A mother's exposure to airborne pollutants may increase the likelihood that her unborn child will later develop asthma, a new study suggests.

In a Danish review of registry data on 45,658 seven-year-old children and their mothers, 18.6 percent of children of mothers who were exposed to low-molecular-weight particles and irritants at work during pregnancy contracted asthma, compared to 16.1 percent of the general population.

Results like these should always be interpreted with caution since they may be caused by confounding from other lifestyle factors that are not easily adjusted for," Dr. Klaus Bonnelykke, who was not involved in the research, told Reuters Health by email.

"However," added Bonnelykke, of the Danish Pediatric Asthma Center, "there is increasing evidence that the prenatal period may be a critical period affecting the offspring's risk for later development of asthma and other (allergic) diseases."

The study, by researchers at the School of Public Health in Denmark, was presented September 26 at the European Respiratory Society's annual congress in Amsterdam.

For the study, Dr. Berit Christensen and colleagues used mothers' job titles to estimate their exposure to workplace pollutants, with categories for either low- or high-molecular-weight particles, mixed, farmers, "unclassifiable" and students, as well as a reference group of office workers for comparison.

After adjusting for age, body mass index, allergy and hypersensitivities, smoking, medication and pets, there was a slightly higher risk -- about 11 percent -- for asthma in children when their pregnant mothers were exposed to particles of both low molecular weight and high molecular weight.

The researchers found no asthma associations in the other exposure groups.

"This is the first large-scale study which has shown an association between maternal exposures during work and asthma in children," Dr. Christensen said in a statement.

"Whilst a link has been found, our results at this stage are modest and further research is needed into specific chemicals and substances to determine those that could be most harmful."