Scientists have found microplastics in human penises for the first time, as concerns over the tiny particles’ proliferation and potential health effects mount.

Seven different kinds of microplastics were found in four out of five samples of penis tissue taken from five different men as part of a study published in IJIR: Your Sexual Medicine Journal on Wednesday.

Microplastics are polymer fragments that can range from less than 0.2 inch (5 millimeters) down to 1/25,000th of an inch (1 micrometer). Anything smaller is a nanoplastic that must be measured in billionths of a meter. They form when larger plastics break down, either by chemically degrading or physically wearing down into smaller pieces.