Determination of the Moisturizing Performance of Facial Toners: Testing Standards, Methods, and Labo

The determination of the moisturizing performance of facial toners is a core laboratory evaluation used to substantiate hydration claims for water-based, leave-on skin care products. By combining capacitance-based measurement of stratum corneum hydration with in vitro hygroscopicity and moisture retention studies, accredited third-party laboratories can quantify both the immediate and long-lasting moisturizing effects of toner formulations. This article outlines the applicable standards, sample types, test items, methodologies, and practical applications of moisturizing performance testing for facial toners.

Technical Overview

Facial toners are water-based, leave-on cosmetic products applied after cleansing to hydrate the skin surface, rebalance the complexion, and prepare the skin for subsequent treatment steps. Because the overwhelming majority of toner products carry moisturizing claims, the objective determination of their moisturizing performance has become an essential component of formulation development, quality control, and regulatory compliance.

Moisturizing performance determination measures the capacity of a toner to increase the water content of the stratum corneum and to maintain that hydration level over time. The evaluation is primarily performed through instrumental in vivo testing on human volunteers, supported by in vitro physicochemical methods that characterize the hygroscopic and water-retention behavior of the formulation and its functional ingredients.

In China, the principal technical reference is QB/T 4256-2011, the light industry guideline for the evaluation of the moisturizing efficacy of cosmetics, which specifies the use of capacitance-based skin hydration instrumentation. In addition, under the Chinese cosmetic efficacy claim evaluation framework introduced in 2021, moisturizing claims on product labels must be supported by scientific evidence, and instrumental human testing conducted in accredited third-party laboratories constitutes the most widely accepted form of substantiation.

Testing Samples

Finished Toner Products

Hydrating toners, softening lotions, astringent lotions, essence waters, and other water-based leave-on products containing humectants such as glycerin, hyaluronic acid, butylene glycol, betaine, and panthenol.

Formulation Prototypes and Comparative Bases

Laboratory pilot batches used during formulation screening, placebo or vehicle bases without the moisturizing active, and benchmark formulas used in comparative or competitor benchmarking studies.

Raw Materials and Functional Ingredients

Hyaluronic acid of varying molecular weights, polyols, amino acid derivatives, saccharide-based moisturizers, and other active ingredients, which are typically characterized by in vitro hygroscopicity and moisture retention methods before incorporation into a toner formula.

Sample submission generally requires a sufficient quantity (commonly no less than 50 mL per test), together with batch information, production date, full ingredient composition, and confirmation that the sample is microbiologically and toxicologically safe for application on human skin.

Testing Items

The core testing items for moisturizing performance determination of toners include:

• Stratum corneum hydration value at baseline and at each post-application time point
• Instant hydration increase rate measured shortly after application
• Sustained hydration effect and effective duration at 2, 4, 6, and 8 hours after application
• Moisture retention capacity of the skin over the measurement period
• Transepidermal water loss (TEWL) as a supporting indicator of skin barrier condition
• In vitro hygroscopicity, that is, moisture absorption capacity at defined relative humidity levels
• In vitro moisture retention capacity under dry environmental conditions
• Statistical comparison of treated sites versus untreated control sites and versus baseline values

Testing Methods

In Vivo Capacitance-Based Hydration Measurement

The most widely adopted in vivo method is based on the capacitance principle. Because the dielectric constant of the stratum corneum changes proportionally with its water content, a capacitance-type skin hydration probe pressed gently against the skin surface registers readings in arbitrary units that correlate directly with the degree of skin hydration. Measurements taken before and after product application allow the hydration change induced by the toner to be calculated objectively.

Test Subjects and Environmental Conditions

• Subjects are healthy adult volunteers, typically 18 to 60 years of age, with normal skin on the test area and no active dermatological disease, allergy, or recent use of medications affecting skin condition.
• Testing is usually conducted with no fewer than 20 to 30 valid subjects to ensure statistical reliability, consistent with the requirements of QB/T 4256-2011.
• The test room is maintained at approximately 20 to 22 °C and 40 to 60 percent relative humidity, and subjects acclimate in the room for at least 15 to 30 minutes before baseline measurement.
• Test sites are marked on the flexor side of the forearms, with sites randomly assigned to the test product, a blank or untreated control, and a reference product where applicable.
• The toner is applied at a defined dose, commonly around 2 mg/cm² or an equivalent liquid volume, and allowed to dry naturally.

Measurement Schedule and Data Analysis

Hydration readings are recorded at baseline and at standardized intervals after application, typically 15 to 30 minutes, 1 hour, 2 hours, 4 hours, 6 hours, and 8 hours. The hydration change rate at each time point is calculated as the difference between the post-application value and the baseline value divided by the baseline value, expressed as a percentage. Results are analyzed using paired statistical tests such as the paired t-test or non-parametric equivalents, with a significance level of p < 0.05. A toner is judged to have demonstrated moisturizing performance when the hydration increase at the defined time points is statistically significant relative to both baseline and the untreated control site.

Transepidermal Water Loss Measurement

In complementary protocols, TEWL is measured using open-chamber or closed-chamber evaporimetry probes that quantify the diffusion of water vapor through the skin barrier. A reduction or stabilization of TEWL after toner application provides supporting evidence that the product helps preserve skin barrier function and reduce passive moisture loss.

In Vitro Hygroscopicity and Moisture Retention Tests

In vitro gravimetric methods are used to characterize the humectant behavior of the toner or its ingredients. In the moisture absorption test, weighed samples are placed in chambers held at defined relative humidity levels, commonly approximately 43 percent and 81 percent using saturated salt solutions, and mass gain is recorded at set intervals. In the moisture retention test, samples pre-loaded with a defined amount of water are placed in a low-humidity chamber containing a drying agent, and mass loss is monitored over time. The resulting absorption and retention curves allow the water-binding and water-holding capacity of the formulation to be ranked and compared.

Quality Control and Validity Criteria

• All skin bioinstrumentation is calibrated before each measurement session according to the manufacturer's specifications.
• Untreated control sites and repeated measurements per site are used to reduce variability.
• Subjects follow standardized restrictions, avoiding other skincare products, caffeine, and vigorous exercise before and during the test day.
• Deviations from protocol, adverse reactions, and dropouts are documented and assessed for their impact on validity.

Applications

• Efficacy claim substantiation: generating the technical evidence package required to support moisturizing claims under Chinese cosmetic efficacy claim regulations and equivalent frameworks in other markets.
• Formulation optimization: comparing hydration performance across humectant types, concentrations, and molecular weights to guide product development decisions.
• Marketing and communication support: providing quantitative data such as percentage hydration increase and duration of effect for use in scientific communication and advertising substantiation.
• Benchmarking: positioning a new toner against reference products or competitor formulas under identical test conditions.
• Quality control: verifying batch-to-batch consistency of the moisturizing performance of commercial production batches.
• Ingredient selection: screening raw materials such as hyaluronic acid, glycerin, and betaine through in vitro absorption and retention profiles before inclusion in finished formulas.

FAQ

Which standard governs the moisturizing performance testing of toners?

The principal reference is QB/T 4256-2011, the guideline for the evaluation of moisturizing efficacy of cosmetics, which specifies capacitance-based instrumental measurement. Testing is also performed in alignment with Chinese cosmetic efficacy claim evaluation requirements introduced in 2021 and with internationally recognized guidance on skin bioengineering measurements.

How many volunteers are required for an in vivo moisturizing test?

A typical protocol enrolls no fewer than 20 to 30 valid subjects with healthy skin. Larger panels improve statistical power and are recommended when subtle differences between formulations are expected.

What time points are measured after toner application?

Common schedules include baseline plus 15 to 30 minutes, 1 hour, 2 hours, 4 hours, 6 hours, and 8 hours after application. Shorter intervals capture the instant hydration boost, while later intervals demonstrate the sustained moisturizing effect.

What is the difference between in vivo and in vitro moisturizing tests?

In vivo tests measure actual hydration changes on human skin under controlled conditions and directly substantiate product claims, whereas in vitro gravimetric tests measure the moisture absorption and retention behavior of the formula or ingredient itself and are mainly used for screening and characterization.

Can a moisturizing claim be supported without human testing?

Under the Chinese efficacy claim framework, moisturizing claims may in certain cases be supported by literature or research data; however, instrumental human efficacy testing provides the strongest and most widely accepted evidence and is the industry-standard approach for toner products.

How long does a complete evaluation take?

The in vivo measurement itself is typically completed within a single test day of up to 8 hours, or across several days for extended-duration protocols. Including panel recruitment, screening, statistical analysis, and reporting, a full study is generally completed within two to four weeks.