It is a popular myth that snus or any other forms of smokeless tobacco contains fiberglass, or glass particles, as an aid to the absorption of nicotine by the user's blood. This is not true. Mucous membranes readily absorb free nicotine. The burning sensation is caused by the nicotine itself (similar to the tingle of nicotine gum) and some food additives such as sodium carbonate (E500). Sodium carbonate is a food additive used to increase the pH of the tobacco (reduce the acidity). This increases the bioavailability of the nicotine, meaning more is available for absorption. Some flavorings (mints in particular) are astringent and may increase the tingling or burning sensation.