Aromatic compounds known as terpenes are natural hydrocarbons responsible for the distinctive scents and flavors in nearly all plants, including fruits, herbs, and cannabis. Manufacturers often incorporate “botanical” terpenes, sourced from non-cannabis plants, into vape liquids to customize flavor profiles or mimic specific cannabis strains. The use of these compounds is widespread because they are perceived as natural and safe additives. However, heating and inhaling these plant extracts introduces chemical and physiological complexities that challenge the assumption of safety.
Defining Botanical Terpenes and Their Use in Vape Products
Terpenes are a large class of organic compounds derived from isoprene units, acting as the building blocks for thousands of aromatic molecules found throughout the plant kingdom. These hydrocarbons are the primary constituents of essential oils, giving a lemon its citrus scent (limonene) or lavender its calming aroma (linalool). Botanical terpenes are chemically identical to those found in cannabis but are extracted from common sources like pine, orange peel, or black pepper, making them cost-effective for commercial use.
In the context of vaping, these botanical extracts serve several distinct functions within the liquid formulation. Their primary purpose is to enhance flavor and aroma, sometimes masking the taste of the base oil or solvent. They are also used to recreate the unique profiles of popular cannabis varieties, allowing manufacturers to standardize the taste. Furthermore, terpenes can alter the viscosity of the vape liquid, which improves its flow and wicking properties in the cartridge hardware.
The Difference Between Ingesting and Inhaling Terpenes
The fundamental challenge to the safety of vaping terpenes lies in the route of exposure, as the body processes compounds differently when they are consumed versus when they are inhaled. Many terpenes are designated as “Generally Recognized As Safe” (GRAS) by regulatory bodies, but this designation applies almost exclusively to their use as flavor additives in food and beverages. When ingested, these compounds are metabolized and broken down by enzymes in the digestive system and the liver before entering systemic circulation.
The lungs, however, lack the extensive metabolic machinery found in the liver necessary to detoxify and process complex oil-based compounds. Inhalation bypasses the body’s primary filtration systems, delivering the compounds directly to the alveolar cells of the lung. This direct exposure can lead to local irritation and cellular damage from substances the respiratory system is not designed to handle.
Chemical Degradation: Breakdown Products from Heating
The most significant safety concern arises not from the terpenes themselves, but from their chemical transformation when subjected to the heat of a vape coil. Vaping devices heat the liquid to a high temperature, often exceeding 200°C (392°F), which causes thermal degradation, or pyrolysis, of the organic molecules. Terpenes are thermally labile, meaning they break down easily under heat, leading to the formation of new, potentially harmful byproducts.
Common terpenes, such as limonene and pinene, are particularly prone to breaking down into irritants and toxic compounds. Studies have shown that this thermal process can generate reactive carbonyl compounds, including formaldehyde, acetaldehyde, and acrolein. Methacrolein, a known respiratory irritant structurally similar to acrolein, is a product of particular concern that forms when certain terpenes are heated to high temperatures.
The temperature of the heating element is a direct factor in the quantity of these toxic degradation products generated. Higher coil temperatures, reached through high-wattage device settings or continuous, rapid use (“chain vaping”), increase the rate of pyrolysis. Therefore, the safety of a terpene-containing vape product is highly dependent on the device’s operational parameters and the user’s vaping style.
Current Scientific Understanding of Inhalation Risks
Scientific studies investigating the inhalation of heated terpenes have highlighted several distinct health outcomes, primarily related to respiratory irritation. Users often report acute effects such as coughing, throat burning, and a raw feeling in the lungs, which are consistent with exposure to the aldehyde and methacrolein breakdown products. In vitro studies using human lung cell models have demonstrated that even unheated terpenes, such as limonene, exhibit cytotoxicity, meaning they can be toxic to lung cells, with the degree of toxicity increasing with concentration.
While terpenes were not the primary cause of the severe lung injuries (EVALI) linked to Vitamin E acetate, their presence contributes to the overall risk by causing inflammation and cellular stress. The lack of comprehensive regulatory oversight for these additives means that the purity, concentration, and formulation of botanical terpenes in vape products vary widely. There is currently a significant gap in long-term epidemiological data to fully understand the effects of chronic, daily inhalation of these thermally degraded compounds.