ISO 24444 · In vivo
ISO 24444: in vivo SPF
The reference method for the SPF on the label. The SPF is the ratio between the UV dose that produces a minimal erythema on protected and on unprotected skin of volunteers, under a standardised and ethically controlled procedure.
Advantages
Compared with in vitro and hybrid methods (ISO 23675, ISO 23698)
- Measures the real biological end point: erythema on human skin
- Applicable to all galenic forms
- Compatible with water-resistance claims (ISO 16217 / ISO 18861)
- Long history of use and a large body of comparative data
Limitations
- Invasive: induces erythema in volunteers, with ethical and safety constraints
- Slow: two sessions per subject, 10–20 valid subjects, several weeks per product
- Higher cost per product (panel recruitment, medical supervision)
- Higher inter-laboratory variability than in vitro methods
- Does not report UVA-PF or critical wavelength
Principle
2.00 mg/cm² of product is applied on test areas of the back of healthy volunteers. Each area is exposed to a geometric series of increasing UV doses from a filtered xenon-arc solar simulator. The minimal erythema dose (MED) is read 16–24 h later on unprotected skin (MEDu) and on protected skin (MEDp).
The individual SPF is MEDp/MEDu; the product SPF is the arithmetic mean over 10–20 valid subjects. A reference sunscreen is tested in parallel, and the result is accepted only if the 95 % confidence interval is within ± 17 % of the mean.
How it is performed
Subject selection
Healthy volunteers aged 18–70, skin colour measured as ITA° (≥ 28° on the back, panel mean 41°–55°).
Unprotected MED
At least 5 sub-sites exposed with a geometric dose progression (typically 1.25×).
Application and exposure
2.00 ± 0.05 mg/cm² applied by weight, 15–30 min drying, exposure centred on MEDu × expected SPF.
Reading and statistics
MED read at 20 ± 4 h under uniform light; SPF calculated and validated statistically.
Related reading
How many volunteers does an HDRS screening need?EU context
ISO 24444 is the reference method for the SPF on the label in the EU and in most markets.
It is also the anchor of the in vitro UVA method ISO 24443, which scales its curve to the in vivo SPF.
At ISD Research
Our in vivo room is equipped with a Solar Light Multiport 601 solar simulator, a calibrated UV radiometer and a skin colorimeter for ITA° measurement. Our volunteer panel is managed on an online platform.
Frequently asked questions
How many volunteers does an ISO 24444 study need?
Between 10 and 20 valid subjects (maximum 25 enrolled). We also offer a screening with 3 subjects to guide formulation before the full study.
How long does it take?
Each subject attends two sessions. Results are usually delivered in 1–2 weeks for a screening and 3–4 weeks for a full study with ISO report, subject to availability.
Can ISO 24444 support a water-resistance claim?
Yes. Combined with the water immersion procedure (ISO 16217) and the calculation of ISO 18861, it supports “water resistant” and “very water resistant” claims.
References
- ISO 24444:2019 + Amd 1:2022 (UNE-EN ISO 24444:2020/A1:2022). Cosmetics, Sun protection test methods, In vivo determination of the SPF.
Indicative summary of the standard: it does not reproduce or replace it, nor the ISD Research SOP or the study protocol.
Other methods
Is this the right method for your product?
Tell us your product, claim and market. We will propose the method and option that fit.