Brazilian researchers exposed newborn rats to cigarette smoke for five minutes, twice a day, starting on the second day of life. When the experiment ended 26 days later, they pulled the animals' teeth and photographed them. The exposed animals' incisors looked slightly yellower than the controls', but there were no chips, no opacities, and no missing structures. By the standard a dentist would apply in a clinic, nothing was wrong.
Only when the team ran the enamel through scanning electron microscopy and energy-dispersive X-ray spectroscopy did a difference appear: in the outermost layer, the microscopic scaffolding was more widely spaced, and in the molars, phosphorus was down and carbon was up.
The findings, from the Ribeirão Preto School of Dentistry at the University of São Paulo, were published in Calcified Tissue International in May and detailed in a research report from São Paulo's state funding agency in late July.
Five Minutes of Smoke, Twice a Day, From Day Two of Life
The design was modest. Thirty-four Wistar rat pups from four litters were split into a control group of 15 and an exposure group of 19. The exposed animals were placed in a transparent acrylic chamber where the smoke of five cigarettes was released under controlled conditions, twice daily with a 12-hour gap, from postnatal day 2 through day 28. Researchers tracked weight and body length throughout.
The exposed pups gained less weight, averaging about 77 grams at day 28, compared with 86 grams in controls. Body length did not differ. Rodents are useful subjects here for a specific reason: their incisors grow continuously, allowing scientists to observe enamel formation rather than only inspect the finished product.
"Even minimal exposure to cigarette smoke impacted the growth of these animals," said Francisco Wanderley Garcia de Paula-Silva, the study's advisor, in the Agência FAPESP report. The research was conducted during Juliana de Lima Gonçalves' doctoral studies and supported by the São Paulo Research Foundation.
The Damage Showed Up Only Under a Microscope
Three changes stood out. In the aprismatic outer layer of the incisors, the gaps between enamel prisms widened. In the molars, phosphorus fell, and carbon rose. Calcium, oxygen, and the calcium-to-phosphorus ratio were unchanged.
Phosphorus matters because it is a building block of hydroxyapatite, the mineral that makes enamel the hardest substance in the human body. The carbon result is strange. Mature enamel contains very little organic material, so excess carbon suggests something was left behind that shouldn't have been there.
Enamel volume was unchanged on micro-computed tomography, and the ameloblasts that lay down enamel showed no morphological differences. The researchers read that as a clue about timing. Formation appeared to proceed normally; it was the later mineralization and maturation phase that looked disturbed.
One result cuts against the alarm. Microhardness testing found no reduction in hardness at the incisal tip, the cervical region, or the molar surface. The authors offer a methodological caveat of their own: microhardness is not a sensitive measure for detecting mild structural changes of this kind. Paula-Silva adds a second explanation: the animals barely used those teeth, having been weaned at 21 days, with the experiment ending a week later.
"What we need to understand now is how this enamel will behave with use over time," he said. Whether an altered chemical signature translates into a tooth that chips or decays under years of chewing is, by his own account, still an open question.
Enamel Keeps a Record That Bone Does Not
The reason a dental finding like this attracts attention beyond dentistry is due to a peculiarity of the tissue. Bone remodels throughout life, continually rebuilding and erasing its own history. Enamel does not. Once laid down, it stays as it was.
That makes it a biological archive of a narrow developmental window, which is why the research sits inside the Developmental Origins of Health and Disease framework, the idea that adult illness can trace back to very early exposures.
The practical question is whether this connects to molar-incisor hypomineralization, a defect that leaves enamel fragile and prone to fractures and cavities. Paula-Silva estimates it affects roughly one in five children, and its causes remain genuinely unexplained. He is careful to call the link a hypothesis, not a finding.
Secondhand smoke exposure is not uncommon in the United States. According to the Centers for Disease Control and Prevention, almost two of every five children aged 3 to 11 were exposed during 2017 to 2018, with more than half of non-Hispanic Black children affected. A meta-analysis of passive tobacco exposure has separately linked it to caries in children and adolescents.
What This Study Cannot Say
It cannot be said that secondhand smoke weakens children's teeth. The subjects were rats, the exposure was controlled and artificial, and the sample was small.
It cannot be said that the observed changes lead to fractures or decay. Hardness was preserved at the point of measurement, and whether the altered chemistry matters under years of chewing is unresolved.
What it demonstrates is that a specific early environmental exposure was associated with measurable changes in how enamel mineralized in an animal model, before the teeth had done any real work. Notably, an earlier rat study using two-hour exposures found reduced enamel thickness and mineral density, suggesting that the dose is doing a great deal of the work here.
The group has begun a second phase with increased exposure, about an hour twice daily, to test whether more intense smoke produces defects more similar to those dentists see in children.
"Smoking is already recognized as a major risk factor for respiratory diseases and child development," Paula-Silva said. What the study adds, in its framing, is the possibility that teeth also carry marks of those exposures. Parents with concerns about a child's enamel should raise them with a pediatric dentist.
Key Questions Answered
What did the study find?
Rat pups exposed to secondhand cigarette smoke from day two of life developed enamel with wider spacing between prisms in the outer layer, less phosphorus, and more carbon, without visible structural defects.
Were the teeth actually weaker?
Not at the time of testing. Enamel hardness and volume were unchanged. The authors note that microhardness testing is not sensitive to mild structural changes.
Did the teeth look different at all?
Slightly. The exposed animals had yellower incisors, but no opacities, fractures or loss of enamel structure were seen.
Does this prove secondhand smoke harms children's teeth?
No. The study was conducted in rats and does not establish a link to dental defects in humans.
Why study enamel specifically?
Enamel does not remodel after it forms, so it preserves a permanent chemical record of a narrow developmental window.
What is molar-incisor hypomineralization?
A common enamel defect that leaves teeth fragile and prone to fractures and cavities. Its causes are not well understood.
What happens next?
Researchers have started a follow-up using roughly an hour of smoke exposure twice daily to see whether more pronounced enamel changes emerge.
