Testing Moisturizing Claims for Skin


Editor’s note: Cosmetics & Toiletries revived its former “Testing Tactics” column last month with new columnist Trefor Evans, PhD, who focused on hair. Here, rotating with Evans, we feature Chris McLeod, who will provide expertise on the skin care side of testing. Readers are invited to engage with this article on the Cosmetics & Toiletries Magazine LinkedIn Group, or to submit comments to CTEdit@allured.com.

Consumer product testing, along with procedures for implementing claims substantiation protocols, is increasingly becoming one of the most talked about topics in the product development process. Whether a company produces cosmetic products for small, independent boutique brands or for large multinational corporations, the race to enhance (or at the very least, match) a product’s on-package claims to its competitors’ is of paramount importance to gain a crucial foothold in the relevant market and target demographic. One of the main claim substantiation areas in modern cosmetics—although one of the least verbosely exhilarating for marketing departments—is moisturization in skin. As with the majority of cosmetic products and claims in the 21st century, marketing, research, development and formulation departments within companies aim to differentiate their product from competitors in one way or another, to create a successful brand and generate profit.

Although this author cannot deny that moisturization capabilities can provide relief for those who suffer from flaky, itching or irritated and dry skin, it would be careless not to acknowledge the marketing-based beguilement within this moisturization sector. Even if a product is legitimately substantiated for providing 96-hr moisturization, it always begs the questions: Why would such a “mindful-of-their-skin” consumer need unremitting moisturization without re-application or cleansing for four continuous days? And does everyone actually need to artificially moisturize their skin?

While this author has written many a journal on the Sensationalization of Sensation, this column will not be one of them. Henceforth, this article and series will hopefully provide necessary information to assist readers in understanding the testing processes that occur to legitimize on-pack claims of Product X’s capabilities. In an ideal world, every product development team member should understand this process in order to launch the most efficient and cost-effective product. Lest it be forgotten, the claim substantiation procedure, whether for safety or efficacy, is essential to market on-pack claims within the cosmetics industry.

Moisturization and Skin

At this stage, it would be good practice to review the basic science behind skin’s moisturization properties. The main purposes of the body’s largest organ, i.e., the skin, are to stop transepidermal water loss (TEWL) and protect and waterproof the body’s inner functions. Oily sebum secreted from the sebaceous glands within the dermis seeps out of the skin’s pores, providing an extra automatic defense mechanism to protect the skin and body from unwanted vitamin, nutrient and blood loss, among other things, and to stop alien substances, liquids or gases from penetrating into foreign inner-corporal territory.

As all good hand washers know, an oily surface provides a rather mischievous layer to remove or penetrate. Nevertheless, sebum can clog pores as well, and so surfactant-type products are used to remove these oils in addition to the horny cell layer, i.e., upper stratum corneum (SC), to clean the skin. This treatment is particularly useful for highly active sebaceous glands, too. Once this layer is removed, the moisture in the epidermis needs to be replenished and/or the natural barrier restored to allow the skin to function as nature intended. Moisturizers ameliorate the skin’s suppleness by facilitating its fluidity, which may help to slow the aging process and, for those with naturally dry skin, help to increase the skin’s functionality by providing a resolution to unwanted irritancies.

From this background it becomes clear that moisturizers work in two ways: either by attracting moisture through water vapor from the atmosphere into the skin, or by creating a barrier on top of the skin to restrict water loss. These respective functionalities are the difference between humectants and emollients, and clearly define which moisturization test is required. In relation, when outsourcing to a product testing house, it is imperative that cosmetic manufacturers have their intended claims in mind, and not vice versa. Some houses may suggest including many different claims that also require many different (and more costly) tests. These may not necessarily serve the honed product the manufacturer plans to launch. Moreover, it is recommended always to be prepared with a few choice claims, in case one is not possible to substantiate or will involve very costly procedures. Following are the most efficient, cost-effective and most robust processes to substantiate all-important moisturization claims.

Humectant Method

Humectancy is typically found in formulations with a high glycerol (glycerine, glycerin) content, which is the standard humectant used in cosmetic formulations barring “natural” ingredient concepts such as avocado, shea butter and argan oil. Claims for these types of products that this type of test can substantiate are: “Moisturizes for up to X hours”; “Intensively hydrates the skin”; “Increases skin’s water content by up to X% for up to X hours”; or any claim that somehow states an increase in the skin’s moisture levels. To test that the product under inspection can legitimately behold these claims, an in vivo and controlled testing procedure is required.

Subjects: This test uses 15 human subjects who enter the lab on day 0 of the assessment. The number of subjects used for certain protocols is chosen based on inter-regulatory and regulatory guidance as to what constitutes a legitimate test; statistically, however, the industry generally knows that for a moisturization study, a minimum of 15 subjects is required to produce the fairest statistical breakout upon post-study data analysis. Since obviously one cannot test the entire possible population of targeted consumers, tests aim to apply the most efficient sample size to reflect results that compare appropriately to the population as a whole with 95% confidence.

Prior to the 15 subjects’ entry on day 0, each would have followed a three-day dry down period during which they use only a simple soap and refrain from applying other products on the test site. In this case, the lower leg is the test site, due to the natural dryness in this area. This site is chosen to bring parity across the sites in order to promote a fair test with the most robust and accurate results possible. Note that some product testing houses have a moisturization test season because of variations in temperature between winter and summer.

Assessment types: Every protocol will have an assessment type of some sort, be it an instrumental or expert visual assessment; quantitative or qualitative; or subjective or objective. For a humectant moisturizing procedure, an instrumental, quantitative, objective assessment type is used—namely capacitance instruments such as a corneometer or dermal phase meter, depending on the manufacturer’s preference. Other instruments can measure electrical conductance through the skin.

The corneometer and dermal phase meter have a wand attached to the main instrument with a probe that is pressed onto the intended skin site. Using a capacitance method, the wand takes a reading of the skin’s dielectric constant. This is compared with the dielectric constant of water, approx 81; thus the greater the moisture level in skin, the closer the dielectric constant will be to 81. For most substances, it is less than 7. One would hope to find an increase in this value after a moisturizer has been applied. This reading is translated into an arbitrary instrumental value that designates the moisture level of the site epidermis.

Procedure: Each subject would have naïve sites (untreated) on their lower leg instrumentally assessed by clinical technicians to get a baseline, pre-treatment moisture level reading; application of the instrument and the product(s) on the human test site are specified by stringent inter-regulatory protocols that cannot be specified here, for confidentiality purposes. The product under inspection is then applied onto the participant by a technician at numerous sites on the lower legs. The positive control is pure glycerol and the negative control is an untreated site.

After product application, the technician takes various measurements of each site’s moisture level at specific time points over a time-period dependent upon the claim being substantiated. For example, a 12-hour moisturization claim will mean the final time point is taken 12 hours after product application; more typical claims include 24 hr and 48 hr moisturization.


Figure 1 shows example results from a 12-hour study. All baseline measurements start around the same point because the small, dried-down sites on the lower leg are untreated at this stage and thus should yield similar moisturization levels. An average dry skin reading on the corneometer is 20–30. The top line here is the positive control, glycerol, and the bottom line is the negative control, untreated. Technicians look for a non-variable graph as variable lines would denote a change in environment, temperature or humidity, which would make the test null and void.

The positive and negative control performing as they are intended also demonstrates that the skin is reacting as expected in a controlled environment. One hour after product application, technicians anticipate the glycerol to spike, especially as the positive control since the humectancy of the product rapidly draws moisture from the atmosphere into the epidermis. The skin’s moisture level then plateaus upon near saturation. The drier the skin, the greater the spike due to the greater need to quench the skin’s moisture needs. The highest spike for moisturization tends to be 70–90.

The lines in between are the six testing sites. To find out whether the products do indeed moisturize the skin and to what percentage over a certain period of time, researchers would then look to statistically compare the results of pretreatment vs. post-treatment values against the controls. A data comparison p-value below 0.05 over the different time point averages is needed in order to claim statistical significance between pretreatment and post-treatment values. If this occurs, the claims can be substantiated and the percentages of moisturization levels are calculated appropriately from the levels obtained.

Emollient Method

The procedure of emollient analysis is conducted in the same way as a humectancy protocol but with the use of different instruments to assess skin’s TEWL rather than transepidermal water gain (TEWG). This test may be conducted using a closed chamber method, where a moisture probe is pressed onto the skin and calculates an in-chamber humidity reading. Another approach is the open chamber method using either an evaporimeter, which measures temperature and relative humidity, or TEWL meter, which measures the transepidermal vapor pressure gradient by pressing two moisture and temperature sensors inside a cylindrical probe on the skin. The TEWL rate is calculated as grams per square meter per hour (g/m2hr). A successful procedure will allow for claims such as: “Improves skin’s natural barrier”; “Reduces transepidermal water loss by X% over X hours”; “Retains skin moisture X% longer than without product X.”

Figure 2 shows readings from a TEWL meter emolliency study. The results show an opposite trend to the humectancy study because instead of looking at moisture increase in the epidermis, the rate of moisture loss is being calculated. These starting values are obtained by a dry down period similar to the humectancy study but with an extra procedure involving tape- stripping of skin from the test sites to expose deeper SC levels. Intact skin reads around 8–10 on the TEWL meter scale. Tape-stripping removes the protective layer and helps to increase the water loss process, facilitating this study. The positive control in this assay is petroleum jelly, which reads as the bottom line on the graph, whereas the negative control (untreated) is the top relatively unvaried, plateaued line. Usually within an hour, TEWL rates decrease from 13 to 6 as the product will, hopefully, provide a dam-like effect against excessive water loss. Once more, the necessary statistical significance analysis is conducted post-data collection to assess the legitimacy of the intended claims.


Hopefully, this information has enlightened readers as to the process behind the testing of moisturizers; however, this column must end on an important note. The duration of these testing procedures is around six weeks from study authorization to the final, signed report release, and will normally cost a few thousand dollars. This is vital to be aware of at the start of the new product development process in order to fully plan and budget each project. Contacting a product testing house at initial conception will ensure the most efficient product reaches the market—and this is even more important with other, lengthier and more expensive testing procedures. Understanding these and other testing procedures will create a more knowledgeable, transparent and trusting industry in a time where this is becoming the main prerogative for all regulatory bodies. Thus, this author looks forward to receiving readers’ inquiries, in due course.

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