Toluene
In plain terms
A fast-evaporating solvent in nail polish, one of the nail toxic trio. It is a reproductive and nervous-system toxicant at high or repeated exposure, but an occasional coat in a ventilated room is low-risk. The serious harm is occupational and from inhalant abuse.
What the evidence shows
Toluene is a petroleum solvent that gives nail polish its smooth flow and finish, and it is one of the nail toxic trio alongside formaldehyde and dibutyl phthalate. Because it evaporates fast, the main exposure is breathing it in. At high or repeated levels it is a real problem: California lists it under Proposition 65 for reproductive and developmental harm, it crosses the placenta, and chronic exposure damages the nervous system, with nail technicians showing the harms most clearly. The serious birth-defect and brain-injury cases come from occupational exposure and from inhalant abuse, far above normal use. For the occasional consumer the picture is calmer: the safety panel concludes toluene is safe in nail products up to 20 percent, and poison-control experts find little evidence that occasional polish use, including during pregnancy, harms a developing baby, because the exposure sits well below the levels that cause those effects. So this is a depends, set by how much and how often. An occasional manicure in a ventilated room is low-risk, while daily users, salon workers, and the first trimester of pregnancy are where caution and toluene-free polish earn their keep. Three-free and higher polishes simply leave it out, and applying with ventilation handles most of the rest.
On the label
Listed as toluene or methylbenzene, mostly in nail polish and treatments. Choose a toluene-free or 3-free or higher polish if you use it often or are pregnant, and apply in a well-ventilated space. An occasional coat with airflow is low-risk for most people.