A mother's exposure to airborne pollutants at work during her pregnancy may increase the likelihood that her unborn child will later develop asthma, a Danish study says.

The review of registry data on 45,658 children, aged 7, and their mothers found that 18.6 percent of children of mothers who were exposed to low-molecular-weight particles at work during pregnancy developed asthma, compared to 16.1 percent of the general population.

"This is the first large-scale study which has shown an association between maternal exposures during work and asthma in children," says study leader Berit Christensen, at the School of Public Health in Denmark, in a statement.

For the study, 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.