That didn’t take long. A bit over a month ago I discussed, for the 2nd time, the relationship between tattoos and your immune system. The claim being made by pretty much just the one guy is that tattoos might improve immune function and reduce risk of illness. It was, and continues to be, a ridiculous notion with extremely low plausibility based on a poor understanding of how the immune system works and a few comically bad studies.
So why are we here again talking about tattoos and the immune system? Thanks for asking. Since my previous post on the subject, a study out of Sweden was published in eClinicalMedicine, an open access clinical journal from The Lancet, that has been discussed widely on various forms of media, both social and…anti-social? The sexy headline, often not followed by the appropriate full context, is that tattoos increase your risk of malignant lymphoma by 21%. Sounds scary. But do they? Do they really?
The authors of the study paint a compelling picture, or at least they discuss why there is enough plausibility for there to reasonably be some concern and to justify their investigation, which is the first of its kind:
Tattoo inks are co*cktails of organic and inorganic colour pigments, together with precursors and byproducts from the pigment synthesis, and additives. Coloured inks may contain primary aromatic amines (PAA), black inks often contain polycyclic aromatic hydrocarbons (PAH), and metals (i.e., arsenic, chromium, cobalt, lead, and nickel) are found in ink of all colours. A significant and concerning number of chemicals in tattoo ink are classified as carcinogenic by the International Agency for Research on Cancer.
https://www.thelancet.com/journals/eclinm/article/PIIS2589-5370(24)00228-1/fulltext
The IARC, an intergovernmental agency that forms part of the World Health Organization, doesn’t have a perfect track record when it comes to these things. For what it’s worth, they have weighed in on the issue of tattoos and long term risks, particularly cancer. They have this to say in regards to tattoos and their potential carcinogeneity:
Neither tattooing as a lifestyle behaviour nor specific tattoo inks have yet been evaluated by theIARC Monographsprogramme. However, several substances classified as carcinogenic (Group 1), probably carcinogenic (Group 2A), or possibly carcinogenic (Group 2B) to humans have been identified in tattoo inks; these classifications were based on studies that did not specifically consider the intradermal exposure scenario as seen with tattooing. Although some of these classified substances, particularly metals, were once used as colour pigments, the carcinogens present in tattoo inks today are generally contaminants due to impure raw materials or contamination during the production process.
https://tattoo.iarc.who.int/faq/
As the authors in the Swedish study point out, there is some evidence that these potential carcinogens do not simply stay in the skin where they impress friends, family members, and future employers. Research has found that pigments and metals from the tattooing process end up in regional lymph nodes, and lymph nodes are made up of often rapidly proliferating tissue that is believed to be sensitive to carcinogenic chemicals and the development of DNA mutations. They mention that exposures to “solvents, flame retardants, pesticides, and hair dyes” are increasingly believed to be involved in the development of malignant lymphoma, so why not chemicals found in tattoo inks?
As discussed in the many news articles written about this paper, the authors did indeed find that tattoo exposure was associated with a 21% increase in the diagnosis of malignant lymphomas. But we need to look closer at the study methodology before getting too worked up. First off, this was a case-control study that set out to compare all the people in Sweden aged 20-60 years who were diagnosed with malignant lymphoma between 2007 and 2017 to a bunch of random age- and sex-matched controls who weren’t. They then retrospectively determined who had tattoos and who didn’t, along with a number of tattoo exposure variables, and used multivariable logistic regression to determine the incidence of rate ratio of lymphoma in people with tattoos. Huzzah!
Of the nearly 12,000 potential study subjects, a little over half of those with lymphoma (1,398) and just under half of the controls (4,193) responded. 21% of subjects with lymphoma had tattoo exposure compared to 18% of those with none, resulting in an incidence rate ratio of 1.21, which is where the 21% relative risk increase comes from. Yes, we are talking about relative risk. And malignant lymphoma is not a particularly common cancer, with a lifetime cumulative non-Hodgkin lymphoma (about 85-90% of lymphoma cases) risk of 0.72% in men and 0.35% in women worldwide. Lymphoma rates vary from country to country, with some having higher incidence than others, but compare that to colon cancer’s lifetime risk of 4.4% in men and 4.1% in women.
There are many known and suspected risk factors for lymphoma. Family history, medications, autoimmune disorders, and infections with certain viruses, like Epstein-Barr virus, are just a few examples. It is exceedingly difficult to tease out many of these potential variables in a retrospective study, meaning that it is entirely possible that the study finding is just statistical noise. It is perhaps even likely that it is. And a confidence interval (0.99-1.48) that includes 1 does not inspire…trust?
There were a few other tidbits from the study that are worth noting, though they should also be taken with a grain of salt. In fact, they probably make the overall finding more suspicious. Risk of lymphoma was higher when tattoo exposure occurred less than two years before a cancer diagnosis, which strikes me as odd, but also when a first tattoo was at least 11 years old, which makes more sense. Cumulative and higher exposure to mutagens should increase risk, but there was not any apparent increase in risk from having a larger total tattooed body surface area. Finally, some lymphomas appeared to be more common than others in subjects with tattoos compared to those without.
I can’t help but worry that this is all just an example of the Texas Sharpshooter fallacy, the most famous example of which (to me) being the uproar over power lines causing leukemia in Swedish children. To be fair, the authors of the study are aware of the many limitations and attempt to address them. Many of these assumptions are a stretch, in my humble opinion. Ultimately, I think we can all agree that it is impossible to put too much stock in an observational and retrospective study like this one.
More and better research is needed to come close to answering the question of whether or not tattoos play a role in the development of lymphoma or any health problems. According to an interview of the lead researcher, the team is also looking into any link between tattoos and skin cancer as well as autoimmune disease. They really seem to hate these cans…I mean tattoos. The IARC is working on a prospective cohort study that is better designed to shed some light, but we won’t have results for many years.
What should people do in the meantime? Eat a healthy diet. Exercise regularly. And put down those cigarettes and that yard of ale. I honestly wouldn’t lose much sleep over the risk of cancer from a tattoo, but different people may have less comfort with uncertainty. If you do get some ink, just make sure that they use a clean needle.
Clay Jones Clay Jones, M.D. is a pediatrician and has been a regular contributor to the Science-Based Medicine blog since 2012. He primarily cares for healthy newborns and hospitalized children, and devotes his full time to educating pediatric residents and medical students. Dr. Jones first became aware of and interested in pseudoscience in medicine while completing his pediatric residency at Vanderbilt Children’s Hospital twenty years ago and has since focused his efforts on teaching the application of critical thinking and scientific skepticism. Dr. Jones has no conflicts of interest to disclose and no ties to the pharmaceutical industry. He can be found on Twitter as @SBMPediatrics. View all posts
