People exposed to secondhand cigarette smoke during adolescence may be more likely to develop multiple sclerosis (MS) later in life, a study suggests.
The study, “Exposure to passive smoking during adolescence is associated with an increased risk of developing multiple sclerosis,” was published in the Multiple Sclerosis Journal.
Environmental (non-genetic) factors can influence an individual’s risk of developing MS. In particular, smoking cigarettes has been shown to increase MS risk. Exposure to secondhand smoke has also been linked to a higher risk of developing MS, though most research has focused on exposure during adulthood.
Environmental factors may have more of an impact at certain times during a person’s development, such as in adolescence, but the effect of secondhand smoke exposure during this time and MS risk later in life is unclear.
To address this question, researchers at the Copenhagen University Hospital Rigshospitalet, Denmark, analyzed data from standardized questionnaires given to adults with MS and healthy blood donors (control group). All individuals were born in, and had parents from, Denmark, Norway, Sweden, Iceland, or the Faroe Islands — this ethnogeographic limitation was chosen to limit the effects of genetic variation in the sample.
Researchers then classified each individual based on whether they had been exposed to secondhand smoke during adolescence (between the ages of 10 and 19) or not, based on answers to questions such as, “Have you ever lived with one or more persons, who smoked indoor on a daily basis?”
In total, the analysis included data for 919 people with MS (649 female, 270 male) and 3,419 controls (1,477 females, 1,942 males) who had not smoked before the age of 19.
The team compared the rates of secondhand smoke exposure during adolescence between people with or without MS. The comparisons were done within each sex and based on adult (i.e., after age 19) smoking status. Statistical adjustments were made to account for other factors, including education, alcohol consumption, and body mass index.
Statistically significant increases in MS risk were identified for males who were active smokers after age 19, with an increased risk of about 59%, and for females who were not active smokers after age 19, with a roughly 43% higher risk.
“We found exposure to passive smoking in adolescence to be a strong risk factor for developing MS later in the life in women, increasing the odds by 43% in never-active smokers,” the researchers wrote.
For males who were not active smokers after age 19, secondhand smoke exposure during adolescence did not significantly alter MS risk.
“The term active smoking after the age of 19 was not a significant predictor of MS, neither in males nor in females,” the researchers wrote, adding that “exposure to passive smoking at the age of 10–19 did not predict post-adult active smoking in men.”
When combining the groups based on sex and making appropriate statistical adjustments to account for smoking status later in life, the researchers found a statistically significant increase in MS risk for males exposed to secondhand smoke during adolescence, but no statistically significant association was found for females.
Notably, the above results are from analyses that considered secondhand smoke exposure as a binary variable — that is, each individual either was or was not exposed. The researchers also performed an analysis based on how long each individual was exposed to secondhand smoke during adolescence (in years), though they noted that the original questionnaire was not designed to obtain this kind of numeric data, so caution should be taken in interpreting the results.
Among females who did not smoke before age 19, each year of secondhand smoke exposure during adolescence was associated with a statistically significant increase in MS risk of approximately 4.6%. No statistically significant results were found for other groups.
“We found that MS patients more often than healthy controls reported exposure to passive smoking, which supports the findings of previous studies looking at adults,” the researchers said.
“Elaborated knowledge about the pivotal effects of the environmental factors are crucial in the efforts for reducing the risk of MS, since environmental factors in many cases can be altered, and it may call attention to hitherto unclear pathogenic mechanisms,” they added.
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