Adverse birth outcomes (ABO) including low birth weight (LBW), preterm birth, and stillbirths represent an unmet public health need in Nigeria. The World Health Organization (WHO) classes infant birth weight of less than 2.5 kg as LBW [1
]. LBW is a key risk factor for morbidity and mortality as well as being a predictor of survival, normal growth, and cognitive development in children [2
]. In 2015, Nigeria had the third-highest number of neonatal deaths, and an estimated 313,700 stillborn deaths, the second-highest globally, and the highest number of maternal mortalities [4
]. Hence, there is a need to explore the high incidence rate of these ABO in Nigeria compared with other countries.
Household air pollution (HAP) is a prominent global public health concern and is a leading environmental health risk globally [5
]. In 2016, HAP was responsible for 7.7% of the global mortality [6
]. It is estimated that 3.8 million premature deaths occur each year from illnesses attributable to HAP [7
]. Of these deaths, 27% were due to pneumonia, 27% to ischaemic heart disease, 20% to chronic obstructive pulmonary disease (COPD), 18% to stroke, and 8% to lung cancer [7
]. HAP has also been linked to pulmonary tuberculosis [8
], eclampsia [9
], and cataracts [10
]. Regional variations in the particulate composition of HAP has been reported within countries [11
] and between countries [12
], and may also depend on locally accessible fuel type within regions [12
]. Within Nigeria, geographic factors also impact the rate of adverse birth outcomes as the North West region has a significantly higher risk for LBW [3
]. Hence, it is important to highlight the effect of regional variation on the prevalence of ABO due to HAP. Additionally, some studies have shown that parental employment, education level, and other indicators of socio-economic status are associated with LBW [14
A primary source of HAP is the use of unclean fuel in cooking [7
]. Common chronic respiratory symptoms, such as congestion and cough, are directly associated with the hours spent cooking food [16
]. Using unclean fuel is especially common in developing countries where biomass fuel sources are more affordable than healthier alternatives such as electricity, natural gas, and liquefied petroleum gas [17
]. Biomass fuel includes wood, crop residue, charcoal, coal, and dung [7
]. The use of this type of fuel is most prevalent in Africa and Southwest Asia where over 60% of households cook with unclean fuels [18
], and there is evidence of geographical disparities in its use [20
]. Nearly 70% of the Nigerian population cook with unclean fuel [21
Global evidence suggests an association of HAP with increased pregnancy complications [22
], preterm birth [23
], and elevated risk of stillbirth [24
]. It is hypothesized that pollutants, including carbon monoxide, contained in HAP can be inhaled and absorbed into the mother’s blood which could detrimentally affect the fetus [26
]. HAP likely potentiates ABO by promoting a pro-hypoxic phenotype including increased expression of Hofbauer cells, syncytial knots, and a compromised chorionic vascular density in utero [28
]. Uterine hypoxia has also been associated with morbidity and mortality [29
]. Both Hofbauer cells [30
] and syncytial knots [31
] have been associated with ABOs such as pre-eclampsia and intrauterine growth retardation, while impaired chronic vascular density is linked to embryonic death [33
]. In Nigeria, a limited randomized control trial demonstrated the increased risk of chronic hypoxia in pregnant women using unclean cooking fuel compared to those using clean cooking fuel [28
]. Another randomized trial showed an intervention of ethanol as cooking fuel significantly increased mean birth weight and gestational age at delivery [34
]. This may also mitigate the risk of cardiovascular disease such as hypertension by downregulating the expression of tumor necrosis factor alpha, interleukin 6, and interleukin 8 in pregnant women [35
Here, we explore the impact of HAP on adverse birth outcomes, such as stillbirths, pregnancy duration, and LBW in Nigeria. We will evaluate the geographic distribution of the association to identify regions with more significant HAP effects on births. This study will add to the existing literature by analyzing the impact of HAP on ABO in Nigeria while accounting for geographic heterogeneity.
We have shown that 89.3% of the mothers included in this study primarily used unclean fuel for cooking. This suggests that a substantial proportion of the population is still dependent on unclean cooking fuel for cooking in Nigeria. This is a significant public health concern due to the associated risks associated with unclean cooking fuel [5
]. In this study, we investigated the association between HAP and adverse birth outcomes, while accounting for geographical heterogeneity and mother’s effect in our data. We were able to control for the mother’s individual effect and analyze stillbirth, LBW, and preterm outcomes. In the bivariate analysis, unclean cooking fuel was associated with a higher risk of stillbirth and protective of preterm birth. This finding supports a previous study based in Argentina that concludes that lower socio-economic status and education levels are associated with higher rates of LBW and lower rates of preterm birth [15
Our result indicates that HAP was associated with an increased risk of stillbirth but not for LBW and preterm birth. Overall, northern states had a higher rate of unclean cooking fuel use and a higher rate of adverse birth outcomes. Northern states were associated with a higher proportion of stillbirths and LBW than the southern states. Previous studies conducted in Nigeria showed a link between HAP and childhood acute respiratory tract infections, which was a leading cause of deaths in children under 5 years [48
]. Our study extends these results to include the effects of HAP in Nigeria to adverse birth outcomes. This study contributes to previous research on HAP’s association with pregnancy outcomes. Relevant literature offers contradictory findings on the significance of unclean cooking fuel when analyzing LBW. In a meta-analysis, it was concluded that indoor air pollution increased the risk of LBW [27
]. A study in Bangladesh reported that indoor solid fuel use was significantly associated with LBW, but not with neonatal mortality or stillbirth when controlling for demographic variables [22
]. Our study concluded that HAP did not significantly increase the risk of LBW. This may be due to the low prevalence of smoking in our sample, as smoking is correlated to increased risk for LBW [50
]. This finding coincides with a Ghanaian cohort study that failed to support an association between HAP and LBW after adjusting for confounding variables [25
Our study suggests that HAP is significantly associated with an increased risk for stillbirth and is supported by a recent meta-analysis that concluded that HAP increased the risk of stillbirth in developing countries [27
]. However, other studies did not find a significant relationship between HAP and stillbirth [22
].In a cross-sectional study using the 2007 Ghana Maternal Health Survey, the authors concluded that unclean cooking fuel could be on the causal pathway between lower socio-economic status and stillbirth [24
]. It is hypothesized that smoke produced from unclean cooking fuels contains pollutants, including carbon monoxide, which can be inhaled and absorbed into the mother’s blood and possibly cause detrimental effects on the fetus [26
]. Our study suggests that these effects on the fetus can result in an increased risk for stillbirth.
Poverty has also been suggested to be a key factor in preventing access to clean fuel but also compounds the burden associated with HAP by worsening access to adequate health care [51
]. This may be a contributing factor to the regional disparity seen between the mostly lower socio-economic demographic of the North compared to the mostly higher socio-economic demographic of the South. For example, a previous study concluded that the Northern region had the highest prevalence of underutilization of antenatal care services [52
] and lowest immunization uptake [53
]. When geographic variation is controlled for in the models, HAP continues to significantly increase the risk of stillbirth. Addressing the use of unclean cooking fuel in Nigeria may lead to decreased rates of stillbirth.
We acknowledge the following limitation in this study. The response rates for some adverse birth outcomes were relatively low. There was a 100% response rate for stillbirth outcomes, but there were low response rates for birth weight (6.1%) and pregnancy duration (31.2%). The smoking status of the mother was only recorded in a small proportion (0.2%) of mothers who identified as smokers. Reporting pregnancy in duration in months could introduce measurement bias in our analysis. Measuring pregnancy duration in weeks would result in a more accurate and sensitive model to variability in pregnancy duration. We also could not distinguish when the pregnancy ended. There may be different associations for perinatal mortality and miscarriages if analyzed separately. Amount of time spent around HAP could contribute to the magnitude of its effect on adverse birth outcomes, but we were unable to include this variable in this study. We could not control for all possible confounding variables such as access to medical services, medical history, and BMI.
Despite these limitations, this study has several strengths. As the study was based on a large representative dataset, the 2018 Nigeria DHS, we are confident that our sample is an excellent representation of the Nigerian population. The use of STAR models for analysis allows for flexible modelling of possible nonlinear effects of independent variables and the geographical effects of the data [41
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.