**5. Conclusions**

This paper studied the association between HAP and adverse birth outcomes using the 2018 NDHS. It highlighted the risk of adverse birth outcomes due to mothers using unclean cooking fuel. In order to decrease the prevalence of adverse birth outcomes in Nigeria, efforts should address the dependence on unclean cooking fuel. Disparities in Nigerian states account for disproportionate risks of stillbirth and LBW, even when the effects of wealth and education are controlled for. This shows that decreasing national levels of adverse birth outcomes depends on working toward addressing the disparities between states.

Further research should be performed to analyze the effects that our study could not control for, such as access to prenatal medical services, and the mother's medical history. This includes accounting for a combination of different fuels instead of studying only the primary cooking fuel. Analyzing dose-response using the combination of cooking fuels

would clarify the strength of the relationship between HAP and adverse birth outcomes. As our study is a cross-sectional study, we cannot analyze causal pathways between HAP, adverse birth outcomes, and other explanatory variables. Further research should be done to study these causal pathways and include a larger selection of adverse birth outcomes, as we limited our study to three outcomes.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/1660-460 1/18/2/634/s1, Table S1: Bivariate analysis of adverse birth outcomes and HAP exposure, mother's level, and household characteristics. Table S2: Criterion for model selection.

**Author Contributions:** Conceptualization, J.R., T.I.E., O.A.A.; methodology, J.R., T.I.E., O.A.A.; software, J.R., O.A.A.; validation, J.R., T.I.E., O.A.A.; formal analysis, J.R., O.A.A.; investigation, J.R., T.I.E., O.A.A.; resources, J.R., O.A.A.; data curation, J.R., O.A.A.; writing—original draft preparation, J.R., O.A.A.; writing—review and editing, J.R., T.I.E., O.A.A.; visualization, J.R., O.A.A.; supervision, T.I.E., O.A.A.; funding acquisition, O.A.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** This study did not require any formal ethics approval because it was a secondary analysis based on publicly available survey data sets from the DHS Program (www.dhsprogram.com).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The survey data sets used in this paper is publicly available from the DHS Program (www.dhsprogram.com).

**Conflicts of Interest:** The authors declare no conflict of interest.
