What is fourthhand smoke, you ask?
I coined the term to represent the effects that fetal exposure to tobacco smoke might have on the subsequent child's children, even though those individuals might have no exposure to firsthand, secondhand, or thirdhand smoke.
What, then, is fifthhand smoke?
I coined the term fifthhand smoke to represent the effects that fetal exposure to tobacco smoke might have on the subsequent child's grandchildren.
Sound farfetched? Is it difficult to imagine a biological mechanism by which fetal exposure to tobacco smoke could cause asthma in a child that has not yet even been conceived, or in the grandchildren of the fetus?
The Rest of the Story
This is precisely what the UCLA researchers are hypothesizing to be the case. The study aims to investigate the potential effects of fourthhand and fifthhand smoke on asthma incidence one and two generations removed from the exposed fetus.
According to the article: "Virender Rehan, M.D., principal investigator at The Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center (LA BioMed), focuses on understanding the detrimental effects of maternal smoking, not only on the exposed offspring but also on the many generations that follow. More specifically, the proposed study will determine if the risk of childhood asthma induced following exposure to cigarette smoke during pregnancy is limited only to the offspring of the exposed pregnancy, or if this risk is carried to grandchildren or even great-grandchildren. “While it is widely known that maternal smoking can pose problems for an unborn child, including asthma, this study is important in that it sheds light on the depth of the issue and raises concerns about the effects of smoke exposure during pregnancy on subsequent generations,” said Dr. Rehan. Studies have shown that exposure to nicotine in utero affects lung growth and differentiation by altering specific mechanisms that are necessary for fetal lung development, which often results in an offspring’s predisposition to asthma."
A study published in 2005 in the journal Chest found that children whose grandmothers smoked during their mother's pregnancy with them were more likely to develop asthma, even after controlling for their own mother's history of asthma and for their own in utero exposure to tobacco smoke. This has led some to speculate that in utero tobacco smoke exposure may have a transgenerational effect on asthma incidence. The hypothesis is that in utero exposure to tobacco smoke could affect generations of offspring down the road through epigenetic mechanisms. Specifically, the authors of the Chest article hypothesize as follows:
"It is possible that by altering DNA methylation patterns in the fetal oocytes, tobacco-derived products may affect both immune function and xenobiotic detoxification mechanisms in the offspring, resulting in an increased susceptibility to asthma affecting one generation to the next."
Epigenetics refers to changes in gene expression that can be passed on to subsequent generations but which are not the result of actual changes to the DNA sequencing in the genes (as are mutations, for example). Instead, epigenetic changes may involve DNA methylation and histone modifications, which are capable of affecting gene expression (though not technically changing the genes themselves).
So if you thought that thirdhand smoke was a stretch, get ready for what I'm sure will be a slew of studies on fourthhand and fifthhand smoke. The emergence of the field of epigenetics has opened the door to the possibility that tobacco smoke exposure now could have effects on generations to come. This could be a field day for the anti-smoking movement.
What's not clear to me, however, is why all of this matters. How is it going to affect public health practice or policy? It seems to me that the results of the research do not matter. If the study finds that smoke exposure has transgenerational effects, then what we need to do is develop and deliver more effective interventions to reduce smoking during pregnancy. If the study finds, instead, that smoke exposure does not have transgenerational effects, then what we need to do is develop and deliver more effective interventions to reduce smoking during pregnancy. Shouldn't the money be going into the development and delivery of interventions to reduce smoking during pregnancy, rather than into studying the methylation patterns of fetal oocytes?
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