As tempting as it may be to believe otherwise, the beautiful dark patina that develops on your copper coffee mug might be considered healthy; after all, its presence is an indicator of age and use; after all, your cup has probably seen more use than someone who bought it new. Unfortunately, however, its “patina” can also hide all sorts of dirt, germs, and grime that could potentially make you sick.

Though an antique copper mug might look cool, its patina can be dangerous to health. Comprised of layers of organic residue from coffee, milk and sugar leftovers – such as coffee grounds – its patina creates the ideal environment for bacteria growth which in turn creates health risks such as foodborne illness and fungal infections.

Jeffrey Starke, an anthropologist at the University of Pennsylvania, told Bustle that unwashed cups may be just fine if only used for coffee (without cream or sugar added), however if left sitting overnight or over a weekend without being cleaned then there will likely be residue buildup that leads to bacteria and fungal growth.

He added that most germs found in coffee cups come from people, rather than from human waste as suggested in one study. His research found 20 percent of mugs harboring bacteria like E coli and Staphylococcus which should only live on sponges or bathroom sinks but can live for three days inside copper mugs! Therefore, in order to stop worrying about your copper mug harboring potentially hazardous germs you’ll need to wash it frequently.