Children exposed in the womb to two chlorinated chemicals widely found in the environment are more likely to develop asthma by the age of 20, according to new research in Denmark.

The study is the first to link asthma to hexachlorobenzene exposure during fetal development, and builds on two earlier studies that linked the respiratory disease to polychlorinated biphenyls.

The blood of 872 Danish women was tested for persistent organic pollutants. The offspring of women in the group with the highest concentrations of HCB and PCB-118 were about twice as likely to be on asthma medication between the ages of 6 and 20 as the offspring of women with the lowest concentrations. A weaker association was found for five other PCB compounds and asthma.

PCBs were widely used for decades, especially in electrical equipment, before they were banned in 1977 because they were building up in the environment. HCB was used as a pesticide until 1965. All commercial use is banned, although some industries release it as a byproduct during chemical manufacturing.

Both compounds, known as persistent organic compounds or POPs, do not easily degrade and can accumulate in the tissue of humans and animals. People are exposed mostly by eating fish.

The authors wrote that the health effects of the banned compounds “will continue to be relevant to populations with high risks of occupational exposures and to communities with high consumption of fish and other marine species.”

Previous human and animal research suggests that PCBs and HCB harm developing immune systems, which may play a key role in asthma. Asthma is a chronic immune disorder that inflames airways and obstructs breathing.

No link to asthma was found in the study for a breakdown product of the pesticide DDT, although two previous studies of younger children had reported a link. “Their findings may therefore at least partly reflect associations with wheezing symptoms that often resolve later in childhood,” the authors wrote.

Other studies examining persistent organic pollutant exposure during development and respiratory problems have looked at children younger than 7 years old.

PCB exposure during development previously has been associated with wheezing and asthma. There are no known other studies of HCB exposure in the womb and asthma.

The researchers said the offspring also may have been exposed to the contaminants during breastfeeding, which complicates the reliability of the link to prenatal exposure.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.