Secondhand Smoke Linked to Myopia

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Secondhand smoke (SHS) exposure in children is associated with in­creased myopic refraction, longer axial length, greater likelihood of develop­ing myopia, and earlier myopia onset, according to the authors of a recent cross-sectional study.

“Our findings add to the evidence of the harmful effects of SHS exposure on children’s ocular health and suggest that eliminating SHS could contribute, albeit little, to myopia prevention among children, particularly in families with young children,” said corresponding author Jason C.S. Yam, MPH, at the Chi­nese University of Hong Kong (CUHK).

DOSE-RESPONSE ASSOCIATION. Exposure to secondhand smoke at a young age was linked with earlier myopia onset.

Methodology and measures. CUHK scientists combed through data from the population-based Hong Kong Children Eye Study collected between March 2015 and September 2021. More than 12,600 children ages 6 to 8 years old were included in the analysis, and all had received comprehensive ophthal­mic and physical exams. Their expo­sure to SHS was evaluated based on a validated questionnaire.¹

After adjusting for covariates—in­cluding age, sex, and parental myopia—SHS exposure was associated with greater myopic refraction and longer axial length. The likelihood of develop­ing moderate (–3 D to –6 D) and high myopia (–6 D or worse) was higher for youngsters with SHS exposure (odds ratios, 1.30 and 2.64, respectively). In addition, SHS exposure was associated with an earlier age at myopia onset (72.8 vs. 74.6 months; P = 0.01).

A dose-dependent risk. The study also showed that every increase in SHS exposure, measured in units of 10 cigarettes per day, was associated with 0.07 D more myopia.

Ian Morgan, PhD, at the Australian National University, who was not involved in the study, wrote in an accompanying commentary, “The statistically significant effect sizes were small, but consistent, and showed a dose-response association with severity of exposure and younger age.”² He said the analysis shows the power of analyzing factors associated with the risk of myopia in a well-designed study that uses a large sample of children of a similar age to minimize age-related confounding. “Performing such studies is hard work, primarily in persuading parents to give informed consent, but the effort pays off. This study is the most precise evidence of an association between exposure to SHS and myopia in children currently available,” Dr. Morgan wrote.

Policies and interventions. Efforts to reduce SHS exposure among children could include educational campaigns in schools to raise awareness about the dangers as well as smoking cessation programs for parents and other family members, said Dr. Yam. “By reducing SHS exposure among children, it may be possible to reduce the risk of childhood myopia and other adverse health outcomes associated with SHS,” he said. The authors noted in the study that SHS exposure has also been associated with asthma, and respiratory tract and ear infections in pediatric populations.

Dr. Morgan wrote that when it comes to reducing myopia in youth, “changes in the school system to reduce early educational pressures and promote more time outdoors, where causality has been established, are likely to be more important.” He pointed to China as an example, noting that President Xi made control of the myopia epidemic a major issue in the education system.

Looking ahead. “Our findings suggest that the risk of SHS exposure to children’s eye health is more serious than we expected, as a higher severity of myopia is related to an increased risk of sight-threatening disease,” Dr. Yam said.

The authors wrote that whether reducing SHS exposure can prevent or slow the progression of myopia in children—and whether there are critical periods during childhood when SHS is most harmful to ocular health—are questions that remain to be determined through research that looks at causality.

Still, they concluded, “Eliminating SHS exposure for eye care among children is important, particularly in families with young children.”

—Christos Evangelou, PhD

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1 Zhang Y et al. JAMA Netw Open. 2023;6(5):e2313006.

2 Morgan IG. JAMA Netw Open. 2023;6(5):e2312995.

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Relevant financial disclosures: Dr. Yam—Research Grants Council, Hong Kong: S; Collaborative Research Fund: S; Innovation and Technology Fund: S; UBS Optimus Foundation: S; Centaline Myopia Fund: S; National Natural Science Foun­dation of China: S; CUHK: S; CUHK Jockey Club Children’s Eye Care Programme: S; CUHK Jockey Club Myopia Prevention Programme: S.

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