Around 35% of tattoo or permanent makeup inks sold in the U.S. are contaminated with bacteria, a new study found.

Researchers tested 75 total inks from 14 manufacturers to identify how much anaerobic bacteria — which thrive in environments without oxygen — and aerobic bacteria — which thrive with oxygen — exist in them.

This is the first time anaerobic bacteria in commercial tattoo inks has been investigated, and the researchers say it's particularly important as the dermal, or middle, layer of the skin is a low-oxygen environment.

Their findings, published in Applied and Environmental Microbiology Tuesday, revealed that sealed, unopened tattoo inks can harbor anaerobic bacteria, and a product label that claimed it was sterile didn't have a clear link to the actual absence of bacteria.

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The researchers say this suggests both types of bacteria can contribute to the increase in ink-related infections that have come with the rising popularity of ink usage and tattooing in the last decade, and the results highlight the importance of monitoring inks for bacteria and pathogenic microorganisms.

Those involved in the study say they're planning to research more efficient microbial detection in tattoo inks and deepen the understanding of the contamination issues.

The U.S. Food and Drug Administration says "tattoos are more popular than ever," with about 30% of all Americans having at least one according to an Ipsos poll.

Inks used in tattooing are subject to the federal agency's oversight, but it says it's received reports of people developing infections from contaminated inks. This led to a 2019 safety alert about certain inks that were contaminated with microorganisms, and in 2023, tattoo ink manufacturers and distributors were given FDA guidance in recognizing and preventing bacteria and microorganism ink contaminations.