Ingredients in e-liquids can be dangerous to inhale even if the Food and Drug Administration (FDA) deems them generally recognized as safe (GRAS) for oral consumption because inhalation exposes the lungs to chemicals in ways that differ significantly from ingestion. The lungs process substances differently than the gut, which can lead to harmful effects not seen with oral intake

. Key reasons why e-liquid ingredients can be harmful when inhaled include:

• Thermal decomposition and formation of toxic compounds: When e-liquids are heated and aerosolized, chemical reactions can produce new, potentially harmful compounds such as carbonyl compounds (formaldehyde, acetaldehyde, acrolein, glyoxal). Formaldehyde is a known human carcinogen, acetaldehyde is possibly carcinogenic, and acrolein can damage lung tissue and cause respiratory irritation

• Inhalation toxicity of flavorings: Many flavoring agents used in e-liquids (e.g., diacetyl, cinnamaldehyde, benzaldehyde) are safe to eat but have been linked to respiratory toxicity when inhaled. For example, diacetyl is associated with bronchiolitis obliterans ("popcorn lung"), a serious lung disease. These flavorings can cause inflammation, cytotoxicity, and immune suppression in the respiratory tract

• Base solvents and their byproducts: Propylene glycol and glycerin, the main solvents in e-liquids, are generally safe to ingest but can cause respiratory irritation and other lung issues when inhaled. Heating glycerin can produce acrolein, a respiratory irritant and probable carcinogen. Propylene glycol can convert to propylene oxide, a possible carcinogen, upon heating

• Presence of harmful substances beyond flavorings and solvents: E-liquids may contain nicotine (addictive and harmful to brain development), heavy metals, volatile organic compounds, and tiny particles that penetrate deep into the lungs. These substances contribute to lung inflammation, oxidative stress, and other adverse health effects

• Lack of inhalation safety data: Many ingredients in e-liquids have not been tested for safety via inhalation. The FDA’s GRAS status applies to ingestion, not inhalation, and inhaling these chemicals can lead to respiratory tract irritation, inflammation, cytotoxicity, and potentially carcinogenic effects

In summary, the danger arises because heating and inhalation transform or expose the lungs to chemicals differently than eating them, producing toxic compounds and respiratory hazards even from ingredients considered safe to eat