California’s top appeals court has ruled that insurance won’t cover gradual pollution, shutting down Montrose Chemical’s bid for broader environmental liability coverage.
A California appeals court has rejected Montrose Chemical Corporation of California’s attempt to expand insurance coverage for environmental damage, ruling that gradual pollution is not covered under the company’s comprehensive general liability (CGL) policies. The decision, issued on September 30, 2025, addresses the meaning of “sudden” in pollution exclusion clauses – a topic that has been the subject of repeated litigation in California.
The case arose from Montrose’s operation of a DDT plant in Torrance, California. Montrose sought a court order requiring its insurers to cover environmental damage, arguing that the word “sudden” in the relevant pollution exclusions should be interpreted to mean “unexpected,” not just “abrupt.” The company attempted to introduce extrinsic evidence, including insurance industry drafting history, to support its interpretation.
The Court of Appeal for the State of California, Second Appellate District, Division Three, held that “sudden” in the context of the qualified pollution exclusions (QPEs) is not reasonably susceptible to include gradual pollution events. The court cited prior California appellate decisions, including Shell Oil Co. v. Winterthur Swiss Ins. Co. and ACL Technologies, Inc. v. Northbrook Property & Casualty Ins. Co., which found that “sudden” and “gradual” are antonyms and that the policy language is not ambiguous.
The insurance policies at issue excluded coverage for bodily injury or property damage arising out of the discharge, dispersal, release, or escape of pollutants, except where such events were “sudden and accidental” or caused by a “sudden, unintended and unexpected happening.” Montrose argued that these phrases could include gradual, unintentional pollution, but the court disagreed, finding that “sudden” requires a temporal, not just unexpected, quality.
The court also addressed the admissibility of extrinsic evidence in interpreting insurance contracts, stating that such evidence is only relevant if the policy language is reasonably susceptible to the meaning advanced by the party. In this case, the court found that the language was not ambiguous and that all prior California appellate courts had rejected Montrose’s interpretation.
As a result, the court denied Montrose’s petition for a writ of mandate and upheld the trial court’s exclusion of extrinsic evidence. The decision leaves Montrose responsible for environmental liabilities arising from gradual pollution events at its DDT plant.
This ruling provides clarity for insurance professionals regarding the interpretation of pollution exclusions in CGL policies, confirming that coverage is limited to abrupt, not gradual, pollution events, in line with established California law.
