Nicotine-Free: E-Cigarettes May Affect Skull Formation During Fetal Development

Researchers tested whether e-cigarette liquids, composed primarily of propylene glycol and vegetable glycerin, even without nicotine, can affect the development of offspring during pregnancy. In mice, a formulation with more vegetable glycerin reduced facial and skull growth, while a balanced formulation did not cause this effect. The study warns that even "nicotine-free" liquids are not necessarily safe for use during pregnancy.

So-called Electronic Nicotine Delivery Systems (ENDS), which include e-cigarettes, have become a major public health challenge. They emerged in the early 2000s and became particularly popular after 2010, when they began to feature more modern designs and advanced technology.

Although companies claim these products were created to help smokers quit traditional cigarettes, they have become very popular among teenagers and young adults. This has created a new group of nicotine users who, in many cases, had never used other tobacco products before.

The use of these devices raises concerns because their long-term health impacts are still unclear. Among the most concerning issues are the risk of nicotine addiction, the frequency with which these products are used, and the possibility that they serve as a gateway to other types of tobacco use.

Furthermore, regulation of these products is weak, especially in the United States, where the industry has resorted to the courts to block or delay new regulations. Meanwhile, the e-cigarette market is growing rapidly, representing a setback for the progress made in reducing tobacco use in recent decades.

The liquid used in these devices is called e-liquid. It is heated to release a vapor that is inhaled by the user. Most e-liquids contain two main components: propylene glycol and vegetable glycerin. Propylene glycol is used to create the "throat hit" sensation when inhaling, and vegetable glycerin is used to produce more visible vapor. Additionally, e-liquids may contain flavorings, acidic substances to adjust pH, and, in many cases, varying concentrations of nicotine.

While much attention has been paid to nicotine and flavorings, little research has been done on the health effects of the ratios of propylene glycol and vegetable glycerin. These proportions can vary greatly: some products use almost exclusively propylene glycol, others use almost exclusively vegetable glycerin, and many use mixtures of the two.

Early research suggests that when there is less propylene glycol, the amount of nicotine and residual particles released also decreases. This indicates that the composition of the liquid influences not only the user experience but also potential health risks.

Another important point is the impact of these products during pregnancy. It is already known that traditional cigarette use during pregnancy can cause serious problems for the baby, including malformations, especially of the head and face.

These changes, called craniofacial birth defects, can be genetic or caused by external factors, such as exposure to toxic substances. Many of these conditions require complex surgeries and treatments soon after birth, and even then, they are rarely completely corrected.

In the study described, the researchers wanted to understand whether different proportions of propylene glycol and vegetable glycerin in the liquids used in electronic devices, even without nicotine, could cause changes in the facial and skull development of offspring exposed during pregnancy. Two formulations were tested: one with half propylene glycol and half vegetable glycerin, and another with 30% propylene glycol and 70% vegetable glycerin.

The experiment was conducted on mice. Pregnant females were exposed to normal air (control group), a formulation with 50% of each substance, or a formulation with more vegetable glycerin (30% propylene glycol and 70% vegetable glycerin). When the pups were 14 days old, researchers measured their weight and analyzed the dimensions of their faces and skulls.

Change in skull shape due to exposure of the ENDS carrier. White/Gray = Control; Yellow = 30/70 PG/VG; Green = 50/50 PG/VG. Coronal overlay used the sella turcica for anatomical registration.

The results showed that pups exposed to the formulation with more vegetable glycerin experienced slower growth, both in weight and in facial and cranial measurements, compared to the control group. Pups exposed to the formulation with equal parts of both substances showed no significant differences compared to the control group.

This indicates that changing the formulation of liquids to include more vegetable glycerin, a common practice in the industry, does not necessarily reduce health risks. Furthermore, the study reinforces that not even nicotine-free liquids can be considered safe during pregnancy.

Cranial measurements found in animals after exposure to different concentrations of ENDS. A. Cranial length of independent samples revealed no statistically significant differences by exposure. B. Analysis of Variance revealed significant differences in cranial width by exposure. C. Analysis revealed significant differences in cranial height. D. The independent samples test for cranial base length revealed no statistically significant differences by exposure. E. Analysis of cranial base width revealed no statistically significant differences by exposure. F. The independent samples test for craniofacial length revealed no statistically significant differences.

These findings indicate a significant need for further studies on the components and formulations of Electronic Nicotine Delivery Systems, alone and in combination, since adverse effects on craniofacial growth have been observed without the presence of nicotine or other common additives.

READ MORE:

In utero exposure to electronic cigarette carriers alters craniofacial morphology

Ethan Richlak, Logan Shope, Ethan Leonard, Leslie Sewell, Tyler Maykovich, Amr Mohi, Roy A. Miller, Matthew W. Gorr, Loren E. Wold, and James J. Cray 

PLoS One. 20(6): e0327190. June 30, 2025DOI: 10.1371/journal.pone.0327190

Abstract: 

Despite the popularity of electronic nicotine delivery systems (ENDS), there is currently a lack of regulation and consistency regarding the formulation of the e-liquids that undergo combustion in use. The two main constituents of most e-liquids are the humectants propylene glycol (PG) and glycerol (vegetable glycerin, VG). E-liquids consist of a ratio of these two components with PG utilized to increase the “throat hit” effect and VG used to increase visible vapor. As PG-based e-liquids are known to generate more carcinogenic carbonyls and increase the uptake of nicotine, many commercial products have moved toward a more VG-centric formulation to reduce potential harm. The purpose of this study was to test the hypothesis that a common VG-based formulation (30/70 PG/VG) would result in fewer negative effects on craniofacial growth compared to an evenly concentrated formulation (50/50 PG/VG) in the absence of nicotine. Adult breeder mice were utilized to generate in utero ENDS component exposed litters including free air exposure (control), 30/70 PG/VG, and 50/50 PG/VG groups. The resulting pups were assessed at postnatal day 14 for skull morphology. Data demonstrate significant reductions in body weight, facial, and cranial dimensions, where there was a significant reduction in growth for the 30/70 PG/VG exposed group. There were no significant differences found between control and 50/50 PG/VG. These results suggest the overall movement to a more VG-centric ENDS formulation may not result in reduced profile for health concerns. Further, it suggests that PG/VG are not a harmless carrier and now popular nicotine-free ENDS formulation may not be considered safe for use in pregnant populations.