It's no secret that smoking during pregnancy can pose health risks to babies, but US scientists believe it could also harm those babies' future children too.
Nicotine appears to switch on bad genes that increase a person's likelihood of suffering asthma, which is then passed on to future generations.
Studies have shown nicotine affects a foetus's developing lungs, which increases their risk of childhood asthma. Now researchers from the Harbor University of California Los Angeles Medical Centre believe nicotine affects an unborn baby's ovaries and testes, which could alter the health of their future children.
To test this, the researchers exposed pregnant rats to nicotine, then studied their pups (called F1) and their pup's pups (called F2).
They found the F1 and F2 pups both had reduced lung function, even though the F2 pups were not exposed to nicotine by their own mother.
"The effects of smoking during pregnancy are, it seems, very long lasting," said study leader Dr Virender Rehan.
"These epigenetic marks may be the mechanism behind how nicotine-induced asthma is transmitted from one generation to the next."
Dr Richard Saffery, who runs an epigenetics lab at the Murdoch Children's Research Institute, told ninemsn epigenetics literally means "on top of the DNA".
"It's all of the factors that associate with DNA to control gene expression," he said.
"Without epigenetics we wouldn't have different cell types. They've all got the same DNA but it is read different ways in different cells."
Epigenetics can switch genes on or off to control how different cells work –– and now it appears nicotine can harm that function in future generations.
"The epigenetic markers are sensitive to the environment and now it looks like that environmental sensitivity is transferable across generations," Dr Saffery said.
"This study is taking it to the next generation quite compellingly. It shows quite conclusively that you've got a two-generation effect. So grandmothers smoking while they are pregnant has the potential to alter the health of their grandchildren."
Dr Saffery said the research field is quite new.
"Up until a few years ago, we thought that sperm and eggs were basically epigenetically clean –– all of the epigenetic marks were thought to be erased, so when you get an embryo, it's starting with a clean slate," he said.
"Now we know that's not the case. Some really nice studies have shown in both humans and in rodents that sperm and eggs have their own epigenetic profile."
The research was published in the journal BMC Medicine.