’24h hydration’, ’48h moisture’, ‘deep and lasting hydration’ — these claims are ubiquitous on cosmetic packaging. But what do they actually mean from a clinical and regulatory standpoint? And how are they rigorously tested?
This article breaks down the science behind long-lasting moisturization claims, reviews the study designs required to substantiate them, and explains how formulation choices directly impact testability.
1. Defining ‘Long-Lasting’ and ‘Deep’ Hydration
What does ‘long-lasting’ mean?
From a testing perspective, a ‘long-lasting’ hydration claim requires demonstrating a statistically significant increase in SC water content at a defined time point after a single application — typically T+24h, T+48h or beyond — measured against a non-treated control area or baseline.
There is no universal regulatory threshold defining what ‘long-lasting’ means in terms of hours. However, common industry practice and regulatory precedent suggest that a 24h claim requires sustained significance at T+24h after a single application, with no intervening product use.
What does ‘deep’ mean?
The word ‘deep’ is scientifically problematic when used alone. Most in vivo hydration methods (corneometry, electrical impedance) measure the upper SC layers. Near-infrared (NIR) spectroscopy and confocal Raman can probe deeper into the dermis, making them the reference methods for ‘deep hydration’ or ‘dermal hydration’ claims.
Using the word ‘deep’ without NIR or Raman data could be considered misleading in the EU context.
2. Key Ingredients and Their Mechanism of Action
| Ingredient | Mechanism | Effect duration | Testing consideration |
| Glycerin | Humectant — attracts water from dermis and environment | Short to medium | Strong corneometry signal at T+1h to T+8h |
| Hyaluronic acid (HMW) | Film-forming — reduces surface TEWL | Medium | Corneometry + TEWL at T+1h to T+24h |
| Hyaluronic acid (LMW) | Penetrates SC — deeper hydration | Medium to long | NIR or Raman for ‘deep’ claim; corneometry for SC |
| Urea (5–10%) | Humectant + keratolytic | Medium | Corneometry at T+4h to T+24h; avoid irritation testing |
| Ceramides + FA + Chol. | Barrier repair — reduces TEWL | Long (structural) | TEWL measured at T+7 days, T+14 days, T+28 days |
| Polyglutamic acid | Stronger humectant than HA | Long | Corneometry + consumer perception at T+24h, T+48h |
| Panthenol (Vit B5) | Pro-humectant + barrier support | Medium | Combined corneometry + TEWL protocol |
3. Study Design for Long-Lasting Hydration Claims
Single application vs. repeated application
A single-application protocol (one product application, measurements up to T+48h) is the most demanding design for a long-lasting claim. It isolates the intrinsic persistence of the hydrating effect without the cumulative benefit of repeated use.
A repeated-application protocol (e.g., twice daily for 28 days) is more appropriate for ‘sustained hydration’ or ‘visible improvement over time’ claims, but cannot support a pure ’24h’ single-dose claim.
Recommended time points for 24h and 48h claims
| Claim | Recommended time points | Control condition |
| 24h hydration | T0 (baseline), T+1h, T+4h, T+8h, T+24h | Untreated area or vehicle |
| 48h hydration | T0, T+1h, T+8h, T+24h, T+48h | Untreated area or vehicle |
| Long-lasting deep moisture | T0, T+4h, T+24h, T+48h, T+72h | Untreated area; NIR recommended |
| Sustained 4-week hydration | T0, T+7 days, T+14 days, T+28 days | Placebo or untreated area |
Panel selection
For long-lasting claims, panel selection should reflect the target consumer. Dry to very dry skin subjects typically show a larger delta vs. baseline, strengthening the statistical significance of the effect. However, if the product is marketed for all skin types, a mixed panel is required.
Recommended panel size: minimum 20 subjects for a standard claim; 30–50 for premium dossiers or comparative claims (‘more moisturizing than X’).
4. Regulatory Framing: What You Can and Cannot Claim
Under EU Regulation 655/2013, claims must be truthful and based on adequate evidence. Specific time claims (’24h’) are permissible provided they are supported by in vivo data demonstrating significant hydration at that time point.
Common pitfalls to avoid:
- Claiming ’24h’ when significance disappears at T+12h
- Using ‘deep hydration’ without dermal measurement data
- Extrapolating single-application results to long-term benefits
- Claiming superiority vs. competitor without head-to-head data
Conclusion
Long-lasting and deep hydration claims are achievable — but they require a carefully designed study protocol aligned with both the product’s mechanism of action and the claim’s regulatory definition. The choice of time points, measurement method, and panel type are not technical details: they are the backbone of a defensible claim.
Skinobs references over 500 CROs worldwide. Use www.skinobs.com to identify labs offering validated long-lasting hydration protocols, including NIR and Raman capacities for premium deep-moisture claims.
