Background About half of the global population and over 70% of countries in the Sub-Saharan Africa rely on biomass fuel as a source of household energy. Over 95% of households in Ethiopia use biomass fuel for cooking. Despite the prevailing major concerns among policy makers and professionals on air pollution, the magnitude of air pollution from domestic and traffic sources in Ethiopia is not well established. Objectives This thesis attempted to examine the magnitude of air pollution by measuring 24-hr concentrations of indoor nitrogen dioxide in rural Butajira and daily measurement of ambient carbon monoxide in traffic congested areas of Addis Ababa. Materials and Methods A longitudinal study was conducted to assess the indoor air pollution component between March 2000 and April 2002. Concentrations of NO2 were measured cross-sectionally at about threemonth interval using a modified Willems badge air samplers. Mothers of children in households were interviewed within 24 hours of air sampling about characteristics of fire use, type of fuel and cooking pattern. A Saltzman colorimetric method using a spectrometer calibrated at 540 nm was used to analyze the mass of NO2 in field samples. Roadside traffic air pollution was assessed using portable CO USB data loggers. CO monitor is small electronic equipment installed along 40 roadside sampling points to continuously measure and record CO concentrations at an average interval of 10 seconds for about 10 hours in the daytime. Four on-road traffic light posts were also included to explore the association with the results of roadside CO concentrations. Data were entered and analyzed using EPI INFO version 6.02 statistical software. SPSS version 15.0 was further used to run regression analysis. Data from CO logger were downloaded in Excel format. Summary statistics, graphs, charts, and tables were the main tools used to present findings. One-way ANOVA, multiple regression analysis and linear mixed model analysis were also used to sort out any non-random differences in NO2 and factors affecting the levels of NO2. viii Results Wood, crop residues and animal dung were the main fuels in rural households in the study area. The mean 24-hr concentration of NO2 was 97.3 μg/m3 (95% CI: 95.9, 98.6). The median (IQR) was 68.4 (98.7) μg/m3. Ecology and season have shown differences in the mean concentration of NO2. Households in the highland areas and during wet season had higher indoor NO2 concentration. Biomass fuel type, ecology, purpose of fire use, cooking of at least one type of food in a day, and frequency of fire use were important household variables to explain the variations in the daily NO2 concentration. While ecology was the major predictor, housing physical structures showed little influence on the variation of indoor NO2. In Addis Ababa, the 15-minute mean (+SD) CO concentrations were 2.03 (1.94) and 2.64 (2.53) ppm respectively observed during the wet and dry seasons of 2007 and 2008. The two means did not vary significantly. There were variations in average CO by time and location of sampling. CO tended to be high in early mornings and in the afternoon rash hours. The CO profiles between roadside and on-traffic post light were, however, not different from each other. Conclusions and Recommendations About 70% of NO2 Key words: magnitude, NO indoor measurements were more than double the currently proposed annual mean of WHO air quality guideline. Ecology and fire-fuel use household characteristics were important determinants of indoor air pollution. Although average CO concentrations were below the US-EPA and WHO ambient air quality guidelines, there is a strong indication that CO concentrations will exceed or approach these guidelines shortly. Further studies in the description of burden of diseases attributed to indoor air pollution are highly recommended. Interventions targeting at improving the design and utilization of fuelstove efficiency and ventilation are essential. The measurement of traffic particulate matter in high traffic areas is suggested given the high proportion of on-road diesel-engined vehicles in Addis Ababa. 2, indoor air pollution, agro-ecology, sources, biomass fuel, variation, Addis Ababa, CO, traffic air pollution, Ethiopia.