Over the past century, the age of menarch—when a girl menstruates for the first time—has changed from an average of 16 to 17 years to the current 12 to 13 years. That means girls are getting their periods four years earlier than they did 100 years ago. Why is that? One possible explanation for this decrease in age is increased exposure to estrogen-mimicking toxins; estrogen being a main hormone that regulates menstruation.

In a recent study by the Centers for Disease Control and Prevention (CDC) published in the journal Environmental Health Perspectives, researchers examined data from 440 girls aged 12 to 16 years. Those girls with the highest levels of dichlorobenzene metabolites in their urine began menstruation an average of eight months before girls who had the lowest levels.

Dichlorobenzene is a solvent used in mothballs and solid blocks of toilet bowl deodorizers and air fresheners. It is classified as a possible known carcinogen and prenatal exposure to it has been linked to low birth weight in boys. The resaerchers also looked at levels of other endocrine disrupting hormones, such as bisphenol A (BPA), phthalates, and parabens, but those were not found to be associated with earlier menstruation. Previous research has linked flame retardants and the pesticide DDT to early menstruation, however. Further, pesticide exposure has been linked to irregular menstruation.

The researchers state that it is impossible to know from this study alone whether exposure to the chemical is the actual cause—and not just an association—of early menstruation. “While it’s entirely possible that chemicals with endocrine-disrupting properties can influence timing of puberty, it’s unclear whether chemical exposures during certain periods of child development can have a bigger impact than at other times,” noted lead author Danielle Buttke. Further studies will look at chemical exposure in girls of younger ages.

Early menstruation has been linked to an array of health conditions and psychological disorders. More studies will be needed to tease out the chicken-and-egg factor (which came first?), but in the meantime, reducing chemical exposure is always recommended. This week, opt for essential oils to naturally freshen air, toilets, and clothing. Say goodbye to synthetic mothballs, toilet bowl bricks, and fresheners if you happen to use them. The possible risk doesn’t seem worth it.