Background: The quality of air inside homes where people spend a large part of their life is an important determinant of a healthy life. However, household air pollution (HAP) from biomass fuel combustion in traditional cookstoves has currently appeared as one of the most serious threats to public health with a recent burden estimate of 2.6 million premature deaths every year worldwide. Although HAP affects all stages of life, children are particularly vulnerable and sensitive to the damaging health effects of HAP due to both their physiology as well as to the extent of their exposure. In particular, acute lower respiratory infection (ALRI) in the form of pneumonia is recognized as the single largest cause of childhood death globally accounting for 16% of the overall deaths. Studies also reported a higher prevalence of childhood acute respiratory infection in Ethiopia, which ranges from 16% up to 33.5%. The use of improved cookstoves has been widely encouraged to reduce these health burdens. It is, however, unclear as to whether it is possible to prevent the burden of HAP-induced diseases such as childhood ALRI with biomass-fuelled improved cookstove interventions. The evidence regarding the child health effect of biomass-fuelled improved cookstove intervention still attracts wide debate; and the efforts to disseminate various types of improved cookstove technologies have faced challenges in adoption in most sub-Saharan countries, including Ethiopia. Therefore, this thesis research focused on the extent of HAP-induced child health burden as well as on improved cookstove technology adoption and the effect of biomass-fuelled improved cookstove intervention in Northwest Ethiopia; where the ALRI is one of the leading causes of morbidity in children. Objectives: This thesis research aimed at four different but interrelated major research objectives. These are identifying the factors associated with the burden of childhood acute lower respiratory infection, identifying the factors that may act as facilitators or barriers to improved cookstove technology adoption, examining the effect of a biomass-fuelled improved cookstove XII intervention in reducing the concentration of HAP, and investigating the effect of a biomass fuelled improved cookstove intervention on morbidity of childhood ALRI. Methods: All studies of this thesis research were conducted in Mecha Health and Demographic Surveillance site in Northwest Ethiopia. In general, studies comprised of cross-sectional and cluster-randomized controlled trials with a longitudinal experimental design were conducted to answer different but interrelated research questions such as whether the use of biomass-fuelled “improved cookstove intervention leads to a decrease in the concentrations of HAP and morbidity of childhood acute lower respiratory infection. All households under the “Mecha” Health and Demographic Surveillance site with at least one less than 4 years old child were eligible for participation in the cross-sectional studies. Among which households who were exclusive users of the traditional cookstove method for “Injera” baking were eligible for participation in the longitudinal trial studies. In order to attain the advantage of having a larger sample size, we considered a sample size of about 5830 households for Paper I and II from the baseline data of the wider stove trial project in Paper IV; and sample sizes of 990 households within 18 clusters and 2,750 children within 50 clusters per arm were considered for Paper III and IV respectively. Baseline data collection on childhood ALRI related variables were carried out by trained local nurses through face-to-face interviews with the mothers/caregivers of children using a pre-tested questionnaire as well as through direct verification based on the Integrated Management of Childhood Illnesses algorithm. Also, HAP related data were collected at baseline by measuring the concentration of indoor particulate matter with a diameter of less than 2.5 micrometers using digital Dylos DC1700 air quality monitors. Replacing the traditional cookstove method with an improved cookstove method, locally called “Mirt” (“best”) improved cookstove, was the intervention. The households that were randomized to the intervention arm were identified using the permanent “Mecha” Health and Demographic Surveillance System house number, and the intervention was delivered at the beginning of the XIII study period to all eligible households allocated in the intervention arm. Households with the traditional cookstove method were served as control arms. After baseline data collection and implementation of the intervention, a total of four follow-up surveys were conducted for one-year at approximately three-month intervals for longitudinal HAP and childhood ALRI data collection. The effects of several potential predictor variables on childhood ALRI and improved cookstove adoption outcomes at baseline were analyzed by fitting binary logistic regression models. Whereas, the effect of improved cookstove intervention on HAP and childhood ALRI were estimated using the Generalized Estimation Equation modeling approach among the intention-to-treat population. Results: Paper I describes the pre-intervention prevalence of childhood ALRI and associated factors among a total of 5830 children aged less than five years old. The pre-intervention prevalence rate of childhood ALRI was 19.2% (95% CI: 18.2-20.2) and found to decrease among children living in homes with chimney, eaves space, and improved cookstove than children living in households with no chimney, eaves space, and improved cookstove with estimated AOR of 0.60 (95% CI: 0.51-0.70), 0.70 (95% CI: 0.60-0.84) and 0.43 (95% CI: 0.28- 0.67) respectively. Whereas, it was found to increase among children living in homes with cow dung fuel type [AOR=1.54 (95% CI: 1.02-2.33)] and extra indoor burning event [AOR= 2.19 (95% CI: 1.41-3.40)] when compared with children living in homes with charcoal fuel and no extra indoor burning event respectively. Paper II describes the magnitude and factors associated with improved cookstove adoption among a total of 5,830 households. The improved cookstove technology adoption rate was 12.3% (95% CI: 11.5-13.2). Factors such as households headed by females (AOR 1.96; 95% CI: 1.24-3.10), private house ownership (AOR 4.58; 95% CI: 3.89-6.19), fuel purchasing as the main source of fuel (AOR 2.13; 95% CI: 1.64-2.76), perceived cheaper price of cookstove (AOR 2.48; 95% CI: 1.91-3.21), and experience of cookstove use demonstration (AOR 2.47; 95% CI: 1.98- 3.07) were played a significant role as facilitators to adoption when compared with male-headed households, rented households, respondents who reported free fuel collection method as the main source of fuel, households headed by those who reported perceived expensive price of cookstove XIV technology, and households headed by respondents who did not report an experience of stove use demonstration respectively. Whereas, lower educational level of household head (AOR 0.31; 95% CI: 0.23-0.42) and prior fuel processing requirements of cookstove technology (AOR 0.55; 95% CI: 0.44-0.70) were found to be barriers for adoption. Paper III presents the effect of improved cookstove intervention on the concentration of HAP among 1977 households within 18 clusters in each arm (984 households in the control and 993 in the intervention arm). The baseline concentration of HAP was 855µg/m3 (95% CI: 839-870) (Intervention = 859, Control = 850); and the post-intervention concentration of HAP was estimated to be 635µg/m3 (95% CI: 627-642) (Intervention = 465, Control=805) among the intention-to-treat households. On average, the use of the current biomass-fuelled improved cookstove in the intervention arm significantly reduces the post-intervention HAP by about 343μg/m3 compared with the continuation of the traditional cookstove method in the control arm [Ḃ = -343 (95% CI: -350,-336)]. Paper IV presents the health effect of the biomass-fuelled improved cookstove intervention on ALRI among 5,333 children across 100 clusters in both arms (2,659 in the intervention and 2,674 in the control arm). The baseline childhood ALRI morbidity rate among the intention-to-treat population was 19.31% (95% CI: 18.30-20.40) with 19.18% and 19.45% in the intervention and control arms respectively. The average post-intervention childhood ALRI morbidity rate was found to be 17.9% (95% CI: 17.4-18.4) with 17.5% and 18.3% in the intervention and control arm respectively. Nevertheless, the biomass-fuelled improved cookstove intervention was not found to have a statistically significant effect on the longitudinal ALRI morbidity among under 5 years old children with an estimated odds ratio of 0.95 (95% CI: 0.89-1.02) compared with the continuation of a traditional cookstove method. Conclusions: High HAP was observed in this study at baseline which might expose family members to a risk of various health impacts in the study area. Correspondingly, a high burden of childhood ALRI morbidity was demonstrated by our baseline study which was found to be associated with lack of chimney, eaves space, and improved cookstove as well as with other cooking-related factors such as cow dung fuel use and presence of extra indoor burning events. XV Based on the findings of the longitudinal HAP assessment, the biomass-fuelled “Mirt” improved cookstove intervention significantly reduces the concentration of HAP compared to the use of the traditional cookstove method. However, the observed post-intervention concentration was still far greater than the current air quality guideline value. This suggests that, although the intervention has the potential to reduce HAP exposure; it may not maintain the potential to significantly reduce the incidence of HAP induced morbidities. Correspondingly, in the child health part of the trial presented in this thesis, we found no evidence that the “Mirt” intervention reduces the risk of childhood ALRI morbidity compared with the continuation of a traditional cookstove method. In general, we interpret this as empirical evidence for lack of intervention effect, and the “Mirt” improved cookstove technology is likely to have no positive child health effect. Recommendations: Families, cooking technology planners, implementers, and policymakers in Ethiopia and other developing countries should consider the implementation of alternative biomass-fuelled improved cookstove technologies that could potentially reduce HAP exposure sufficiently by removing emissions to the outdoor environment with a chimney to avert HAP induced ALRI morbidity among children, who are the most vulnerable and sensitive segment of the population to the damaging health effects of HAP. From a future researches point of view, studies should further examine the health effect of biomass-fuelled improved cookstove interventions using a personal exposure monitoring method to establish whether the use of the current biomass-fuelled improved cookstove intervention could be translated into meaningful health benefits. Trial registration: The trial was registered on August 2, 2018, at clinical trials.gov registry database (registration identifier number: NCT03612362). Keywords: Acute lower respiratory infection, adoption, biomass-fuelled, childhood acute lower respiratory infection, chimney, cooking fuel, cookstove, household air pollution, improved cookstove, indoor burning, particulate matter.