With the rising popularity of vaping and electronic cigarettes, many might consider turning to them to stop smoking or to at least divert their habit to a healthier alternative. However, the liquids are made using food products. Would the liquids need to be kosher certified even though they aren't being digested? Is the activity itself allowed?
TL;DR According to YU Rosh Yeshiva R' Daniel Stein (who cites the opinions of R' Herschel Schachter & R' Mordechai Willig among others), vaping should be avoided entirely, as it entails the inhalation of known toxins and possibly forbidden foods.
R' Daniel Stein published a TorahWeb.org article titled "Are Vaping and E-Cigarettes Kosher?" in 2018, in which he discusses two main issues with vaping:
1) Inhalation of dangerous toxins:
In January of 2018 The National Academies of Sciences, Engineering, and Medicine published a comprehensive study which concluded that while the use of e-cigarettes appears less harmful than conventional cigarettes, e-cigarettes emit numerous known toxins aside from nicotine. Studies have shown that e-cigarette emissions can include potentially toxic levels of formaldehyde, arsenic, benzene, chromium, manganese, nickel, lead, zinc, and diacetyl, etc. Even though some of the ingredients in e-juice have been designated as safe when consumed at room temperature, when they are heated by a metal coil they produce toxic substances that are potentially dangerous. These findings have been discussed in many recent news columns including Medical News Today, Newsweek, and The New York Times.
R' Stein goes on to cite more studies that seem to show that prolonged inhalation of these toxins can lead to an increased risk of cancer, heart disease, stroke, and respiratory distress.
Even though the precise long-term effects of regular e-cigarette use remains largely undefined, since it involves the frequent inhalation of known toxins, Rav Hershel Schachter shlit"a and Rav Mordechai Willig shlit"a believe that there is already ample basis to assert that the use of e-cigarettes by non-smokers should provisionally be forbidden by virtue of the prohibition against self-endangerment found in the pasuk "And you shall watch yourselves very well," pending the collection of definitive data.
2) Kashrus Issues:
Often one of the core ingredients in the e-liquid is glycerin, a clear, odorless liquid with a thick consistency and sweet taste... Glycerin can be derived from vegetable oils or animal fats and they are used interchangeably. Therefore, a food product containing glycerin typically requires kashrus supervision despite the insistence of the manufacturer that it uses strictly vegetable glycerin. Arguably, a glycerin product that is turned into an aerosol or vapor and then inhaled should also require kashrus certification. Indeed, this is the opinion of the Chicago Rabbinical Council and reported to be the position of Rav Shlomo Miller shlit"a as well.
R' Stein continues:
Additionally, the Magen Avraham (467:10) prohibits cigarettes containing beer-soaked tobacco on Pesach, even though the tobacco is presently inedible (see Beis Meir.) Rav Moshe Schick (Maharam Schick, Orach Chaim 242) explains that since the beer-soaked tobacco is designed for inhalation and it has a pleasant flavor when inhaled, the tobacco can still be considered fit for consumption and remains prohibited. This is presumably predicated upon the notion that inhalation can sometimes be considered a form of imbibing (see also Magen Avraham 210:9). Similarly, since the e-juice is intended for inhalation and enjoyable when inhaled, it would retain its status as a forbidden food and as a result it should require kashrus supervision.
According to the answer to this question :
The materials used in the cigarettes are not edible
I realize that the question was regarding its use on Pesach, and the answer states that they are not Chametz, b/c it is inedible. Therefore, something that is not considered food, has no concerns of Kashrut.