Background: Formaldehyde is found in virtually all homes and buildings. Low-income populations are disproportionately at risk of exposure. The relationship between asthma and exposure to formaldehyde has been under evaluation by government agencies for the last few decades; however asthma was not included in EPA's economic analysis of the benefits of regulating formaldehyde. Objective: We conducted a systematic review to answer the question: "is exposure to formaldehyde associated with diagnosis, signs, symptoms, exacerbation, or other measures of asthma in humans?" Methods: We conducted a comprehensive search of articles published up to February 2018. We included original studies that investigated indoor or outdoor sources of airborne inhalation exposure to formaldehyde incurred any time prior or concurrent to diagnosis or exacerbation of asthma. We developed and registered a protocol in PROSPERO, evaluated the potential risk of bias for each included study, identified a subset of studies combinable in a metaanalysis, and rated the quality and strength of the body of evidence. Results: We screened 4,482 references and identified 148 human studies that met our inclusion criteria; of these, 81 reported on the asthma status of participants; and of these, 10 studies (12 datasets) were deemed combinable in a meta-analysis for development of childhood asthma, 15 were combinable in a meta-analysis for exacerbation of childhood asthma, and 4 were combinable in a meta-analysis for adult asthma outcomes. Studies generally had low to probably-low risk of bias across most domains. A 10-fold increase in formaldehyde exposure was associated with an increased risk of asthma development for both indoor formaldehyde exposure (OR= 2.27 95% CI: [1.26, 4.09]) as well as ambient exposures (OR=1.03, 95% CI: [1.02, 1.04]). The final results of rating the quality and strength of the evidence and recommendations for prevention will be presented.
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