Absorbance
A measure of the extent to which light of a specific wavelength is blocked by a sunscreen. Mathematically, the absorbance is the logarithm of the Monochromatic Protection Factor (MPF).
Absorption
A mechanism by which a sunscreen active ingredient blocks UV light by converting it into another form of energy, usually heat.
Active ingredient
Generally, an ingredient which provides a functional benefit in a personal care product. In the case of sun care, active ingredients are UV filters.
Acute sun exposure effects
Effects on skin arising from short-term sun exposure, for example erythema.
Aggregate
A particle comprising strongly bonded or fused particles where the resulting external surface area may be significantly smaller than the sum of calculated surface areas of the individual components.
Agglomerate
A collection of loosely bound particles or aggregates or mixtures of the two where the resulting external surface area is similar to the sum of the surface areas of the individual components.
Attenuation
Generic term for the blocking of light by a sunscreen. Absorption and scattering are both mechanisms of attenuation.
Basal cell carcinoma (BCC)
A type of skin cancer that arises from the basal cells, small cells found in the lower part (or base) of the epidermis, the outer layer of the skin. BCC occurs most often on the face. About 70% of all skin cancers are BCC’s.
Boots Star Rating
Labeling system used in the UK to denote the level of UVA protection offered by sun protection products. The assigned star rating is based on the mean absorbance ratio as measured in an in-vitro SPF test. The maximum rating is 5 stars, corresponding to a mean absorbance ratio of greater than 0.9. The star rating is used only in the UK as a basis for labelling, but some manufacturers in other countries use it as an internal standard to set targets for UVA protection in new product developments.
Broad spectrum protection
This is a claim which means a product protects effectively against both UVA and UVB.
Bronzers
Alternative name for self-tanners.
Chemical sunscreens
See organic sunscreens.
Chemical-free sunscreen
Marketing term sometimes used as a promotional slogan for a sunscreen product which contains no organic sunscreens. Technically a misnomer, as only a vacuum is truly “chemical-free” !!
Chronic sun exposure effects
Effects on the skin arising from long-term sun exposure, for example premature aging and skin cancer.
COLIPA SPF method
A method developed in Europe for determining the in-vivo SPF of a sun protection product. Fore-runner of the International method.
COLIPA UVA protocol
An in-vitro method for determining the UVA protection factor of a sunscreen product.
Critical wavelength
A method for assessing to what degree a sun protection product provides broad spectrum protection. The absorbance curve is measured from a wavelength of 290 nm to 400nm, and the area under the curve is calculated. The critical wavelength, λc, is such that the area from 290 nm to λc is 90% of the total area. The broader the spectrum, the larger the value of λc. Current EU guidelines for testing and labelling of sunscreen products require that the critical wavelength should be greater than 370nm.
Crystal size
The size of the fundamental crystals which make up a particulate material. These crystals are usually aggregated to some degree to form larger particles. Therefore the particle size of a particulate material (for example, a physical sunscreen) is almost always greater than the crystal size.
Dispersion
A mixture consisting of small particles or droplets of one material within an external carrier medium. Most commonly, this term refers to solid particles within a liquid carrier. Croda sunscreen products (Tioveil™, Tioveil 50, Solaveil™ Clarus, and Spectraveil™ product ranges) are dispersions of inorganic sunscreens.
Erythema
Reddening of the skin, most commonly caused by over-exposure to UV light, in which case it is commonly known as sunburn.
Erythemal action spectrum
A spectrum which indicates the extent to which different wavelengths of light can cause sunburn (erythema). In sunlight at the Earth’s surface, the most erythemally-active wavelengths are in the UVB part of the spectrum.
Erythemal effectiveness
A spectrum derived by combining the erythemal action spectrum with the solar spectrum under defined conditions, in order to describe the overall “sunburn hazard” level at each wavelength at the Earth’s surface.
Extinction coefficient
A parameter which defines the effectiveness (per unit of concentration) of a UV filter in attenuating (blocking) light of a specific wavelength.
Isoelectric point (IEP)
pH value at which the charges on the surface of inorganic particles balance out so that the overall charge on the surface is zero. In the case of physical sunscreens dispersed in water, this pH should be avoided as there is no electrostatic repulsion between particles, and hence there is a greater potential for agglomeration. For particles dispersed in oil (or hydrophobically-coated particles in water), the IEP is not relevant because the surface charges are insulated and a good dispersion is maintained by steric repulsion rather than electrostatic repulsion.
Immediate pigment darkening (IPD)
A transient, brownish-grey discolouration of the skin which occurs after exposure to UVA light. It begins within 60 seconds and last for a few minutes. IPD has been used as the end-point for one in-vivo method of measuring protection against UVA, but it is not popular because the end-point is difficult to read and highly subjective.
In-vitro SPF
The Sun Protection Factor of a sun protection product, as measured by instrumental methods. Such methods are a valuable tool in developing sun care formulations, as they are much cheaper and quicker than in-vivo SPF test methods, and therefore are more suitable for testing large numbers of samples. Various methods have been used but by far the most commonly-used method is that developed by Diffey and Robson. This involves applying the test product onto a suitable textured substrate to form a thin film, and then measuring the absorbance of UV light by this film at various wavelengths. The SPF is calculated from these absorbance measurements by taking into account the erythemal action spectrum and the solar spectrum. Other performance parameters can also be calculated, such as the mean absorbance ratio and the critical wavelength. There are two commercially-available instruments based on this method, the Optometrics SPF290 and the Labsphere UV1000S.
In-vivo SPF
The Sun Protection Factor of a product as measured by tests on live subjects, ie. human volunteers. This is the definitive measure of SPF, and any commercial formulation which makes an SPF claim should always be tested in-vivo. There are different protocols for doing the test (eg. International method, FDA method), but the basic principle is the same in each case. Small sites on the subject’s back are exposed to varying doses of UV radiation from a solar simulator in order to determine the minimum erythemal dose (MED) for that person with no protection (“unprotected” MED). The test product is then applied to a different area of the back, and a further series of exposures is carried out to determine the “protected” MED. The ratio between the protected MED and the unprotected MED is the SPF. The measurement is repeated for a number of volunteers and the results averaged to give the final product SPF.
Inorganic sunscreens
Inorganic materials which block UV light by a combination of absorption and scattering. Various inorganic compounds can do this, but by far the most effective (and most commonly-used) are titanium dioxide and zinc oxide. Tioveil™, Tioveil 50, and Solaveil™ CT products are dispersions of titanium dioxide, while Spectraveil™ and Solaveil CZ products are dispersions of zinc oxide. Inorganic sunscreens are also known as physical sunscreens.
International method
Protocol for measuring the in-vivo SPF of a sun protection product, agreed between various regulatory bodies and trade associations around the world. Now accepted as the standard method for SPF determination in Europe.
Malignant melanoma (MM)
A form of skin cancer that arises in the melanocytes. This is the least common but most lethal form of skin cancer, accounting for only about 10% of cases but about 90% of skin cancer deaths.
Mean absorbance ratio
The ratio between the mean absorbance given by a product in the UVA (320 – 400 nm), and the mean absorbance in the UVB. This ratio is determined by in-vitro SPF test methods, and is used as the basis for the Boots Star Rating in the UK. The ratio is also known as the UVA/UVB ratio.
Melanin
The pigment naturally present in human skin, which darkens on exposure to UV light.
Melanocytes
Cells within the skin which produce melanin.
Melanogenesis
The process by which melanocytes produce melanin, resulting in tanning of the skin.
Microfine
A term often used to describe a particulate material which has a very small particle size, such as an inorganic sunscreen. Other similar terms include micronised and ultrafine; these words are often used interchangeably.
Micronised
This tends to be used as a generic term to describe any fine particle material, for example an inorganic sunscreen. However this is a misnomer; micronising is a specific physical process by which larger particles are broken down into smaller ones by a high-energy impact process. This type of process is sometimes used to make inorganic sunscreens but not always; for example, the particles in Tioveil™ and Solaveil™ Clarus products are not micronised; the small particle size is achieved by thermodynamic control of the precipitation process, which gives much better control over particle size and size distribution than micronising does.
Minimum Erythemal Dose (MED)
The MED is defined as the minimum dose of UV radiation required to produce the first detectable reddening of fair human skin. In other words, it is the minimum UV dose required to produce the first sign of sunburn in pale skin. This varies from one person to another, according to skin type, age and sex. Determination of the MED forms a critical part of measuring the in-vivo SPF of a sun protection product.
Monochromatic Protection Factor (MPF)
Parameter which indicates the degree of protection given by a product against a specific wavelength of light. Mathematically, the MPF is the reciprocal of the transmittance.
Organic sunscreens
Organic molecules which function as UV absorbers, ie. they have the capability of absorbing UV radiation, and are used for this function in personal care products. Also known as chemical sunscreens.
OTC drugs
An abbreviation for “Over-The-Counter” drugs, ie. drug products which can be bought without prescription. This is relevant to sun care because in some countries, most notably the USA, sun protection products are regarded as (and regulated as) OTC drugs (as opposed to the situation in Europe for example, where sun protection products are regulated as cosmetics).
PA rating
Japanese standard for labelling the UVA protection given by a product. The PA rating is determined by measuring the UVA protection factor (PFA), according to the Persistent Pigment Darkening method. A PA rating of “PA+” corresponds to a PFA of 2-4, “PA++” indicates a PFA of 4-8, and the maximum rating “PA+++” denotes a PFA of greater than 8.
Particle size
The size of particles in a sample of a particulate material. Each particle is usually an agglomerate or aggregate of more than one crystal, so it is important to be aware of the difference between particle size and crystal size. Crystal size means the size of the fundamental crystals, whereas particle size indicates the size of the agglomerates. Particle size can be measured by a wide variety of different methods, but different methods give different answers, so particle size data measured by different methods cannot meaningfully be compared with each other.
Persistent Pigment Darkening (PPD)
A long-lasting response of the skin to UV exposure, resulting in darkening of pigmented skin. Melanin already present is further darkened, and production of new melanin is enhanced. PPD begins within hours of exposure and may last for days to weeks. As it is believed to be mainly due to UVA exposure, PPD is used as the end-point for the most popular in-vivo method of measuring UVA protection; for example this is the Japanese standard method, used to determine PA rating.
PFA
Abbreviation for UVA Protection Factor.
Photoaging
Aging of the skin as a result of chronic sun exposure, resulting in for example dryness, deep wrinkles, accentuated skin furrows, sagging, loss of elasticity, and mottled pigmentation. This is quite distinct from chronological, or intrinsic aging.
Photoallergy
A skin reaction caused by an interaction of a drug, light, and the immune system in susceptible individuals. This is the less common form of drug-induced photosensitivity (the other is phototoxicity) and is often caused by UVB. Photoallergic reactions, unlike phototoxic reactions, represent an immunologic reaction and require a latent period of 24-48 hours during which sensitisation occurs.
Photocatalyst
A material which can promote a light-induced chemical reaction in species it comes into contact with, while undergoing no overall chemical change itself. Both titanium dioxide and zinc oxide have the potential to act as photocatalysts, but this is an undesirable property in personal care products. Sunscreen grades of TiO2 are coated to prevent photocatalytic activity. The crystal form of the TiO2 is also important; all Tioveil and Solaveil Clarus TiO2 dispersions use the rutile form, as this gives much lower photocatalytic activity than the anatase form. Zinc oxide is actually a very inefficient photocatalyst, and does not require coating to inhibit photocatalytic activity.
Photosensitivity
A condition whereby an individual is particularly susceptible to adverse reactions caused by UV light. There are a number of specific skin disorders which are triggered or exacerbated by UV radiation. A photosensitive reaction may also occur due to a chemically induced alteration in the skin (eg. from taking a drug) that makes an individual more sensitive to UV radiation. Such reactions may result in phototoxicity or photoallergy.
Photostability
This term has two meanings in the sunscreen industry. The first is concerned with how well a UV filter maintains it’s efficacy on exposure to UV light. Some organic sunscreens photodecay, ie. they undergo chemical changes when they absorb UV, and as a result lose efficacy. In this context, a photostable sunscreen is one which does not photodecay. All inorganic sunscreens are inherently photostable in this sense, as they work by physical means and therefore undergo no chemical change. The second meaning of the term “photostability” relates to the resistance of inorganic sunscreens to act as photocatalysts. In this context, a photostable inorganic is one which shows very low photocatalytic activity. The coatings used in Tioveil™ and Solaveil™ Clarus products are designed to make the titanium dioxide as photostable as possible.
Phototoxicity
Phototoxic reactions are the most common form of drug-induced photosensitivity and are commonly caused by UVA. This is not a form of allergy; theoretically, this type of reaction can occur in anyone exposed to sufficient quantities of a photosensitizing agent and light. The reaction is dose-dependent for both.
Physical sunscreens
See inorganic sunscreens.
Pigment
A material which gives colour or opacity. Inorganic sunscreens are often referred to as microfine, micronised, or ultrafine pigments, but this is not strictly correct as inorganic sunscreens are usually designed to give no colour and as little opacity (to visible light) as possible. As it relates to titanium dioxide and zinc oxide, the term “pigment” more correctly refers to larger particle size materials which are designed to be opaque to visible light.
Sand proof
This is a claim which is now appearing on a number of sun protection products, and implies that the product will remain on the skin and maintain protection even after the user gets sand on them, which is subsequently rubbed off. However there is no defined protocol or performance criterion for justifying a sand proof claim.
Scattering
One of the mechanisms by which inorganic sunscreens attenuate UV light. Scattering occurs as a result of a difference in refractive index between a material and it’s surroundings. Physical sunscreens (titanium dioxide, zinc oxide) have high refractive indices and are therefore very effective in scattering light. It is this mechanism which differentiates inorganic sunscreens from organic sunscreens, which work purely by chemical absorption of light.
Self-tanners
Products which provide a tan without exposure to sunlight or artificial UV radiation. Such products are also known as bronzers, sunless tanners, or tanning accelerators. Early self-tan products simply contained a dye or pigment that stained the skin; these products were not popular because they tended to give a very unnatural colour and were difficult to apply evenly, resulting in a streaked appearance. Modern self-tan products use an active ingredient (the most common one is dihydroxyacetone, DHA) which stimulates production of melanin in the skin, giving a more natural and longer-lasting colour. The efficacy of such products is enhanced by ensuring that the formulation contains an effective solvent for the active; Croda’s product Arlasolve™ DMI is especially effective in this regard. It is important to remember that self-tanners do not, in themselves, provide any UV protection; however there are a few products on the market which combine UV filters and a self-tanning active in the same formulation.
Skin cancer
A disease involving a cancerous growth in or on the skin. There are three main types of skin cancer : basal cell carcinoma, malignant melanoma, and squamous cell carcinoma. Most skin cancers are believed to be caused at least in part by exposure to UV radiation.
Solar simulator
An instrument which emits a beam of UV light with a spectrum approximating that of the sun, but with higher intensity. Solar simulators are used to irradiate the skin for in-vivo SPF measurements.
Solar spectrum
A spectrum showing the intensity of natural sunlight at various wavelengths throughout the UV region, under defined conditions (time of day, time of year, geographical latitude).
SPF Efficacy
This is an unofficial term which refers to the effectiveness of a sunscreen active or formulation, in terms of SPF per unit concentration of the active. It is usually expressed as “SPF per % active”; for example, if a formulation contains 10% active and gives and SPF of 25, the SPF efficacy would be 2.5 units per % active.
Squamous cell carcinoma (SCC)
A form of skin cancer that begins in squamous cells, which are thin, flat cells resembling fish scales found in the outermost cells of the skin. SCC accounts for about 20% of skin cancer cases.
Standard Erythema Dose (SED)
The SED is a measure of erythemal UV radiation; it is equivalent to an erythemal effective radiant exposure of 100 J/m2. The SED is used as a convenient unit to describe UV doses and ambient levels of erythemally effective UV. For a minimal erythema on previously unexposed skin of a Northern European skin type (skin types II or III), a dose of about 2-4 SED is required.
Sun Protection Factor (SPF)
SPF is the primary measure of the performance of a sun protection product. It is defined as:-
| Sun Protection Factor |
= |
MED with sunscreen applied |
| MED without sunscreen |
where MED is the Minimum Erythemal Dose. For commercial products, the SPF should always be established by means of an in-vivo SPF test. However when measuring or comparing the efficacy of a large number of formulations, for example during product development, it is more convenient to use in-vitro SPF methods.
Sunscreen
A chemical, or a cosmetic formulation, which provides protection from UV radiation. In common usage, the term “sunscreen” is used to refer to both finished formulations, and the UV filters which are the active ingredients in such formulations.
Sunscreen Monograph
A set of rules, being developed by the United States’ Food and Drug Administration (FDA), to govern the formulation, testing and labelling of sunscreen products manufactures in, or imported into, the USA. The first Proposed Rules were published in 1978; this was followed in 1993 by the “Tentative Final Monograph”, and then, in May 1999, the “Final Monograph”. However, this monograph has still not been implemented, as it is incomplete; the process of completing the Monograph is still ongoing. Most recently, in August 2007, the FDA proposed a number of amendments to the monograph, including proposals for UVA testing and labelling. As yet there is no definite date for implementation of the monograph.
Sunless tanners
See self-tanners.
Sweat-resistant or sweat-proof
This is another claim which is found on a number of sun care products, and implies that the product is not removed from the skin by sweating. Test methods for this have been proposed, but there is no officially recognised method.
Tanning
Darkening of the skin due to exposure to UV radiation, often seen as attractive and socially desirable in western society. UV causes melanin already present in the skin to darken, and also stimulates production of further melanin.
Tanning accelerators
See self-tanners.
Transmittance
In optical physics, transmittance is the fraction of light of a specific wavelength which passes through a screening or absorbing medium.
Ultrafine
See microfine.
Ultra-violet radiation (UV)
Electromagnetic radiation in the wavelength range 100 nm to 400nm. In the sunscreen industry, this is divided into three regions : UVA, UVB, and UVC.
UV absorbers
Often used as a generic term for sunscreen active ingredients, but strictly speaking should only be applied to organic sunscreens.
UV filters
Generic term for any chemical which attenuates or blocks UV radiation.
UVA
UV radiation in the wavelength range 320 nm to 400 nm. UVA plays a major role in tanning of the skin, but also contributes to premature aging and skin cancer.
UVB
UV radiation in the wavelength range 290 nm to 320 nm. UVB plays the major role in causing erythema (sunburn) and also contributes to skin cancer.
UVC
UV radiation in the wavelength range 100 nm to 290 nm. UVC is actually the most dangerous form of UV light, but it is blocked by the ozone layer and hence does not reach the Earth’s surface.
UVA/UVB ratio
See mean absorbance ratio.
UVA protection factor
A parameter which indicates the level of UVA protection given by a product. There are various different definitions of UVA protection factor; the most commonly-used is that measured in-vivo, by the Persistent Pigment Darkening method.
Water-resistant
This is a claim found on many sun care products, and indicates that the product maintains protection after swimming. However the criteria and test methods for a water-resistant claim vary from country to country.
Waterproof
This is a stronger version of the water-resistant claim, and implies either that protection is maintained for longer periods of water immersion, or that a greater proportion of the original SPF is maintained. However this claim is now not allowed in many countries, as it is considered that such an “absolute” claim cannot be justified. In some countries “waterproof” has been replaced by the claim “very water-resistant”.
Whitening index
This is a parameter used by Croda to compare differences in transparency between different grades of inorganic sunscreens or between different formulations. It is measured by performing a “draw-down” of a test formulation to give a thin film on a black card. The colour of the card is measured before and after application of the test product; the parameter of interest is the L-value, which indicates whiteness. The whitening index is the difference between the L-value of the black card (Ls) and the L-value of the formulation on the card (Lf):-
Whitening index, ΔL = Lf - Ls
This test can also be performed in-vivo, by applying the product on skin with a gloved finger. In this case Ls is the L-value of the skin before product application.