Acetaldehyde (systematic name ethanal) is an organic chemical compound with the formula CH₃CHO. It is a colourless, highly reactive liquid with a fruity odour at low concentrations and a pungent, irritating smell at higher levels. Acetaldehyde is a volatile organic compound (VOC) that evaporates easily and is flammable. It mixes readily with water, alcohols, and other organic solvents.
Acetaldehyde vapours are heavier than air and can travel long distances, potentially causing flashbacks from ignition sources. It decomposes upon heating to release toxic gases like carbon monoxide and carbon dioxide. It can form unstable peroxides upon prolonged storage, particularly in moist environments. Acetaldehyde reacts vigorously with acids, bases, oxidising agents, reducing agents, and many other compounds, including nitric acid, isocyanates, halogens, and alcohols. Explosive reactions may occur with certain metal salts, and polymerisation is possible in the presence of trace metals or acids. Although it can degrade rubber products, it generally does not corrode most metals.
Acetaldehyde is naturally present in foods like coffee, bread, and ripe fruit and is produced in small amounts in the human body as a metabolite of alcohol. Industrially, it is manufactured on a large scale, primarily via ethylene oxidation.
Acetaldehyde is widely used as a chemical intermediate in the production of various products such as:
Additionally, it is employed as a synthetic flavouring and food preservative in products such as dairy items, fruit juices, baked goods, and soft drinks.
Industrial Sources: Acetaldehyde is emitted during its production, use, storage, and transport. Common industrial sources include power plants, internal combustion engines, oil refineries, chemical manufacturing, cement kilns, and paper mills.
Diffuse Sources: Photochemical oxidation of other VOCs in the air is the primary source of acetaldehyde in ambient air. Emissions also come from fireplaces, wood stoves, agricultural burning, and bushfires.
Natural Sources: Acetaldehyde is produced during plant respiration and microbial fermentation. It occurs in fruits like apples, in tobacco, in oak, and in a wide variety of essential oils. Natural events such as forest fires and volcanic activity also release acetaldehyde.
Transport Sources: Vehicle exhaust from gasoline and diesel engines contributes to environmental acetaldehyde.
Consumer Products: Found in heated dairy products, meats, rum, and baked goods. It is also used in flavourings and as a preservative. Indoor exposure may result from burning wood or using air deodorizers.
The main routes of human exposure to acetaldehyde are:
Short-term exposure to acetaldehyde primarily affects the eyes, skin, and respiratory tract, causing irritation. At higher levels, individuals may experience redness, coughing, pulmonary edoema, and tissue necrosis. In animal studies, acute inhalation has resulted in slowed breathing and increased blood pressure. Toxicity tests in rats, rabbits, and hamsters suggest low toxicity via inhalation and moderate toxicity through oral or skin exposure.
Prolonged exposure to acetaldehyde may lead to symptoms similar to chronic alcohol intoxication. Animal studies in hamsters have shown tissue changes in the nasal cavity and trachea, slowed growth, mild anaemia, and increased kidney weight. The U.S. EPA has set a Reference Concentration (RfC) of 0.009 mg/m³ based on nasal epithelium damage in rats. No Reference Dose (RfD) has been established.
While no human data is available, studies in animals have shown that acetaldehyde can cross the placenta and accumulate in foetal tissue. Exposure has been associated with developmental effects such as skeletal malformations, lower birth weight, increased postnatal mortality, and embryonic resorptions.
Human data on acetaldehyde's carcinogenicity is limited and inconclusive due to insufficient sample sizes and confounding exposures. Animal studies have shown increased rates of nasal tumours in rats and laryngeal tumours in hamsters after inhalation exposure. The EPA classifies acetaldehyde as a Group B2 probable human carcinogen.
Acetaldehyde is highly flammable and forms explosive peroxides when exposed to air, heat, or light.
Firefighting Recommendations:
Decomposition produces acrid smoke and fumes. Hazardous polymerisation may occur with acids, bases, or trace metals.
For Large Spills:
Consult with a safety specialist to verify adequate protection measures are in place before handling acetaldehyde.
OSHA: The United States Occupational Safety & Health Administration has set the following Permissible Exposure Limits (PEL) for acetaldehyde:
ACGIH: The American Conference of Governmental Industrial Hygienists has set a Threshold Limit Value (TLV) for acetaldehyde of: 25 ppm, 45 mg/m³ Ceiling; Appendix A3 - Confirmed Animal Carcinogen with Unknown Relevance to Humans
NIOSH: The National Institute for Occupational Safety and Health has not established a Recommended Exposure Limit (REL) for acetaldehyde, Appendix A - NIOSH Potential Occupational Carcinogens; Appendix C - supplementary Exposure Limits (Aldehydes)
Safe Work Australia: Safe Work Australia has established the following exposure limits for acetaldehyde:
