Asthma rates have steadily increased across the globe for the past several decades. According to the CDC asthma now affects 9.3% of kids in the USA. Now, a new study from Sweden claims part of the problem could be smoking grandmothers. According to the study, if your grandmother smoked during her pregnancy, you’re 10 to 22% more likely to develop asthma, even if your mother didn’t smoke during hers.

The study looked at data from the Swedish Registry on 44,853 different grandmothers and their 66,271 grandchildren. “We found that smoking in previous generations can influence the risk of asthma in subsequent generations,” said Dr. Caroline Lodge, one the of the study’s authors. “This may also be important in the transmission of other exposures and diseases.”

How might this work? Exposure to tobacco can affect the not-yet-born mother’s genes through a process called epigenetic modification. These genes are then passed onto the grandchild, making him or her more at risk for developing asthma. Some scientists have theorized that multigenerational transmission may explain “why 98% of inherited human diseases are unaccounted for by the prevailing view of genetic trait transmission, known as Mendelian genetics.”

But, it’s also possible that growing up with a smoking grandmother is what causes the increased asthma rate. After all, secondhand smoke is a risk factor for developing asthma.

Next, the scientists plan to research whether a grandma smoking during pregnancy can affect their grandchildren’s asthma rate if the grandma was pregnant with the grandchild’s father and not their mother at the time.