PARIS: Regular exposure to secondhand smoke is more common among nonsmoking teens with depression symptoms, according to a survey across 22 low- and middle-income countries.
While there has been plenty of research suggesting a direct link between smoking and depression in teenagers, little is known about how passive smoking might also be linked to depression, said study coauthor Louis Jacob of the University of Versailles Saint-Quentin-en-Yvelines in Montigny-le-Bretonneux, France.
"Adolescence is a critical period of life and adolescent depression increases the odds of mental and physical conditions later in life," Jacob told Reuters Health in an email.
Jacob's team analysed publicly available data from annual student health surveys administered to school-going children in 2003-2008.
Among other questions, participants were asked on how many days they were exposed to secondhand smoke during the past week.
Depressive symptoms were assessed with the question, "During the past 12 months, did you ever feel so sad or hopeless almost every day for 2 weeks or more in a row that you stopped doing your usual activities?"
Among 37,505 nonsmoking participants, ages 12 to 15, 23 per cent of those with no secondhand smoke exposure reported depressive symptoms, compared with 29 per cent of adolescents exposed to secondhand smoke every day in the past week, Jacob's team reports in the American Journal of Preventive Medicine.
After adjusting for age, family poverty level and other factors that might influence depression risk, researchers calculated that kids exposed to secondhand smoke on three to six days a week were 38 per cent more likely to report depression symptoms compared to kids who were not exposed. Kids exposed seven days a week were at 68 per cent higher risk.
The authors cite several factors that could link exposure to smoke to depressive symptoms, including stress brought on by physical discomfort such as coughing or eye irritation and nicotine exposure from being around smokers that could affect neurotransmitters, noted Frank Bandiera, a health scientist administrator at the U.S. National Institutes of Health, who wasn't involved in the study.
"I agree that those might be reasons for their findings," Bandiera, who has researched secondhand smoke exposure and mental health in children and teens, said in a phone interview.
"The brain is still developing at this age, (adolescents) can be exposed to secondhand smoking through their parents and for prevention it is better to catch them at an earlier age."
However, Dr Jonathan Samet, dean of the Colorado School of Public Health in Aurora, who was not involved in the study, said the findings don't make clear how secondhand smoke by itself could cause depressive symptoms.
"They talk about ways that could happen, but the mechanism is a little bit challenging to identify," he noted.
The authors acknowledge that the study looked at a single point in time, so it cannot establish cause and effect.
They also lacked information about other factors that could influence depression risk, smoke exposure, or both, such as family mental health history and family structure and circumstances.
In addition, the researchers lacked details about the location, duration and intensity of the participants' secondhand smoke exposure.
Even so, Jacob believes the data support calls for better implementation of smoke-free air policies and more strategies aimed at reducing the number of smokers in developing nations.
"Although one has to bear in mind that the association between secondhand smoking and depressive symptoms may be bidirectional, we believe that depressive symptoms being caused by secondhand smoking is the most likely hypothesis," Jacob said.