Sunscreens (or sunblocks) are a combination of agents available as creams, lotions, sprays, gels etc. for application on the skin to protect from the ultraviolet (UV rays) of the sun.
The sun’s rays consist of the visible spectrum of light (VIBGYOR) and the invisible ultraviolet (UV) and infra-red rays. UV rays are of 3 types:
UVA radiation (long wave: 320-400nm) has the lowest energy and highest penetration (can reach beyond the epidermis into the inner skin layer – the dermis). UVA causes tanning (temporary darkening of sun-exposed areas), and photoaging (skin aging due to gradual and long-term oxidative skin damage resulting from the release of reactive oxygen species – ROS, also called free radicals). UVA1 rays are between 340-400 nm while UVA2 rays are between 320-340 nm in wavelength.
UVB radiation (short wave: 280-320nm) has higher energy than UVA but has low penetration being largely absorbed by the epidermis (outer skin layer) itself. UVB aids in Vitamin D synthesis. However, too much exposure to UVB can cause sunburns as an immediate effect, and also increase the risk of skin cancers as a long-term effect.
UVC (ultra-short wave: 100-280nm) have very high energy but fortunately for us, all of the UVC radiation, and >95% of UVB radiation is absorbed in the earth’s atmosphere by ozone (absorbs 200-315nm) and oxygen (absorbs <200nm). So, what reaches us is largely UVA and 3-4% of the UVB rays. Therefore, preserving ozone in the atmosphere is important to prevent over-exposure to high energy harmful UV rays.
DESIRED PROPERTIES OF SUNSCREENS
An ideal sunscreen should be the following
- Highly protective – against both UVA and UVB
- Stable in sunlight & heat
- Resistant to sweat
- Well-tolerated, non-irritating, and comfortable
- Smooth and not patchy or clumpy on application
- Cosmetically acceptable
- Minimally absorbed into the blood and environment friendly
The standard dose of applying sunscreen (FDA Standard) is 2 mg/cm2 or 30ml/full body. Most sunscreens need to be re-applied after bathing and swimming as the wash-out time is around 30-45 minutes of water exposure. Some newer sunscreens have polymeric hydrophobic (water repellent) added substances that can increase this wash-off time by 20-30% to more than an hour. However, it’s best to reapply sunscreens after being in water.
UNDERSTANDING THE AMOUNT OF UV PROTECTION
The action of sunscreens can be quantified against UVA and UVB.
Sun Protection Factor (SPF)
It measures the protection against UVB radiation.
Sun Protection Factor (SPF) = MED photo-protected/MED unprotected skin
(Ratio of the minimum effective dose of UVB radiation that produces redness in skin protected by the given sunscreen versus unprotected skin)
UVB radiation absorbed:
- SPF 15 = 93% (SPF:2-15 is considered low)
- SPF 30 = 96.7% (SPF: 15-30 is considered medium)
- SPF 50 = 98% (SPF: 30-50 is considered high)
With SPF > 50, there is not much of added value of UVB absorbed, therefore such sunscreens may just mention SPF 50+ indicating very high SPF.
For most people, a medium SPF sunscreen gives adequate protection from UVB. However, those predisposed to sunburns or those with melasma and hyperpigmentation conditions should go for high/very high SPF sunscreens.
PPD and PA system
PPD stands for persistent pigment darkening and measures how protective the sunscreen is against a long-term tan caused by UVA by the ratio of minimum radiation dose on sunscreen-treated skin and minimum dose on unprotected skin.
The Protection Grade of UVA (PA) system given by the Japan Cosmetic Industry Association, and used now in large parts of Asia and by other global brands, measures the UVA protection of sunscreens based on the PPD reaction, and is indicated as below:
- PA+ = PPD 2-4,
- PA++ = PPD 4-8,
- PA+++ = PPD >8
For most people, a PA++ sunscreen gives adequate protection from UVA. However, those predisposed to tanning or those with melasma and hyperpigmentation conditions should go for PA+++ sunscreens.
Boot Star Rating
Often sunscreens are procured or prescribed only based on SPF, however now there is growing evidence that the PA factor is equally important and an effective sunscreen gives adequate protection from both UVA and UVB.
The Boot Star rating is a British system and is according to the UVA/UVB protection ratio, and therefore it gives a good indication of the overall protection by the sunscreen.
- *** 0.6-0.79
- **** 0.8-0.89
- ***** 0.9 or higher
Five stars on the Boots system means that UVA protection is >90% UVB protection.
Critical Wavelength and Broad Spectrum
The critical wavelength value tells consumers how far out on either side (starting at the beginning of the UVB range at 320 nm) a sunscreen’s “protective umbrella” extends; or more precisely how far out 90% of this “umbrella” extends.
The FDA has defined “broad-spectrum sunscreen” as products with a critical wavelength value of over 370nm, and only in such cases can the ‘broad spectrum sunscreen’ claim be made.
COMPOSITION OF SUNSCREENS
Sunscreens consist of physical and chemical filters. While physical filters block and reflect radiation, chemical filters absorb radiation.
These are opaque formulations containing zinc oxide (ZnO), titanium dioxide (TiO2), talc, kaolin, etc. They scatter or reflect UV radiation due to their large particle size. Traditional ZnO and TiO2 clusters are large in the range of 100-300nm, so, therefore, block and reflect in the range of 200-600nm (part of visible light in addition to both UVA and UVB). Maximum scattering occurs for wavelengths twice as large as the clusters. While ZnO blocks UVA and UVB almost completely, TiO2 blocks UVB and UVA2.
Using only physical blockers as sunscreens would require them to be applied in larger quantities or higher concentrations for adequate protection. This poses problems of needing a thick layer of application that often looks powdery or cosmetically unacceptable, stains clothing, and does not wash off easily. Now microfine and nanoparticle formulations are available that have different particle shapes with a broad size distribution range of 30–150 nm. These are cosmetically more appealing, easier to apply, and higher concentrations of these compounds can be used.
Physical sunscreens ZnO and TiO2 have been considered GRASE (generally recognized as safe and effective) by FDA. People with sensitive skin should use physical sunscreens.
These act by absorbing UV radiation. Different chemicals absorb a particular range of UV radiation and therefore a combination of chemicals gives adequate protection from both UVA and UVB. For example, avobenzone mainly absorbs UVA while oxybenzone and octinoxate mainly absorb UVB, so these have been traditionally used as a combination in sunscreens. Newer agents like Tinosorb M and S absorb both UVA and UVB adequately.
Concerns have been expressed over the human body’s absorption and blood levels, as well as environmental toxicity especially to marine life, of some of the chemical sunscreens like avobenzone, oxybenzone, octinoxate, homosalate, octisalate, and octocrylene. Therefore, chemical sunscreens do not have FDA GRASE status like physical sunscreens, but there is also no data to show adverse health effects yet, except for oxybenzone that may affect hormonal balance in women.
It is better to use these chemical sunscreens judiciously, and also avoid them during pregnancy/lactation. Earlier chemical sunscreens aminobenzoic acid (PABA) and trolamine salicylate had been declared unsafe by FDA. However, for the other mentioned 6 chemicals, more research is underway and as of now, they continue to be regarded as acceptable. The Uvinul and Tinosorb based sunscreens are more photostable than conventional chemical sunscreens. Each sunscreen formulation is individually evaluated for its components, concentration, and UV protection before being approved by regulatory authorities of each country.
Reference: USFDA 2021 Sunscreens