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ImportanceSecondhand smoke (SHS) exposure potentially threatens ocular health; however, its association with myopia is unknown.ObjectiveTo examine the association between SHS exposure and childhood myopia.Design, Setting, and ParticipantsCross-sectional data from the population-based Hong Kong Children Eye Study were used. Data were collected from March 5, 2015, to September 12, 2021, at The Chinese University of Hong Kong Eye Center. Participants included children aged 6 to 8 years. Secondhand smoke exposure was evaluated using a validated questionnaire. All participants underwent comprehensive ophthalmic and physical examinations.ExposureSecondhand smoke exposure.Main Outcomes and MeasuresGeneralized estimating equations were constructed to examine the association of SHS exposure with spherical equivalent and axial length; logistic regression models, with myopia rate; and linear regression models, with myopia onset.ResultsA total of 12 630 children (mean [SD] age, 7.37 [0.88] years; 53.2% boys) were included in the analysis. Among the participants, 4092 (32.4%) had SHS exposure. After adjusting for age, sex, parental myopia, body mass index, near-work time, outdoor time, and family income, SHS exposure was associated with greater myopic refraction (β = −0.09 [95% CI, −0.14 to −0.03]) and longer axial length (β = 0.05 [95% CI, 0.02-0.08]). Children with SHS exposure were more likely to develop moderate (odds ratio [OR], 1.30 [95% CI, 1.06-1.59]) and high myopia (OR, 2.64 [95% CI, 1.48-4.69]). The association of SHS exposure with spherical equivalence and axial length was magnified in younger children. For each younger year of a child’s exposure to SHS, SHS exposure was associated with a 0.07-D decrease in spherical equivalence (β = 0.07 [95% CI, 0.01-0.13]) and a 0.05-mm increase in axial length (β = −0.05 [95% CI, −0.08 to −0.01]). Exposure to SHS was associated with an earlier mean (SD) age at onset of myopia (72.8 [0.9] vs 74.6 [0.6] months; P = .01). Every increase in SHS exposure in units of 10 cigarettes per day was associated with greater myopic refraction (β = −0.07 [95% CI, −0.11 to −0.02]), axial length (β = 0.04 [95% CI, 0.01-0.06]), and likelihood of developing moderate (OR, 1.23 [95% CI, 1.05-1.44]) and high myopia (OR, 1.75 [95% CI, 1.20-2.56]), and earlier myopia onset (β = −1.30 [95% CI, −2.32 to −0.27]).Conclusions and RelevanceThe findings of this cross-sectional study suggest that SHS exposure was associated with greater myopic refraction, longer axial length, greater likelihood of developing moderate and high myopia, and earlier myopia onset. The larger the quantity of SHS exposure and the younger the child, the more advanced myopia development and progression with which SHS exposure is associated.