De Ligt and colleagues conducted a follow-up human in vivo study measuring systemic absorption of aluminum from a representative roll-on antiperspirant formulation containing aluminum chlorohydrate at 6.25 percent equivalent aluminum, using an accelerator mass spectrometry-based 26Al radiotracer at a 25-fold higher applied activity than the 2018 original study. This is the more refined of two 26Al microtracer studies for this product type, and its absorption fraction was adopted by the Scientific Committee on Consumer Safety in its 2019 updated opinion.
Key numbers
Systemic absorption fraction (Part A, Table 1, p. 515)
The fraction absorbed was calculated as the fraction of the applied 26Al dose recovered in urine and feces, normalized against an intravenous 26Al dose representing 100 percent bioavailability.
Skin washes at 24 hours recovered 62.0 percent of the applied dose (range 54.1 to 73.6 percent); skin washes at 48 hours recovered 1.6 percent (range 0.8 to 3.0 percent). The t-shirt collected 6.0 percent at 24 hours (range 1.1 to 14.6 percent) and 0.09 percent at 48 hours. Combined non-absorbed subtotal: 69.7 percent (range 58.7 to 76.7 percent).
Urinary recovery: 0.0003 percent of the applied dose (range 0.0001 to 0.0007 percent). Fecal recovery: 0.0017 percent (range 0.0008 to 0.0057 percent). Tape strips at 168 hours: 0.0097 percent (range 0.0019 to 0.0417 percent). Tape strips at 840 hours: 0.0090 percent (range 0.00004 to 0.0525 percent). Skin biopsy at 840 hours: 0.00003 to 0.00004 percent.
The combined potentially absorbed subtotal (urine plus feces plus skin biopsy plus late tape strips) was 0.021 percent (range 0.004 to 0.095 percent, Table 1, p. 515).
The calculated dermal bioavailability fraction (F_abs), using the LLOQ substitution method: conservative mean estimate 0.00052 percent (range 0.00026 to 0.00108 percent, p. 516). This is approximately 18-fold lower than the estimate from the 2018 study (0.0094 percent) and results from the 25-fold higher applied radioactivity enabling detection of systemically absorbed aluminum that was previously below instrumental detection limits.
Local fate on skin (Part B, p. 515)
At 20 minutes after application, most radioactivity was in the outermost tape strip. By 24 hours, less than 2 percent of the normalized applied dose remained recoverable. Skin biopsy at 24 hours: 0.08 percent of dose (n equals 4; 2 were below the lower limit of quantification). The authors state that the vast majority of aluminum in the formulation remains associated with the external layers of the skin without penetrating through the skin (p. 518).
Blood analysis (pp. 515 to 516)
186 blood samples were analyzed by AMS. Only 12 samples from 2 subjects exceeded the lower limit of quantification of 0.118 fg per mL, all well below the intravenous reference concentrations. This confirms minimal systemic uptake.
Methods (brief)
Design: randomized crossover, 12 healthy Caucasian women, Netherlands. Formulation: representative roll-on containing aluminum chlorohydrate, 6.25 percent equivalent aluminum, labeled with 26Al at approximately 2,500 Bq per axilla (25-fold above the 2018 study). Part A: repeated real-life use with and without prior shaving; urine, feces, skin washes, and t-shirts collected up to 672 hours. Part B: single application, tape stripping at 20 minutes, 1 hour, 6 hours, and 24 hours, then skin biopsy. An intravenous 26Al dose (0.1 Bq) given after Part B to normalize urinary and fecal excretion to 100 percent bioavailability. 26Al detected by accelerator mass spectrometry. LLOQ for blood: 0.118 fg per mL. CRM recoveries not reported per se; the design uses the IV dose as the internal bioavailability reference. No certified reference material for blood or urine 26Al is commercially available for this matrix.
Evidence fitness
This source is the strongest available evidence for the systemic bioavailability of aluminum from a topically applied antiperspirant in humans. The mean F_abs of 0.00052 percent is the value adopted by the SCCS in its 2019 updated opinion (SCCS/1613/19). It supports the conclusion that dermal aluminum absorption from antiperspirant is extremely small relative to the applied dose. This source does not measure aluminum concentrations in the product as marketed; it uses an experimental radiolabeled formulation at 6.25 percent equivalent aluminum. It cannot establish whether absorption differs in damaged, shaved, or irritated skin at scale, in non-Caucasian populations, or in elderly or pediatric users, as the authors note.
Limitations
Population limited to 12 healthy Caucasian women in the Netherlands, as stated by the authors; any difference in absorption due to race, age, skin condition, or climate is not captured. The formulation is a representative roll-on; absorption from stick, spray, or solid alum formats may differ. Part A measured repeated use under semi-occlusive real-life conditions; Part B was a single application with sequential tape stripping and biopsy, and results from the two parts are not directly comparable. Fecal aluminum may include secondary contamination from non-systemic routes rather than true gastrointestinal transit. The 25-fold higher applied radioactivity versus the 2018 study still resulted in most blood and urine samples below the LLOQ.
Related evidence
- Antiperspirant (Al-salt, dirty sibling of r9 deodorant)
- Aluminum
- Assessment of Dermal Absorption of Aluminum from a Representative Antiperspirant Formulation Using a 26Al Microtracer Approach
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