Many aldehydes are either gases or volatile liquids, and are highly flammable (with flash points below 100 degrees F). Especially dangerous are formaldehyde, acetaldehyde, acrolein, crotonaldehyde, and other aldehydes that have wide flammability limits.

Aldehydes are frequently involved in self-condensation or polymerization reactions. These reactions are exothermic; they are often catalyzed by acid and bases. Aldehydes are readily oxidized to give carboxylic acids. Flammable and/or toxic gases are generated by the combination of aldehydes with azo, diazo compounds, dithiocarbamates, nitrides, and strong reducing agents. Aldehydes can react with air to give peroxo acids initially, and ultimately carboxylic acids. These autoxidation reactions are activated by light, catalyzed by salts of transition metals, and are autocatalytic (catalyzed by the products of the reaction). The addition of stabilizers (anti-oxidants) to shipments of aldehydes retards autoxidation. Autoxidation can be accelerated if the aldehyde is dispersed on a high surface area solid (ex. paper towels, rags, absorbents, insulation, etc.), potentially leading to a fire in air.

Variable. Aldehydes act by inhalation, by ingestion, and by contact with the skin. Their exact metabolic effect depends upon the route of exposure. Among the most toxic are acrolein and crotonaldehyde, which are known carcinogens. Aldehydes are known sensitizers for small populations of humans and serve to cause chemically induced allergic reactions. The effects of these reactions can be dramatic at rather low concentrations. The effects of formaldehyde and acetaldehyde have been well documented because these aldehydes are found in many synthetic products, such as clothing, insulation, and building products. These two aldehydes are also naturally occurring as combustion products (e.g., of forest fires).

Aldehydes contain a carbonyl group (C=O group) to which one hydrogen atom and either an aliphatic or aromatic organic group are attached. Compounds of this group are formed by the oxidation of alcohols.

Formaldehyde, butyraldehyde, acetaldehyde, benzaldehyde, crotonaldehyde, acrolein, propionaldehyde, valeraldehyde, butyraldehyde, isodecaldehyde.