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Smoking Raises Grandkids' Asthma Risks Long Before They're Born

A new study finds a link between grandmothers' smoking and their grandchildren developing asthma and wheezing.
(Photo: Jack Lyons/Flickr)

(Photo: Jack Lyons/Flickr)

Mothers-to-be, here's another reason to put down the cigarettes: You're increasing the risk any future grandchildren will develop asthma, according to research presented this week at the European Respiratory Society International Conference. In fact, maternal grandmothers who smoke could increase their grandchildren's risk by up to 27 percent, the study argues.

Exactly what causes asthma isn't entirely clear. It's less common in rural areas than urban ones, suggesting the "hygiene hypothesis"—basically that our modern emphasis on disinfecting everything has changed the way children's immune systems develop. But researchers have known (perhaps since the 17th century) that asthma runs in families, and twin studies suggest it's hereditary, though the patterns and mechanism of its inheritance aren't well understood.

The rough idea is that smoking affects not only a fetus' genes, but also the reproductive material already under development in a fetus while still in the womb.

One possibility, a team led by Caroline Lodge, a researcher at the University of Melbourne School of Population and Global Health, posits, is that the inheritance might be epigenetic. Basically, epigenetics refers to the complex chemical interactions that switch genes off and on—for example, the interactions responsible for eye color or the onset of puberty. The idea isn't new: Despite recent hype, the chemical processes that underlie epigenetics were first linked to cancer in 1983. But recently, epigenetics has gained attention as a way to understand previously confusing genetic effects, including asthma. Back in 2012, researchers discovered that when they exposed rats to cigarette smoke during pregnancy, both their children and their grandchildren were more likely to develop asthma. The rough idea is that smoking affects not only a fetus' genes, but also the reproductive material—the eggs, that is—already under development in a fetus while still in the womb.

Lodge and her team's question, then, was whether the same could happen in people. Fortunately, they had a good source of data: the Swedish Registries, a vast collection of Swedes' health information maintained by the Swedish government. The researchers focused on data covering 44,583 women who gave birth to a child between 1982 and 1986; 46,197 of their daughters who had children of their own; and 66,271 of those kids. The Registries also included whether children had been treated for asthma—specifically, whether they'd purchased asthma medications—and how much their grandmothers and mothers smoked.

Analyzing the data, Lodge and her colleagues found that young children whose grandmothers smoked—and whose mothers didn't—were between 13 and 27 percent more likely to develop asthma and wheezing, compared to children whose grandmothers and mothers didn't smoke. (The exact number depended on the kids' age.) Interestingly, grandmaternal smoking was linked to persistent asthma and not transient asthma, a milder form that doesn't require long-term treatment. Mothers' smoking, in contrast, was more likely to lead to transient asthma, but had no effect on persistent asthma.

In addition to furthering medical researchers' understanding of asthma and other heritable diseases, the results may help explain why asthma incidence is on the rise, the authors argue.

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