Researchers have discovered that prenatal exposure to secondhand smoke can have adverse effects on lung health that persist into adulthood.
A report on the study - which was led by researchers at Louisiana State University in Baton Rouge - is published in the journal Respiratory Research.
Secondhand smoke is that produced by the burning of tobacco products such as cigars, cigarettes, and pipes that can be inhaled by people nearby.
Breathing in secondhand smoke is also known as passive smoking. Smoke that is exhaled by someone who is smoking is also classed as secondhand smoke.
Hundreds of the 7,000 chemicals in tobacco smoke are toxic - that is, they cause some degree of harm to the body. These include 70 that can cause cancer.
Around 890,000 of the 7 million people killed by tobacco worldwide every year are nonsmokers who are exposed to secondhand smoke.
In the United States, around 2.5 million nonsmokers have died in the past 50 years from diseases caused by inhaling secondhand smoke.
Exposure is declining
Tests for tobacco biomarkers in the saliva, urine, and blood of nonsmokers show that exposure to secondhand smoke has fallen steadily in the U.S. in recent decades.
In the years 2011 to 2012, around 25 percent of nonsmokers had measurable levels of tobacco biomarkers, compared with nearly 90 percent during the years between 1988 and 1991.
However, despite measures to reduce smoking, a significant number of nonsmokers - approximately 58 million between 2011 and 2012 - are still exposed to secondhand smoke in the U.S.
In their study report, the researchers cite well documented evidence that offspring of women who smoked during pregnancy have altered lung function and a higher rate of respiratory problems.
They also note that there is growing evidence that exposure to secondhand smoke during pregnancy can affect the development of the unborn baby and result in a low birthweight and a higher risk of disease in adulthood.
Damage to lung tissue
However, what is not so well known is the extent to which exposure to secondhand smoke in pregnancy may affect fetal lung development, and if it does, how long the effects endure.
To investigate this further, the team exposed one group of pregnant mice to secondhand smoke mixed with filtered air and exposed another group just to filtered air. When the offspring were born, they were then raised only in filtered air.
The researchers used filtered air in order to remove any potential influence from air particles not arising from secondhand smoke.
When the offspring reached adulthood at 15 weeks old, the researchers carried out a series of exams on their lungs, including measurement of lung function, tests for tissue damage, and molecular analysis. They compared the results of the exposed mice to those of the non-exposed mice.
The results showed that adult mice that had been exposed to secondhand smoke before birth had changes in their lungs that suggested tissue damage.
Lung function tests in adult mice showed significantly lower tidal volume (how much air is inhaled and exhaled per breath during normal breathing) and minute volume (how much is inhaled and exhaled per minute) in the males that had been exposed to secondhand smoke before birth, but not the females.
The molecular analysis showed that the lung tissue of the exposed mice had several altered genes.
One of the altered genes, called alpha-1-antitrypsin, is also common to humans. Deficiency in the protein that the gene codes for can raise the risk of emphysema and other diseases.
The researchers note that their findings are consistent with studies that show lung development differs in male and female fetuses, and they suggest that "male mice may be more susceptible than female mice to insults occurring during lung development."
The study is limited by the fact that it was carried out on mice under experimental conditions. Secondhand smoke may not have the same effect on human fetuses, and the levels used in the experiment may not be the same as those encountered by pregnant women in real life.
Nonetheless, the authors suggest that their findings may explain the link between secondhand smoke exposure before birth and a higher risk of respiratory diseases later in life.
"Our study strongly indicates that in utero secondhand smoke exposure alone has significant persistent repercussions on the respiratory system, suggesting that in utero secondhand smoke exposure can predispose to adult lung diseases in mice."
Senior author Prof. Arthur Penn, Louisiana State University