Everyday Toxins and Early Puberty: What the California PAH Study Tells Us
Polycyclic aromatic hydrocarbons, or PAHs, are harmful chemicals created whenever organic materials like wood, oil, or coal are burnt. PAHs are also formed during grilling of meat or fish. This leaves behind an invisible, toxic footprint that most of us breathe in or eat every single day. Exposures come through outdoor and indoor sources like cigarette smoking, wood burning, wildfires, vehicle traffic, and grilling meat. These sources are often unavoidable in the modern world.
PAHs are known to have a direct link to reproductive hormone disruption in women and increase the risk of breast cancer. Though most PAH studies have focused on adults, recent papers published by Dr. Esther M. John at Stanford University and her colleagues focused on PAH exposure in young girls during puberty. The researchers explored whether PAHs are linked to early puberty, which increases the risk of breast cancer later in life.
What sources of PAHs are girls exposed to?
In a study started in 2011 called Lessons in Epidemiology and Genetics of Adult Cancer from Youth (LEGACY)Lessons in Epidemiology and Genetics of Adult Cancer from Youth (LEGACY), researchers enrolled a group of 1,040 girls ages six to 13 years old across the US and Canada in a follow-up study. Around half of the girls had a history of breast cancer in the family, and the other half didn’t. They investigated the role of early life factors in pubertal development, and they collected extensive survey data and urine and blood samples at the start of the study from these girls, and continued collecting data every six months over five years.
From this larger study, focused research was conducted at the Northern California LEGACY site in a project called the California PAH study.
In this project, researchers focused on seven different PAH metabolites in urine samples of 359 girls. Metabolites are the chemical leftovers found in the urine after the body breaks down PAHs. The study found that exposure to PAHs is incredibly common. In fact, certain widespread PAH metabolites were detected in over 98% of the girls tested.
For the different kinds of metabolites, they found that different levels depended on the girls’ age at urine collection, body mass index (BMI), indoor and outdoor lifestyles, and diets. While higher chemical levels were generally noticed in girls from lower-income families or racial minority groups, these specific background differences were not large enough to be statistically significant.
What can urine samples tell us about the impact of PAHs?
Using these data, researchers looked at when these young girls hit puberty milestones to see if there was a link to their chemical exposure. They focused on four puberty outcomes: breast development, pubic hair growth, age at first period, and the time interval between the start of breast development and first period. To assess breast development and pubic hair growth, they used Tanner scales, which are charts/guides with drawings that helped participants and their parents/guardians to determine what stage of development the girl is in.
An analysis of 176 girls who had not started puberty yet when they enrolled in the study, the researchers found a link between these chemicals and early development, especially depending on a girl's weight:
- For girls with overweight: Those with high concentrations of certain PAH metabolites in their urine had double the chance of earlier breast development and triple the chance of earlier pubic hair development compared to girls with lower exposure.
- For girls without overweight: High concentration of one PAH metabolite was associated with early pubic hair development. For other metabolites, pubic hair development was earlier in girls with low concentrations.
There was no link between high concentrations of PAH metabolites and a girl getting her period early, but lower concentrations of certain metabolites in girls with overweight were associated with a girl getting an earlier period. Overall, the study found that higher concentrations of certain PAH metabolites were associated with earlier puberty.
What can blood samples tell us about the impact of PAHs?
An additional analysis published in 2025 included data from 215 of the girls, this time focusing on PAHs absorbed directly into the bloodstream. They measured chemical markers left behind when PAHs lock onto proteins in the blood.
Of the 215 samples collected, all but two were found to have these PAH markers in their blood. The blood tests confirmed the overall trends seen in the urine tests, showing that everyday exposures are widespread.
Furthermore, higher levels of PAHs in the blood were directly linked with earlier breast development. Because this part of the project was based on a smaller group of girls, the researchers couldn't determine how body weight factored into these specific blood results, but the overall developmental link was clear.
Why is this important and what can we do?
Early puberty in young girls is linked to higher risk of breast cancer, mental illnesses, cardiovascular disease, and Type 2 diabetes. By studying PAHs in urine and blood, researchers have found a link between exposure to these harmful chemicals and early puberty.
PAH exposure is unfortunately not easy for an individual to avoid. There are many activities people do in their lives that expose them to PAHs. Many young girls may have a simple campfire while making smores, exposing them to smoke from wood burning. They might eat grilled meat off a barbecue their family was hosting. They may be walking to school in a busy area with car exhaust fumes in the air.
Although exposure comes from some sources we can control – like cigarette smoke, wood burning, and grilling meats – it also comes from many sources we can’t control, like vehicle traffic and wildfires. As these recent studies show, focusing on reducing those exposures we can avoid is crucial to protecting young girls' future health. Policy changes are also necessary to reduce exposure to PAHs outside of individual control.
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The California PAH Study was conducted by Dr. Esther M. John at Stanford University and was funded by the California Breast Cancer Research Program. Zero Breast Cancer collaborated in the study as the Advocacy Organization.
Written by Jeannine Elmasri, Collaborative for Health & Environment (CHE) Science Communications Intern, and Lianna Hartmour, MA, NBC-HWC, Zero Breast Cancer Program and Communications Director at CHE.
Reviewed by Esther John, PhD at Stanford University
