Exposure to even small amounts of the chemical benzene may pose a health risk, say scientists.
Benzene is contained in traffic fumes
They have shown that workers who inhaled less than one part per million had fewer white blood cells than those who were not exposed.
Benzene is found in many sources, including second-hand cigarette smoke, petrol vapours and air pollution.
The research, by US and Chinese scientists, is published in the journal Science.
Scientists have known that workers in industries like oil and shipping who are
exposed to high doses of the substance run an increased risk of developing
But the potential dangers from smaller amounts of the chemical have been
The new study shows that even exposure to levels of the chemical that are considered safe under US guidelines appear to cause changes to the bone marrow.
The researchers compared 250 workers exposed to benzene-laden glues in two shoe factories in China to 140 workers who sewed clothes in other Chinese factories, but who did not come into contact with the chemical.
They measured benzene exposure by taking urine and blood samples and testing air in the factories, as well as at each worker's home.
As expected, workers exposed to benzene at levels of 1ppm and higher had fewer
white blood cells, such as granulocytes and B cells, than did unexposed workers.
But this also held true for the 109 workers exposed to less than 1ppm of benzene - even after controlling for smoking and other potential confounding factors.
These workers had on average 15% to 18% fewer granulocytes and B cells than
did unexposed workers.
The researchers say that although these workers showed no signs of ill health, the findings suggest that low doses of benzene may have a damaging impact on bone marrow which could lead to health problems.
White blood cells, which are produced in the bone marrow, play a key role in the body's ability to fight off infection and disease.
However, Dr Richard Irons, of the University of Colorado, who is leading an industry-funded study into the effect of benzene, said it was possible that the findings recorded by the study might be due to exposure to other chemicals, or factors such as nutrition.
The researchers also studied the effect of benzene on the progenitor cells found in the bone marrow that give rise to blood cells.
They found that the ability of progenitor cells to grow and multiply declined with higher exposures.
More work needed
Dr Richard McNally, of the Cancer Research UK Paediatric and Familial Cancer Research Group, said it would be wrong to draw firm conclusions from the study.
He said: "It does not show how low-level exposure to benzene affects the risk of leukaemia, if at all.
"What it does show is that low-level exposure to benzene can lead to reduced blood cell counts.
"The observed dose-response relationship suggests this is a real result.
"However, the effects occurred within a particular genetic sub-population.
"Whether exposure to benzene would have similar effects in the UK population would depend on how many people in the UK have the genetic susceptibility described in the report, and would need to be tested through further research."
Among the institutions who took part in the research were the US National Cancer
Institute in Bethesda, Maryland, the Chinese Centre for Disease Control
and Prevention in Beijing, the University of California, Berkeley.