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Tattoos may be a risk factor for malignant lymphoma, according to a Swedish population-based case-control study.

Of nearly 12,000 people included in the study, those with tattoos had a higher risk of overall lymphoma compared with non-tattooed individuals (incidence rate ratio [IRR] 1.21, 95% CI 0.99-1.48) after adjusting for factors such as education, age, income, and smoking status, reported Christel Nielsen, PhD, of Lund University in Sweden, and colleagues.

This association was strongest for those with diffuse large B-cell lymphoma (IRR 1.30, 95% CI 0.99-1.71), followed by follicular lymphoma (IRR 1.29, 95% CI 0.92-1.82), they noted in .

Of note, the risk of lymphoma was highest in those with less than 2 years between their first tattoo and the index year (IRR 1.81, 95% CI 1.03-3.20), and decreased with intermediate exposure duration (3 to 10 years), but increased again in those who received their first tattoo ≥11 years before the index year (IRR 1.19, 95% CI 0.94-1.50).

"More epidemiologic research is urgently needed to establish causality," Nielsen and colleagues wrote. "The study underscores the importance of regulatory measures to control the chemical composition of tattoo ink."

"If these findings can be corroborated by further studies, they would indicate that exposure to tattoo ink may be associated with both tumor initiation, which is often associated with a latency of several years, and tumor promotion where effects occur much faster," they added.

In a , Nielsen noted that "people will likely want to continue to express their identity through tattoos, and therefore it is very important that we as a society can make sure that it is safe. For the individual, it is good to know that tattoos can affect your health, and that you should turn to your healthcare provider if you experience symptoms that you believe could be related to your tattoo."

The authors pointed out that the popularity of tattoos has increased dramatically over the last several decades, with prevalence rates reported to be over 20% in several European countries, and about 30% in the U.S.

Since people often start getting tattoos at younger ages, this means they could be exposed to the ingredients of tattoo ink -- which often contains carcinogenic chemicals -- "over almost the entire life course," Nielsen and team wrote. "Yet, research has only begun to scratch the surface of understanding the long-term health effects of tattoos."

For this study, the researchers identified all incident cases of malignant lymphoma diagnosed from 2007 to 2017 in patients ages 20 to 60 years in the Swedish National Cancer Register. Three random age- and sex-matched controls per patient were sampled from the Total Population Register using incidence density sampling.

The study included 11,905 people (2,938 with lymphoma and 8,967 controls). Among those with lymphoma, 54% answered a questionnaire regarding tattoo exposure compared with 47% in the control group, and 21% versus 18%, respectively, were tattooed.

The most common subtypes of lymphoma were diffuse large B-cell lymphoma (28%), Hodgkin lymphoma (21%), and follicular lymphoma (18%). Median age at diagnosis ranged from 51 to 57, except among patients with Hodgkin lymphoma (median age 36 years). Men made up more than half of each group.

Respondents provided their age at first tattoo, as well as tattoo characteristics such as color, area of tattooed body surface (<1, 1-5, or >5 hand palms), skill level of tattoo artist (professional or amateur), and the part of the body tattooed.

The authors observed that while it "seems intuitive" that a larger tattooed body surface would be associated with a greater risk of lymphoma, they found no evidence of an exposure-response relationship.

Instead, they found that the highest lymphoma risk was in people with tattoos smaller than one hand palm (IRR 1.27, 95% CI 0.99-1.63).

Nielsen and colleagues acknowledged that, as in any case-control study, "elective participation is a generic concern that may distort the results. The fact that we could not formally assess potential selection bias because we did not have access to individual-level data on the nonparticipants is a limitation."

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