Overview
ISO 18861:2020 - Cosmetics - Sun protection test methods - Percentage of water resistance - specifies a standardized procedure to evaluate the water resistance retention of sunscreen products. The standard defines how to compare the sun protection factor (SPF) measured before water immersion (static SPF) with the SPF measured after a defined water immersion sequence (wet SPF). ISO 18861:2020 is intended for in vivo testing and links to the established SPF and immersion methods to ensure reproducible, auditable results for sunscreen claim substantiation.
Keywords: ISO 18861:2020, water resistance testing, sunscreen SPF, sun protection factor, in vivo SPF, simulated swim test.
Key topics and requirements
- Purpose: Calculate percentage of water resistance (%WR) by comparing wet SPF to static SPF (ratio between SPF after immersion and SPF before immersion).
- Referenced methods: Requires the SPF in vivo method in ISO 24444 and the water immersion procedure in ISO 16217.
- Test population: At least 10 subjects selected per ISO 24444; up to 25 may be enrolled. A minimum of 10 and up to 20 valid subjects should complete the test depending on statistical acceptance criteria.
- Immersion sequence: Uses a simulated swim test device with 20 min immersion / 5–20 min drying cycles. Repeat sequences appropriate to target claims (e.g., 40 min = two cycles; 80 min = four cycles). Final drying of at least 15 min (no towelling) before post-immersion SPF measurement.
- Data and calculations: Individual %WR values are calculated per subject and averaged to yield the mean %WR. Static SPF requires a 95% confidence interval; the test is acceptable if this CI is within ±17% of the mean static SPF. Laboratories must compute standard statistics and lower confidence limits as specified.
- Reference product: Uses the P2 High SPF Reference Standard (ISO 24444 Annex C) to validate water resistance performance; validation frequency at least every 8 weeks or every 10 tests.
Applications
- Substantiating marketing claims such as “water resistant” or “very water resistant” for sunscreens and sun-care cosmetics.
- Product development and formulation optimization to assess how well active UV filters persist after water exposure.
- Quality control and batch verification in manufacturing when water resistance is a critical attribute.
- Regulatory and compliance dossiers where standardized, comparable in vivo evidence of water resistance is required.
Who should use it
- Cosmetic formulators and R&D teams
- Contract testing laboratories and clinical SPF testing facilities
- Regulatory affairs and compliance specialists
- Marketing teams needing robust claim substantiation
Related standards
- ISO 24444:2019 - In vivo determination of the sun protection factor (SPF)
- ISO 16217:2020 - Water immersion procedure for determining water resistance
Use ISO 18861:2020 together with ISO 24444 and ISO 16217 to produce reproducible, internationally recognized evidence for sunscreen water-resistance claims.