Under-five mortality (U5M), defined as the number of children dying before the fifth birthday (0–59 months) has universally remained unacceptably high. On the average, between 1990 and 2017, there was a 58% decline in U5M across the globe, 118 countries have already achieved the sustainable development goal (SDG) 3 target of the U5M rate of less than 25 per 1,000 live births . Although the world witnessed a tremendous reduction in child mortality, sub-Saharan Africa which Nigeria belongs is the only region with U5M rates of 76 deaths per 1,000 live births [1, 2]. These deaths remain considerably high in Nigeria as the eighth highest in the world  and the highest in Africa . Even going by the current U5M rate of 132 deaths per 1,000 live births, this implies that more than 1 in every 8 children in Nigeria dies before age 5 . Despite the decline from 213 deaths per 1,000 live births in 1990 to 132 deaths per 1,000 live births in 2018 , the slow pace of decline in U5M has been attributed to poor progress in child survival interventions , including exposure to household air pollution due to inefficient cooking practices .
Globally, around 3 billion people cook with polluting open fires fuelled by biomass including wood, animal dung, crop waste and coal and most of these people are poor and live in low- and middle-income countries . Consequently, 3.8 million people die prematurely annually from illness attributable to the household air pollution caused by the inefficient cooking practices of using solid fuels, while exposure to household air pollution almost doubles the risks for childhood pneumonia and is responsible for 45% of all pneumonia deaths in children less than 5 years old . In Nigeria, the percentage of households using solid cooking fuels is high (79%), with 61% using wood. This includes 87% of households in rural areas and 47.7% of households in urban areas . Studies have shown that the use of some solid fuels has been associated with indoor air pollution, unsafe levels of toxic and mortality related cases in both adults and children [7–11]. Children are more susceptible to housing air pollution than adults since they spend most time indoors. They are at risk because they require a higher amount of air inhalation than adults, and their organs are not fully developed [12–13]. Hence, children from the households where solid fuels are the main sources of cooking amenities are more likely to experience acute respiratory infections (ARIs) because indoor air pollution is closely associated with the location of kitchen .
Poverty and child’s health are connected in diverse ways. Children from less privileged households are more vulnerable to air pollution, poor sanitation among others, which are some of the risks of diverse diseases resulting in U5M [3, 15]. A study has shown that poverty is negatively associated with child health . No doubt, the absence substances that make life comfortable in the household as a result of poverty could have an adverse effect on a child’s health outcome. Children who are living in poverty are more likely to experience respiratory health problems . Previous studies observed that the majority of susceptible children are exposed to harmful emissions of biomass smoke at home, which significantly increases the risk for acute lower respiratory tract infections, upper respiratory tract infections and asthma, and pneumonia [18–20]. Besides, most under-five children’s deaths caused by ARIs are closely associated with environmental factors including the use of solid fuels [21–22]. Therefore, avoiding the use of solid fuels such as wood stove seems as preventive measures to reduce the risks of ARIs for under-five children .
Socio-economic and demographic factors are akin to the status of children’s health. Mother’s education is negatively associated with the incidence of children’s diseases which could result in deaths [24–25]. In addition to the mother’s education, household wealth influences the choice of household fuel for cooking . Thus, increasing the level of education among women and in association with other socio-economic and demographic factors significantly reduces the incidence of U5M . Educated mothers tend to seek a healthy environment and health care facility for their children. Under-five children’s health outcomes are tied to mother’s education, occupation, and wealth status among others . The choice of cooking amenities and rural location are making much difference in terms of health challenges including ARIs affecting children’s health . Hence, the high prevalence of under-five deaths in rural areas has been attributed to the predominant negative practices, as well as a low level of education and wealth status . In Nigeria, the geo-political zones have significant effects on the unevenness in childhood mortality rates . Attributably, areas with enhanced accessibility to clean cooking fuels tend to record reduced under-five deaths.
There have been few studies that investigated the association between household cooking fuels and health outcome of under-five children in Nigeria. Most of these studies were limited to solid cooking fuels by adopting binary logistic regression for data analyses ; contributory factors to regional inequalities in ARIs symptoms among under-five children  and household materials as predictors of U5M using 2013 NDHS . Also, the observations of some studies from other countries that examined the effect of household air pollution on people’s health reached contradictory conclusions [34–36]. However, in the context of global child health priority, no previous studies have examined the interaction effects of neighbourhood poverty and using solid cooking fuels within the house on U5M. The implication is that the findings of these studies might be limited in informing strategic interventions and policy formulation concerning the interaction effects of neighbourhood poverty and use of solid cooking fuels on under-five children’s health outcome. The need for this study becomes pertinent, especially in Nigeria which is one of the five countries with the largest populations living in extreme poverty and are home to about 23% of the world’s poor . No doubt, poverty plays a role in the choice of solid cooking fuels that exposes under-five children to harmful emissions of biomass smoke at home.
Assessing the contributory role of neighbourhood poverty simply described as the percentage of households in the poorest quintile of the wealth index  and deprivation from a broader perspective which is considered as a lack of basic needs, possibly resulting to the use of solid cooking fuels , which directly impact people’s health  becomes important. Though, studies have shown the significant association between poverty and child health outcomes [41–42]. This study investigated the interaction effects of neighbourhood poverty and use of solid cooking fuels on U5M using the latest NDHS data and Cox proportional regression analyses considered most appropriate for examining the risk of U5M. Adopting the implications of the findings of this study is essential for futuristic strategies towards developing more strategies towards ending poverty in all its forms, attaining universal access to affordable, reliable, sustainable and modern energy, as well as reducing U5M in Nigeria by 2030.