California’s workers’ compensation insurers lost more than $1.5 billion in their underwriting in 2009 due to a battered economy and lower workers’ compensation rates, according to a summary of recently released workers’ comp data.

Workers’ comp insurance companies earned premium of $9.1 billion in 2009, a 16.5 percent decrease from the $10.9 billion they earned the previous year, the California Workers’ Compensation Institute noted in an overview of 2009 insurer data released by the California Workers’ Compensation Insurance Rating Bureau of California.

The Rating Bureau, a nonprofit research organization, prepared the summary of calendar year losses and expenses for California’s lawmakers and governor. The California Workers’ Compensation Institute, in turn, condensed and summarized those findings.

The number of workers’ comp claims fell in 2009, but insurers continued to see costlier claims.

“As usual, medical treatment remained the biggest single cost category in California workers’ compensation, accounting for just over one-third of total expenditures last year,” the Oakland-based institute wrote in a bulletin.

Medical care payments rose to $3.68 billion in 2009, from $3.64 billion in 2008.

Payments for lost work time made up a quarter of insurers’ expenditures in 2009, the institute noted. Of those indemnity payments, temporary disability was the largest claims category, accounting for $1.36 billion, or 48.3 percent.

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