Maleic anhydride

Maleic anhydride was first prepared in 1834 and its industrial production began in the 20th century via the vapor-phase catalytic oxidation of benzene (and later, n-butane). [1] It is a high-production-volume chemical — annual US production exceeds 800 million pounds — and is the fourth largest volume organic anhydride produced globally. Its primary use is in the production of unsaturated polyester resins (UPR), which are made by reacting maleic anhydride with glycols; these resins, reinforced with fiberglass, form fiberglass-reinforced plastics (FRP) used in boats, automotive parts, bathtubs, bathroom fixtures, construction panels, and wind turbine blades. Other applications include maleic acid and fumaric acid production (food acidulants), lubricant additives, agricultural chemical intermediates, and as a reactive modifier for polymers and resins. [2] Maleic anhydride is a potent electrophile that can react with nucleophilic groups in proteins and DNA — this reactivity makes it an excellent crosslinker in polymer chemistry, but also the basis for its biological sensitization properties. Occupational asthma from maleic anhydride inhalation has been documented in multiple industries, with both IgE-mediated and non-IgE-mediated mechanisms. [3] The EPA lists it as a HAP based on respiratory tract toxicity and carcinogenicity potential.

Chemical manufacturing workers in maleic anhydride production, UPR manufacturing workers, fiberglass product fabricators (who heat UPR resins releasing maleic anhydride), and workers in lubricant additive production face the highest occupational inhalation and skin exposures. Maleic anhydride fumes can be generated when fiberglass-reinforced products are sanded, cut, or thermally processed. The general public has negligible ambient exposure from environmental releases.

Occupational asthma is the primary concern — once sensitized, workers develop asthmatic reactions to even trace exposures, requiring job change. [3] Skin sensitization and contact dermatitis also occur. Maleic anhydride is a severe eye, skin, and mucous membrane irritant — accidental splashes cause chemical burns. Animal carcinogenicity studies show tumors at high doses, supporting the HAP classification. The global shift to n-butane as the feedstock for maleic anhydride production (replacing benzene) has reduced associated benzene exposures but has not changed the maleic anhydride-specific hazards.

Chemical plant workers manufacturing maleic anhydride and polyester resins, fiberglass composite workers in boat building and automotive parts manufacturing, wind turbine blade fabricators, and workers who thermally process or sand fiberglass parts are most at risk.

1. Engineering controls (local exhaust ventilation, closed systems) are primary — organic vapor respirators must cover maleic anhydride for any residual exposures. 2. Workers should wear protective clothing and chemical splash goggles; maleic anhydride fumes can cause conjunctivitis before they cause lower respiratory symptoms. 3. Pre-placement and periodic spirometry for workers with significant inhalation exposures. 4. Any worker with new-onset respiratory symptoms after maleic anhydride exposure should be removed and evaluated for occupational asthma. 5. Cool maleic anhydride vessels carefully — hot molten anhydride and its vapors are far more hazardous than solid or cool material.