In utero exposure to two liquid ingredients in e-cigarettes - minus the nicotine that drives addiction - can alter skull shape during fetal development, a new study in mice has found.
In a series of experiments, pregnant mice were exposed to a combination of two liquids used to create vaping's throat hit and smoke plume. Compared to two other experimental conditions, the offspring of mothers exposed to a specific ratio of these compounds weighed less and were born with narrowed facial features and shortened skulls.
The finding was surprising to researchers not only because no nicotine was present, but because the ratio of chemicals that produced the skull defect was designed by the e-cigarette industry to be a safer available option than the proportions in earlier products.
"This had no nicotine, and it's still having effects on the development of the skull in our model, which was not anything we expected," said lead study author James Cray, professor of anatomy in The Ohio State University College of Medicine.
The research was published recently in the journal PLOS One.
Cray's developmental biology lab has studied the effects of in utero exposure to nicotine on head and face development for years, reporting in 2020 that exposure to nicotine through breastmilk caused skull defects in mice. About 3% of babies are born annually with a birth defect, according to the Centers for Disease Control and Prevention, and alterations to the head and neck are among the most common - particularly cleft lip and/or cleft palate.
In this study, the team was developing a model to serve as an experimental control for comparison to animals exposed in utero to nicotine through vaping.
Pregnant mice were exposed to either filtered free air or two humectants - hydrating substances that function as carriers of other e-cigarette contents - at differing concentrations: 50% each of propylene glycol and glycerol (also called vegetable glycerin), or 30% propylene glycol and 70% glycerol (30/70 PG/VG).
Because propylene glycol in e-cigarettes has been linked to increasing uptake of nicotine, Cray said, "a lot of companies have moved toward getting away from a high percentage of propylene glycol toward more glycerol, trying to position this as a safer alternative."
Female mice were exposed to these three different conditions at a rate of one puff per minute, four hours per day on five days each week during the roughly 20-day pregnancy.
The cranial width and height of mouse pup skulls were scanned two weeks after birth. The analysis showed statistically significant reduced measures in the offspring of pregnant mice exposed to the presumed safer 30/70 PG/VG mixture compared to mice exposed to free air and the 50/50 formula.
"What we see is a consistent narrowing of all of the facial features, and the same thing as we move back into the cranium as well. So globally they're narrower and a little bit shorter-headed, which does mimic some observable changes that we see in children," Cray said. "We also saw a bit of a reduction in weight. These animals were within the normal range of an animal that age, but they still weighed less."
The findings were consistent across several litters of mice and in both biological sexes.
"The 50/50 mixture had no dramatic statistical changes - and that's where we were looking for the difference. We thought heavier propylene glycol should be causing more effects, and it was the exact opposite," Cray said.
The Food and Drug Administration began regulating electronic nicotine delivery systems in 2016, but has faced legal pushback from e-cigarette companies - which prevailed in a June 20 U.S. Supreme Court ruling that makes it easier to sue the FDA over efforts to block products from the market. In April, the court sided with the FDA's crackdown on sweet-flavored products after a major increase in teen vaping.
Though researchers continue to study the health effects of multiple ingredients detected in e-cigarettes, many unknowns remain as the products grow in popularity.
"The majority of users are young adults and teenagers, so we are talking about people who are in peak reproductive years. And with development of the head happening very early in fetal development, people could be using these products and not even realize that they are pregnant, which is of great concern," Cray said.
"This is a small study that speaks to the possibility that nicotine-free vaping is not safe. And it's a sign that we probably should study the nicotine-free products as much as we study the nicotine-laden products."
This work was funded by the Ohio State colleges of Medicine and Dentistry.
Co-authors include Logan Shope, Ethan Leonard, Leslie Sewell, Tyler Maykovich, Roy Miller, Matthew Gorr and Loren Wold of Ohio State; Ethan Richlak of Nationwide Children's Hospital and the University of Pittsburgh; and Amr Mohi of UT Health San Antonio.
