A common knock against e-cigarettes is that we don’t know the long-term health effects of consuming nicotine vapor. The fact is that while it’s impossible to have complete knowledge about a novel form of tobacco use, scientists already know a great deal about propylene glycol (PG), a key ingredient of the vapor in e-cigarettes.
A comprehensive 2012 PG toxicology review, authored by members of the Cosmetics Ingredient Review (CIR) expert panel, appeared in the International Journal of Toxicology.
The report was an update of a 1994 assessment which concluded that PG was “safe for use in cosmetic products at concentrations up to 50.0%” – the cosmetic industry standard at the time. By 2012, PG was used in 9,094 of 34,391 cosmetic and personal care formulations. One of the most interesting passages in the 1994 document was a description of aerosolized PG in hair sprays:
Propylene glycol is used in hair sprays, and its effects on the lungs that may be induced by aerosolized products containing this ingredient may be of concern. The aerosol properties that determine deposition in the respiratory system are particle size and density. The parameter most closely associated with deposition is the aerodynamic diameter, da, defined as the diameter of a sphere of unit density possessing the same terminal settling velocity as the particle in question. In humans, particles with an aerodynamic diameter of < 10 um [micrometers] are respirable. Particles with a da from 0.1 to 10 um settle in the upper respiratory tract and particles with a da < 0.1 um settle in the lower respiratory tract. Particle diameters of 60 to 80 um and > 80 um have been reported for anhydrous hair sprays and pump hairsprays, respectively. In practice, aerosols should have at least 99% of their particle diameters in the 10 to 110 um range and the mean particle diameter in a typical aerosol spray has been reported as ~ 38 um. Therefore, most aerosol particles are deposited in the nasopharyngeal region and are not respirable.
In contrast, PG aerosols generated by e-cigarettes appear to be smaller, in the range of 0.1 to 0.4 um, according to a study published last year. So, while it likely that most e-cigarette vapor is settling in the upper respiratory tract, some small aerosol particles are likely reaching deeper lung tissue.
The 2012 CIR panel also commented about PG as a food additive:
According to the Joint FAO/WHO Expert Committee on Food Additives (JECFA), the acceptable daily intake of PG is 25 milligrams per kilogram of body weight per day. In Japan, the Ministry of Health, Labour, and Welfare (MHLW) specified that according to the food sanitation law, PG has no potential to cause harm to human health.
The CIR panel also reviewed numerous animal studies regarding potential problems. They concluded:
Both PG and PPGs [polypropylene glycols] were not considered to be acute or chronic toxicants in oral or dermal studies, were not genotoxic or carcinogenic, and were not reproductive or developmental toxicants, supporting that their use in cosmetics would be safe in regard to these end points.
The CIR was established in 1976 by an industry association with the support of the FDA and the Consumer Federation of America; it reviews and assesses the safety of ingredients used in cosmetics. University faculty, industry representatives and the director of the Office of Cosmetics and Colors at the FDA participate in the reviews, which are conducted according to published procedures. The CIR appears to be an excellent example of cooperation between federal regulators and the industry they regulate; it might serve as a template for cooperation among the FDA Center for Tobacco Products and tobacco manufacturers.
The FDA has thus far taken an aggressively antagonistic approach in its regulatory procedures, some of which are dictated by the 2009 enabling legislation. For example, the legislation prohibits members of the FDA Tobacco Product Scientific Advisory Committee (TPSAC) from having any relationship with the tobacco industry in the 18 months preceding their appointment. This is in direct contrast to all other federal advisory committees (including expert CIR panels), in which an industry conflict of interest for a qualified candidate is reportable but not prohibited.
The CIR serves the public interest in producing an informative PG safety review; it also serves as a role model for constructive government-industry interaction.