Factory workers exposed to high levels of formaldehyde were more likely to die of cancers of the blood and lymphatic system than workers with low-level exposures, according to a study by researchers at the National Cancer Institute.

But the risk of dying of these cancers diminished over time after the exposure stopped, said Laura E. Beane Freeman, lead author of the study, which was published online Tuesday in The Journal of the National Cancer Institute.

The research looked at some 14,000 deaths among 25,619 workers, most of them white men, who began working before 1966 at 10 plants that produced formaldehyde and formaldehyde resin.

In the four ensuing decades, the researchers found, workers with the highest peak exposures to formaldehyde had a 37 percent greater risk of death from all blood and lymphatic cancers combined than those with lower peak exposures.