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
(available
here).
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 (abstract
here).
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 (
here).
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 (documented
here).
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 served the public interest in producing an informative
PG safety review; it also serves as a role model for constructive
government-industry interaction.