & TiO2 are categorized as mineral or physical sunscreens. The main UV filter used in them is ZnO which provides broad spectrum & long-lasting protection without penetrating into inner layers of skin.

The physical sunscreens carrying non-nano ZnO or named as sunblocks are more effective in protecting from UVB rays as compared to those containing nano ZnO & TiO2 UV filters.

Sunblocks primary job is to protect you from UVB rays (sunburn). So, if you know you’re going to be under the harsh sun all day, it’s best to use a sunblock. You don’t have to keep reapplying as often but it gives you a white cast. As ZnO has good absorption as compared to TiO2, its important that an effective sunblock must contain 25% ZnO as UV filter. You might have seen cricketers using them 🙂

Why some Non-nano sunscreens give less white-cast?

These days in order to provide less white-cast without compromising with sunscreen efficacy, brands have started creating formulations which have the non-whitening characteristics of nano zinc oxide and the large particle assurance of non-nano zinc oxide.

How this is done? Zinc oxide powder made of smaller particles that are fused together into larger micron-sized (1000+ nanometers) aggregate particles. These aggregates have a porous surface texture, so they don’t reflect as much visible light as standard non-nano particles. Thus less whitening on the skin. Such formulated zinc particles give the same outstanding safety and efficacy of standard non-nano zinc oxide meaning it won’t absorb into your skin, it won’t harm the environment, and it provides excellent UVA and UVB protection.

Hybrid Sunscreens

Since chemical filters are known to provide maximum protection from UVA rays & physical filters provide efficient protection against UVB rays, both are combined together to give a synergistic effect to overall sunscreen formulation, and we name it as hybrid sunscreen. In order to main the thickness level, chemical UV filters are combined with TiO2 to provide the broad spectrum protection against UV rays. TiO2 helps in protecting the chemical filters from degradation too by forming a layer on skin.

Chemical Sunscreen Vs Physical Sunscreen

Though the quality of sunscreens depends a lot on formulations & active ingredients used by brand, I have listed down the generic differences based on the properties they likely to carry. You can check this article for more insights.

Types of Sunscreens

The difference above clearly shows that a hybrid sunscreen is the most effective when it comes to choosing a sunscreen which could fight all wavelength of UV rays. Given this its not necessary that every hybrid sunscreen will proved to be effective & every chemical sunscreen is bad. It always depends on the type of UV filters used in a sunscreen.

Factors for effective sunscreen formulation

An appropriate sunscreen product must fulfil the following critical requirements:

  • Provide efficient protection against UVB and UVA radiation
  • Be stable to heat and to UV radiation (UVR)
  • Friendly to skin type to encourage frequent application
  • Less irritation properties (anti-oxidant rich formula)
  • Minimal white cast for every day wear

Are UV filters Hormone or Endocrine disruptor?

Some UV filters like Oxybenzone, Ensuizole, nano TiO2 etc have been suspected to penetrate the skin & absorbed by body resulting in causing hormonal imbalances & effecting endocrine system. Though many studies found their presence in human blood, Urine & breast milk, none of them could actually prove their side-effects. The reason is amount of product used for research is much more than the those found in the human body. That’s why till now no damage has been observed in real life with the use of chemical filters in sunscreens.

Current FDA Safe sunscreen filters – FDA simply points out that chemical sunscreen filters enter our body and that there is a question mark around their systemic impact which must be analyzed before they can regain the FDA’s “safe and effective” status. At this stage the FDA does not say that chemical filters are harmful or should be avoided.

In short, you can continue to use chemical filters based sunscreens unless further update from FDA. If you are pregnant or breast feeding or have to use on children, be on a safer side and use mineral sunscreens only.

Relation of UV filters with coral reef

Between 6000 and 14,000 tons of sunscreen washes off swimmers, scuba divers, and snorkelers into coral reef environments each year. Even more sunscreen pollution can reach coastal areas via waste water discharges. Up to 10% of the world’s coral reefs are suspected to be threatened by certain chemicals found in most sunscreens.

Common sunscreen ingredients which were shown to kill or bleach coral are Oxybenzone, Octinoxate, Octocrylene, 4-methylbenzylidene camphorButylparaben, Nanoparticles including of zinc oxide or titanium dioxide. You can check various articles discussing harmful effects of sunscreen chemicals on coral reef here. This is a more detailed one.

What you can do to save coral reef from suspected UV filters:

  • If you plan to swim, scuba dive, or snorkel near coral reefs you should use a coral reef friendly or reef safe sunscreen. 
  • Do not use a sunscreen that contains oxybenzone, octinoxate or the other ingredients listed above that are shown to kill coral
  • Use a water resistant sunscreen which will be more likely to stay on your skin and out of the water. 

How much sunscreen should be applied?

A sunscreen efficacy is tested in a laboratory by using a generous amount of quantity ( 2 mg/cm2 ) as compared to what we apply on skin. The protection factors in a sunscreen are relevant only as long as the sunscreen is applied liberally and isn’t washed away by sweat, water or your towel.

At least a medium coin size amount (1/2 tsp )is required to cover your entire face and neck in order to protect your skin from harmful sun rays. The lesser you apply, the more frequently you have to reapply it. For entire body you need an ounce of sunscreen approx. 30 ml. Reapplying is a mandate after every 2 hours while swimming/sweating.

What if you apply less sunscreen? I know I know, the humid weather conditions often make us irritated & we don’t want to apply anything on our skin. But still what if you apply half the recommended amount of sunscreen?

-> You will be getting half of the labelled SPF value. The reason is SPF decreases linearly with less application. It means SPF 30 will give you a protection of SPF 15. Though that’s not an excuse but still in order to increase protection while applying less sunscreen, you can go for SPF 50. I would still stay if you are stepping out, just slather the recommended amount of sunscreen without any excuse for complete protection.

Concluded facts about sunscreen

  • Products with good UVA absorbing filters prevents photo-ageing
  • Low SPF sunscreens with balanced UVA & UVB filters (broad spectrum) provide maximum broad spectrum protection which only higher SPF sunscreens may not provide.
  • Almost all UV filters break down when exposed to sunlight. That’s why you’re told to reapply your sunscreens every few hours. Its just that some filters break down faster than the other.
  • Only higher SPF doesn’t matter : Anything higher than SPF 50+ can tempt you to stay in the sun too long. Even if you don’t burn, your skin may get damaged
  • Sunscreens with both hydrophilic(water-soluble) & lipophilic(oil-soluble) UVA/UVB filters form an effective broad spectrum because they have more synergistic effect.
  • Antioxidants in sunscreens are added to stabilize UV filters & prevent the UV-induced biological damage.

How to choose your sunscreen?

  • If you stay outdoors & expose to sunlight, then always look for non-nano SPF 50 (PA+++) sunscreens with Zinc Oxide along with photo-stable chemical broad spectrum or UVA filters.
  • If you stay indoors with glass windows, then sunscreens above SPF 15 containing chemical UV filters along with either ZnO or TiO2 would do the job.
  • Always look for at least one photo-stable UV filter in sunscreen ingredient list esp. for outdoors.
  • If you have acne prone oily skin, use a water-based sunscreen. These do not cause your skin to break out like oil-based creams.
  • Use mineral sunscreen (with titanium dioxide and zinc oxide) if you’re pregnant or breastfeeding and on children.
  • People who have sensitive skin or skin conditions like rosacea can go for zinc oxide sunscreens instead of irritating chemicals like benzophenones, PBSA etc.
  • Avoid sunscreens with alcohols & fragrances if you have skin allergies & irritation.
  • Look for sunscreens which has antioxidants along with UV filters because they help in fighting free radicals generated by UV filters.
8 Tips to choose an effective sunscreen


After looking into hundreds of articles about sunscreens, I can only say that its a forever debatable topic. Its really not that easy to understand all the science behind them. The only thing I would suggest you to follow is to check the ingredient list of sunscreens along with their lab tested label. You never know the proportion in which brands are adding the UV filters & making an effective formulation which could protect us from sunlight.

If you are staying indoors, you might be less worried about applying only tested sunscreens because there is no risk of sunburn leading to age spots & pigmentation. But let me tell you one thing, UVA rays will still reach your skin making a long term damage, so its really important to choose an authentic sunscreen which could neutralize the UVA rays.

Hope this article helped you in clearing some of your doubts about sunscreens. If you have any doubts or not agree with any of the point I shared, then please hit the comment section. I will try to solve your query & update my blog after doing more research.

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