Phthalates, a type of chemical found in plastic manufacturing parts, are one of the biggest culprits, according to Swan.

That’s because they disrupt how the hormone endocrine is produced in the body. In turn, that disruption can contribute to obesity, lower IQs, premature birth. As Swan found, it can also decrease testosterone production, lower sperm counts, decrease fertility, and contribute to smaller penis size.

These endocrine disruptors can affect babies as they grow in the womb, if the person carrying them has been exposed to chemicals, according to Swan and other researchers’ work. They can also be passed onto babies in breast milk, said Swan.

“Babies are now entering the world already contaminated with chemicals because of the substances they absorb in the womb,” she wrote.