Humidifier disinfectants were household biocidal products used to prevent microbial growth in humidifier water. They were withdrawn from the South Korean market in 2011 after health risks were recognized, but epidemiological studies published since then have reported associations between exposure and a range of respiratory diseases. Until now, the authors note, the overall strength of that evidence and the shape of the exposure–response relationship had not been comprehensively quantified.

The research team, led by Jonghyuk Lee, with co-authors including Hye-Jin Kim, Byoungcheun Lee, Yong-Wook Baek, Kyu Hyuck Chung, Yong Joo Park and In-hwan Baek, searched PubMed, the Excerpta Medica Database and the Cumulative Index to Nursing and Allied Health Literature through September 19, 2025. The primary outcome was the pooled odds ratio for respiratory disease risk among exposed versus non-exposed individuals, with secondary analyses assessing exposure–response relationships using categorical and continuous exposure metrics.

Seventeen observational studies, all conducted in South Korea, met the inclusion criteria. Pooling their results, the researchers estimated that humidifier disinfectant exposure was associated with an odds ratio of 7.02 for respiratory disease (95% confidence interval 1.97 to 24.98).

Age-stratified subgroup analyses showed significantly elevated risks across adult, pediatric and mixed-age populations, suggesting that the association is not confined to a single demographic group. The authors emphasize that these are pooled estimates from observational studies, which quantify associations rather than directly proving causation, although the pattern of results is consistent across subgroups.

Beyond the overall pooled estimate, the review examined exposure–response relationships using several categorical exposure metrics. Significant associations were found for usage duration, daily usage hours, cumulative use hours, airborne concentration, cumulative exposure level, longer use during sleep, and humidifier disinfectant use during sleep. The consistency of associations across these different ways of characterizing exposure was a central feature of the analysis.

Continuous exposure analyses added a further dimension. Individuals affected by respiratory disease had significantly longer cumulative use hours than unaffected individuals, with a pooled standardized mean difference of 0.197 (95% CI 0.069 to 0.325). While the magnitude of this difference is modest, the authors report that it was statistically significant, reinforcing the graded pattern seen in the categorical analyses.

The authors argue that the combination of a strong pooled association and consistent exposure–response relationships across multiple exposure metrics supports a causal relationship between inhalational exposure to humidifier disinfectants and adverse respiratory outcomes. In their impact statement, they write that the findings strengthen causal inference for the inhalational toxicity of these products and highlight the importance of exposure assessment in environmental health risk evaluation.

The review’s conclusions are framed as relevant to environmental health policy, risk assessment and prevention strategies for inhalation exposure to household biocidal products more broadly. Because the analysis is restricted to observational studies conducted in South Korea, where the specific products and exposure circumstances were distinctive, the pooled estimates reflect that context. The datasets generated and analyzed during the study are available from the corresponding author on reasonable request, and the full quantitative details are available in the journal article.

For readers, the study offers a consolidated quantitative picture of an episode that has shaped South Korean environmental health policy since 2011: household products intended to keep humidifiers clean, when inhaled over prolonged periods, were associated with substantially elevated respiratory disease risk in the studies reviewed, with the exposure–response evidence now formally synthesized across seventeen studies.

Subject of Research: Systematic review and meta-analysis of respiratory disease risk associated with humidifier disinfectant exposure in South Korea

Article Title: Respiratory health risks associated with humidifier disinfectant exposure in South Korea: A systematic review and meta-analysis with exposure–response evaluation

Article References: Lee, J., Kim, H.-J., Lee, B., Baek, Y.-W., Chung, K. H., Park, Y. J., & Baek, I.-H. (2026). Respiratory health risks associated with humidifier disinfectant exposure in South Korea: A systematic review and meta-analysis with exposure–response evaluation. Journal of Exposure Science & Environmental Epidemiology. https://doi.org/10.1038/s41370-026-00998-w

Image Credits: AI Generated

DOI: 10.1038/s41370-026-00998-w

Keywords: humidifier disinfectant, South Korea, respiratory disease, lung injury, meta-analysis, systematic review, exposure-response, odds ratio, environmental health, household biocides, inhalation exposure, public health