Tattoo inks, which are generally considered safe, are not biologically inert, according to new scientific evidence. Tattoo pigments interact with the immune system beneath the skin and can lead to allergic reactions and chronic inflammation. Some inks contain trace amounts of heavy metals, including nickel, chromium, cobalt, and occasionally lead, which can trigger allergic reactions. Other tattooing pigments can also contain organic compounds, such as azo dyes and polycyclic aromatic hydrocarbons, which have been linked to cancer and genetic damage in laboratory studies. A recent study found that tattoo pigments could influence immune activity, trigger inflammation, and reduce the effectiveness of certain vaccines. However, there is no strong epidemiological evidence linking tattoos to cancer in humans.