What a standard is
A test standard is a document setting out a method in enough detail that two laboratories following it independently will produce comparable results. It specifies apparatus, materials, conditions, procedure, calculation, validity criteria and reporting.
It is not a law. It creates no offence and confers no permission. It is not a licence, and a laboratory does not need anyone's authority to follow one. And it is not a quality mark: a product tested to a standard is not thereby a good product, it is a product whose properties have been measured in a way that others can compare against.
What standards do is make measurement portable. Without them, a factor determined in one laboratory would be a statement about that laboratory rather than about the product.
Which standards apply
ISO 24444 covers the in vivo determination of the sun protection factor: the human panel method described in what an in vivo SPF test involves. It is the reference method behind the figure on the front of the pack.
ISO 24443 covers the in vitro determination of sunscreen UVA photoprotection: the plate method described in in vitro UVA testing and what it measures. It produces the ultraviolet A protection factor and the critical wavelength.
Water resistance has its own published methods covering the immersion procedure and the calculation of retained protection. The determination of water resistance is examined in depth by other publications in this field; our concern is the claim it supports, covered in waterproof and sweatproof.
Standards are revised. A method carries a year, and a laboratory report should state which version was followed, because revisions change details that affect results.
| Standard | Determination | Carried out on |
|---|---|---|
| ISO 24444 | In vivo sun protection factor | A screened human panel |
| ISO 24443 | In vitro UVA photoprotection | A roughened plate, with pre-irradiation |
| Water resistance methods | Retained protection after a defined immersion sequence | A human panel, with immersion |
| Laboratory accreditation standard | Competence of the laboratory, not the product | The laboratory, assessed by a national accreditation body |
Framework of this publication. It describes how published rules and guidance fit together and is not a measurement, a survey or a study.
How a standard acquires force
This is the part that is regularly misunderstood, including by people writing packaging copy.
The United Kingdom cosmetics regime does not mandate a particular test method for sun protection claims. What it requires is that a claim is supported by adequate and verifiable evidence. The advertising codes require that objective claims are capable of substantiation.
A recognised international standard is how a manufacturer demonstrates that its evidence is adequate and verifiable. Testing to a method that everybody recognises, with a report that states the method and its version, is a straightforward way of meeting the requirement. Testing to an unpublished in house method is not impossible, and it puts the manufacturer in the position of having to justify the method as well as the result.
So the standard is not the rule. It is the accepted way of satisfying the rule, which is a different relationship and a more flexible one.
Referencing a standard is not complying with it
A recurring problem in this area, and one that appears in trade documentation more often than on consumer packs, is a reference to a standard that does more work than it should.
Stating that a product was tested in accordance with a named standard is a claim, and like any claim it needs to be accurate. Testing to some parts of a method, testing a related formulation, or testing to a superseded version and citing the current one are all forms of the same failure. The evidence has to support the claim as made, and a claim naming a standard is a claim that the standard was followed.
There is also a subtler version. A product tested to a standard is not certified, approved or accredited by ISO, which does not test or certify anything. Wording that implies otherwise is describing an authority that does not exist.
Accreditation, which is about the laboratory
Separate from the standard is the question of who did the testing. Laboratory accreditation is an independent assessment that a laboratory is competent to perform particular methods, carried out against a general standard for testing laboratories by a national accreditation body. In the United Kingdom that body is the United Kingdom Accreditation Service.
Accreditation relates to the laboratory and to a scope of methods, not to the product. A report from an accredited laboratory carries more weight because the competence of the laboratory has been assessed, but the accreditation does not attach to the result and cannot be claimed by the manufacturer as a property of the product.
For anyone commissioning testing, the accreditation scope is the thing to check. A laboratory can be accredited for some methods and not for others, and the scope is published.
Why results still vary between laboratories
Even with a written method, an accredited laboratory and a competent operator, the same product tested twice will not give exactly the same figure. That is a property of measurement rather than a failure of the standard.
Sources of variation include the panel, because people differ; the assessor, because the endpoint is read visually; the application, because spreading is a manual operation; and the source, because simulators are matched to a tolerance rather than being identical. The standard's validity criteria and statistical requirements exist to keep that variation within stated bounds rather than to eliminate it.
The practical consequence is covered in measurement uncertainty and what a tested figure is, and it is the main reason the labelling convention groups results into a small set of permitted figures rather than printing them exactly.
National adoption and what BS EN ISO means
A prefix on a standard reference carries information that is regularly ignored. A standard published internationally by ISO can be adopted as a European standard and as a national one, and the reference then accumulates prefixes accordingly.
A reference beginning BS indicates adoption as a British Standard by the national standards body. A reference including EN indicates adoption as a European standard. The technical content of the method is the same document; what the prefixes record is the route by which it became a standard in a particular place, and which national body publishes it.
For a business commissioning testing, the practical point is small but real: a report citing a national adoption of a method and a report citing the international standard are citing the same method, and neither is more authoritative than the other. What matters is the edition and whether the whole method was followed.
For a reader, the point is that a string of letters in front of a number does not indicate a higher grade of testing. It indicates a publication route.
A note on access
Standards are published documents, and they are sold rather than given away. That is how the standards system is funded, and it has a consequence worth naming: the detailed methods behind the most prominent number on a consumer product are not freely readable by consumers.
This is not a scandal, and it is a reason why publications like this one exist. What we can do is describe the shape of a method accurately from what is publicly documented about it, name the standard so that anyone who needs the detail knows what to obtain, and avoid quoting numerical criteria we are not confident about.
What to take from this
ISO 24444 for the in vivo factor, ISO 24443 for in vitro UVA, separate methods for water resistance. A standard is a method, not a law and not an approval. It acquires force because evidence has to be adequate and verifiable, and a recognised method is how that is shown.