Doing the arithmetic properly
Originally published September 28, 2024. Revised with updated figures below.
Aluminum shows up in two very different places most people encounter it: as an adjuvant in some vaccines, and as a contaminant or additive in tattoo ink. Comparing the two sounds simple. It isn’t, and getting it right means being careful about two things in particular: which number is elemental aluminum and which is the compound it’s bound up in, and how much ink actually ends up under the skin in the first place.
Start with the ink. Sozer Karadagli and colleagues, working out of Ege University, dissolved tattoo inks in acid and ran them through mass spectrometry, which measures total elemental aluminum regardless of what chemical form it started in. Their range was 1,191 to 3,425 mg of aluminum per kilogram of ink, across green, black, and red samples from three brands.
The vaccine side needs more care. Infanrix Hexa is labeled as containing 0.82 mg of aluminum per 0.5 mL dose, and that figure is already elemental aluminum, not the weight of the salt compound it’s suspended in. U.S. regulation caps vaccine aluminum content at 0.85 mg per dose, stated in terms of the element, which is the convention behind the labeling. The salt itself, aluminum hydroxide or a related compound, weighs roughly ten times more, around 8 mg, because most of that mass is the rest of the molecule rather than the aluminum atom. Independent lab measurement of six Infanrix Hexa doses found a mean of 0.806 mg, consistent with the label. So 0.82 mg is the right number to work with directly, with no further adjustment needed.
The other figure worth getting right is how much ink actually lands in the skin during tattooing. Engel’s team weighed pig and human skin before and after tattooing and found a range of 0.6 to 9.4 mg of ink per square centimeter, averaging around 2.5. The European Chemicals Agency, working from the same underlying data, used a more conservative regulatory estimate of 14.36 mg per square centimeter when it restricted certain tattoo pigments in 2022.
Put those together and a fairly striking picture appears. Using Engel’s average of 2.5 mg of ink per square centimeter and the measured aluminum range, a square centimeter of tattooed skin carries somewhere between 0.003 and 0.009 mg of elemental aluminum. To match a single 0.82 mg Infanrix Hexa dose, you’d need something between roughly 95 and 275 square centimeters of tattooed area, depending on which ink is involved. That’s the size of a large forearm piece on the low end, not a full sleeve.
Push the ink deposition estimate to ECHA’s more conservative 14.36 mg per square centimeter, and the area needed to match a single dose drops to somewhere between 17 and 48 square centimeters, smaller than a palm.
I want to be honest about what this comparison can and can’t tell you, because a milligram of elemental aluminum is not automatically the same thing wherever it’s found. The chemical form matters, possibly more than the quantity. Vaccine adjuvants are specific, engineered aluminum salts, aluminum hydroxide or aluminum phosphate, prepared as nanoparticles at a particle size deliberately tuned for uptake by immune cells. That design is the entire point of an adjuvant. Whatever aluminum sits inside a tattoo pigment is almost certainly locked into a different structure entirely, whether that’s a trace contaminant from manufacturing, a filler compound, or aluminum bound up as part of a larger pigment molecule such as a phthalocyanine. Nobody has published work characterizing what form the aluminum in tattoo ink actually takes once it’s under the skin, or how available it is to the immune system once macrophages start working on it. Equal milligrams of aluminum are not a guarantee of equal biological consequence, and I don’t want to imply otherwise by putting two numbers next to each other.
The exposure pattern differs too, and in ways that cut in both directions. A vaccination schedule delivers aluminum in discrete, spaced doses, several of them over an infant’s first two years, and the total across a schedule adds up to several milligrams. A tattoo delivers its aluminum once, in a single sitting, and then simply stays. The deposit doesn’t clear. Schreiver’s synchrotron imaging work shows tattoo pigment migrating into the draining lymph nodes years after the original tattoo, carried there by macrophages that engulfed it at the site. Whatever aluminum rode along with that pigment is presumably going wherever the pigment goes.
None of this settles the question of whether either exposure carries meaningful health risk. What it does is put both numbers on the same footing, elemental aluminum measured the same way on both sides, using deposition figures that a regulator and independent lab measurement both stand behind. A modest tattoo, in the range of a large forearm piece or smaller, can plausibly carry as much elemental aluminum as a single infant vaccine dose. What that means biologically, given how differently the two forms of aluminum are likely structured and processed, is a genuinely open question.
References
Sozer Karadagli, S., Cansever, I., Armagan, G., & Sogut, O. (2023). Are Some Metals in Tattoo Inks Harmful to Health? An Analytical Approach. Chemical Research in Toxicology, 36(1), 104–111.
Exley, C. (2020). An aluminium adjuvant in a vaccine is an acute exposure to aluminium. Journal of Trace Elements in Medicine and Biology, 57, 57–59.
Exley, C., & Clarkson, E. (2020). Aluminium in human brain tissue from donors without neurodegenerative disease: A comparison with Alzheimer’s disease, multiple sclerosis and autism. Scientific Reports, 10(1), 7770.
Engel, E., et al. (2008). Modern tattoos cause high concentrations of hazardous pigments in skin. Contact Dermatitis, 58(4), 228–233.
European Chemicals Agency. Tattoo inks and permanent make-up: restriction assessment, using an ink deposition estimate of 14.36 mg/cm² derived from Engel et al.
Schreiver, I., Hesse, B., Seim, C., et al. (2017). Synchrotron-based μ-XRF mapping and μ-FTIR microscopy enable to look into the fate and effects of tattoo pigments in human skin. Scientific Reports, 7(1), 11395.
Laux, P., Tralau, T., Tentschert, J., et al. (2016). A medical-toxicological view of tattooing. The Lancet, 387(10016), 395–402.
