Formaldehyde is a gas, whereas glutaraldehyde is an oil at room temperature. However, both are readily soluble in water. Their solutions are irritating or caustic to tissues, causing coagulation necrosis and protein precipitation, but have potent germicidal properties against all organisms, including spores. Their solutions do not lose appreciable antimicrobial properties in the presence of organic matter and are noncorrosive to metals, paints, and fabric. Both are used as disinfectants. Formalin contains 37% formaldehyde gas in aqueous solution with variable amounts of methyl alcohol to prevent polymerization. A 1%–10% solution of formaldehyde is commonly used as a disinfectant. Glutaral (glutaraldehyde), a 1%–2% alkaline solution (pH 7.5–8.5) in 70% isopropanol, is a more potent germicide than 4% formaldehyde, effective against all microorganisms, including viruses and spores. It is often used to sterilize surgical and endoscopic instruments and plastic and rubber apparatus. It is a known sensitizer, causing occupational contact dermatitis, as well as bronchial and laryngeal mucous membrane irritation.
Orthophthaldehyde (OPA) is an aromatic aldehyde similar to glutaraldehyde but with several potential advantages. Typical 0.55% solutions have excellent stability over a wide pH range (3–9), are less toxic and irritating to eyes and nasal passages, and have a barely perceptible odor. They are compatible with most materials, including flexible endoscopes. OPA solutions are faster acting than glutaraldehyde against mycobacteria but have somewhat less sporicidal activity. A potential disadvantage of OPA is that it stains proteins (including unprotected skin) gray, so it must be handled with caution.
Sulfur dioxide, as a gaseous fumigant, is produced by burning sulfur in closed spaces. For maximal effect, the surface should be moist, because the gas dissolves in water to form sulfurous acid, which is bactericidal. However, this reducing effect of the acid can also corrode metals, rot fabrics, and bleach dyes.