Passive smoking: Does it influence risk of childhood atopic eczema, food allergy?

A systematic review and meta-analysis from Singapore demonstrates that parental smoking is not a strong risk factor for offspring eczema, food allergy, and food sensitization development.

A total of 32 studies with nearly 190K participants were deemed eligible for inclusion in the meta-analysis. [Pediatr Allergy Immunol 2023;34:e14010]

“Despite the widely reported detrimental effects of smoke exposure in children … we demonstrated that parental smoking during the critical phases of development (ie, preconception, pregnancy, postnatal periods) was generally not associated with the risk of eczema, food allergy, and food sensitization development in the offspring by age 3 years,” said the researchers.

Only maternal passive smoking during pregnancy was positively associated with clinician assessment of offspring eczema (risk ratio [RR], 1.38) on subgroup analysis by outcome assessment type. This finding was based on evidence from one study. [Pediatr Allergy Immunol 2019;30:540-546]

“Although the evidence supporting [this result] is limited, it remains an important finding given the high prevalence of pregnant women exposed to second-hand smoke worldwide,” the researchers said. Second-hand smoke can alter immune reactions and increase susceptibility to allergies. It triggers higher levels of immunoglobulin A and basophils, which are associated with allergies and hypersensitivity reactions. [Int J Environ Res Public Health 2018;15:1033; Indian J Clin Biochem 2019;34:143-154]

Negative correlations

Conversely, a negative association was seen between maternal active smoking during pregnancy and self-reported doctor diagnosis of eczema (RR, 0.87). The mechanism linking the two variables may not be biological since the association with clinician assessment of eczema was not significant, the researchers said.

Bias may have also been introduced due to underreporting of active smoking, they added. “[This could have been driven by] the stigma attached to cigarette consumption in pregnant women. The profiles of women who smoke during pregnancy may also skew towards the socio-economically disadvantaged or the less health-conscious; hence, they may be less aware of or concerned about eczema symptoms in their children.”

Nonetheless, this finding should be interpreted with caution and requires further investigation, they said.

Paternal active smoking during the postnatal period was also negatively associated with offspring food sensitization evaluated using serum immunoglobulin E levels in one study (RR, 0.52). [Allergy 1999;54:220-228] However, the researchers noted that this effect could have been minimal given the lack of association on multivariate analysis with adjustments for several confounders.

Halt the atopic march

Allergic diseases impair quality of life and adversely impact physical, mental, and social health. [Expert Rev Pharmacoecon Outcomes Res 2020;20:437-453] “Hence, it is important to prevent and treat allergic diseases at the early stages to halt the atopic march,” said the researchers.

“Our findings highlighted the importance of in utero programming in early-life allergy development and that other environmental factors may potentially play more important roles than smoke exposure in their pathogenesis,” they added.

A key intervention target

Nonetheless, smoking remains a key intervention target to mitigate the risk of allergy development, they noted. “Despite the weak evidence, our results suggest that pregnant women should minimize their contact with second-hand smoke to prevent offspring eczema development.”

“This field remains understudied and there is a need for greater utilization of objective allergy assessments in future studies to reduce response bias and misclassification,” the investigators concluded.