The first study of a large group of workers breathing air with very low levels of benzene suggests that the chemical may harm the bone marrow, the body’s main factory for blood cells, even in amounts below the threshold deemed safe under U.S. law.

The researchers said counts of certain protective white blood cells in 250 Chinese shoe factory workers exposed to small amounts of benzene — less than one part per million in the air — were 15 percent to 18 percent lower than counts in a similar group of 140 garment workers who were not exposed. The lower blood counts were not in a range deemed harmful, but independent experts said the findings strongly hinted that benzene was one of a small group of chemicals for which no safe threshold exists.

The study was conducted by scientists from the National Cancer Institute, China’s Center for Disease Control and Prevention, the University of California Berkeley, and several other institutions. It was published this week in the journal Science.

Benzene is one of the most heavily used solvents in the world, with applications in everything from tires to drugs, paper to refined sugar. It makes up about one percent of gasoline and is also produced when coal and other fuels are burned. Benzene has long been identified as a cause of leukemia and other blood ailments in people exposed to significant amounts over many years.

Experts not affiliated with the new study said it should prompt a re-evaluation of the American workplace standard, which OSHA set at one part per million in 1987, even though the National Institute of Occupational Safety and Health (NIOSH) recommended a standard of 0.1 parts per million.

Groups representing the chemical and oil industries, which have fought the progressive tightening of benzene standards over the last three decades, said they would need to analyze the study before commenting.

In the United States, overall air concentrations have dropped sharply in recent decades, although many workers are still exposed. The last thorough federal survey of worker exposure, released in 1987, estimated that more than 200,000 American workers, from gas station attendants to tire factory workers, were chronically exposed to some amount of benzene.

The new findings are a clear sign that even low levels of exposure pose a risk, said Dr. Robert A. Rinsky, an epidemiologist at Cincinnati Children’s Hospital and Medical Center.

Along with the depression of white-cell counts, the researchers measured a noticeable weakening of the ability of a tiny number of blood-forming cells, called progenitor cells, that circulate outside the bone marrow.

When samples of these cells from the exposed and unexposed workers were cultured in laboratories, those from people with the slight benzene exposure did not grow as readily.