BRIGHTON, England — Several of the world's long-established terrorism insurance pools were set up when the threat of terrorism differed significantly from the situation today, according to a technical guide published Wednesday by Airmic Ltd. and Willis Group Holdings P.L.C.

Many terrorism pooling arrangements were established to cover losses from domestic or separatist terrorist acts, according to the guide launched at Airmic's annual risk management conference in Brighton, England.

For example, the U.K. terrorism pool, London-based Pool Reinsurance Co. Ltd., was established in the 1980s to cover losses from Irish Republican terrorism activity that were not covered by the mainstream insurance market, according to the guide.

Pool Re rates fell as the peace process in Ireland progressed and the threat lessened, but there now are other factors that may affect rates for terrorism coverage.

“Globally speaking, the terrorism threat is complex, nuanced and constantly evolving,” James Borrie, an executive director in Willis' terrorism and political violence practice and one of the authors of the guide, said in a statement.

While exposure to domestic, separatist terrorist attacks may have lessened in certain parts of the world, there are new, different threats, he said.

The threat of a “lone wolf” attack, such as the killing of a British Army soldier in Woolwich, London, last month, represents a different sort of risk compared with that of domestic separatist terrorism.

And while several terrorism insurance plans were set up in response to the Sept. 11, 2001, attacks in the United States, the guide illustrates that not all insurance offers the same coverage.

“This makes the design of a truly comprehensive global terrorism insurance policy very complex,” Mr. Borrie said in the statement.

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The guide also highlights the potentially diverse range of weapons that may be used by terrorists, including chemical weapons, which may result in losses that can include product recalls or business interruption.

According to the guide, a company's decisions on participating in a terrorism pool or purchasing terrorism insurance coverage from the commercial market likely will depend upon the nature of the company and the threat levels in the countries in which it operates.

When evaluating the coverage and terms and conditions offered by terrorism pools, buyers should consider the claims experience of the pool before purchasing coverage, the guide noted.