A “burnt coil,” or “dry hit,” occurs when the heating element of a vaping device overheats the wicking material instead of the e-liquid. This happens when the cotton or wick material is not fully saturated, causing the coil to burn the dry material and residual e-liquid components. The resulting aerosol is dramatically different from the intended vapor, leading to an acrid, unpleasant taste and harsh inhalation. This process produces harmful chemical compounds delivered directly into the respiratory system.
The Mechanism of Thermal Degradation
E-liquid consists mainly of Propylene Glycol (PG) and Vegetable Glycerin (VG), designed to vaporize safely below 200°C (392°F). When insufficient liquid is present, the coil temperature increases far beyond this threshold, causing the intended vaporization process to fail. Instead, the extreme heat causes PG and VG molecules to undergo thermal decomposition, known as pyrolysis.
This chemical breakdown is accelerated by oxygen and the lack of a heat sink, which the liquid normally provides. The components fragment into smaller, highly reactive, and volatile compounds instead of turning into vapor. This mechanism of generating toxic byproducts differentiates a normal vaping experience from a burnt hit.
Hazardous Chemical Production
Thermal degradation of PG and VG generates a significant quantity of harmful carbonyl compounds, primarily aldehydes. The most concerning are Formaldehyde, Acetaldehyde, and Acrolein. These substances are not present in the e-liquid but are created instantly by the excessive heat of a dry coil.
The specific chemical pathways differ slightly between carriers. Vegetable Glycerin tends to yield higher levels of Formaldehyde and Acrolein, while Propylene Glycol breakdown produces more Acetaldehyde. Acrolein is a potent toxicant, classified as an unsaturated aldehyde, which is highly reactive with biological tissues. The concentration of these toxicants in the aerosol from a burnt coil can spike dramatically, exceeding levels found during normal operation.
Immediate Biological Impact on Lung Tissue
Inhaling the aldehyde-rich aerosol causes an immediate and acute biological reaction in the respiratory system. Formaldehyde, Acetaldehyde, and Acrolein are potent irritants that react quickly with the mucous membranes lining the airways. This reaction triggers a protective pulmonary irritant reflex, manifesting as the intense coughing and throat discomfort associated with a dry hit.
At the cellular level, these aldehydes induce oxidative stress and cytotoxicity within the delicate tissues of the bronchioles and alveoli. Acrolein rapidly forms protein adducts, disrupting the function of endothelial cells that line blood vessels in the lungs. This cellular damage can lead to acute inflammation and compromise lung function. Chronic exposure to such compounds is linked to the development of serious respiratory conditions like Chronic Obstructive Pulmonary Disease (COPD) and acute lung injury.
Prevention and Mitigation
The most effective way to avoid inhaling these toxic compounds is to prevent the coil from burning. This begins with proper “priming” of a new coil, which involves manually dripping e-liquid onto the exposed cotton wicking material before installation. After filling the tank, allowing the device to sit for five to ten minutes ensures the wick is completely saturated before firing.
Users should consistently monitor the e-liquid level, ensuring the liquid never drops below the wicking ports or the minimum fill line. Avoiding “chain vaping”—taking multiple, rapid puffs—is also important, as this prevents the wick from re-saturating quickly enough. Finally, use a wattage setting appropriate for the coil’s resistance, as recommended by the manufacturer, to prevent the heating element from overpowering the liquid supply.