Biomass fuel is the primary source of household energy in developing countries. Fifty two percent of the global population and more than 90% of rural homes in developing countries use solid biomass fuels for cooking, heating, and lighting purposes 1. Biomass fuel, also designated as unprocessed or dirty solid biofuel, mainly includes firewood, animal dung, agricultural residues and plant leaves.

Indoor air pollution is considered as one of the risk factors causing high burden of diseases and of premature deaths in developing countries 2345. IAP is recognized as a silent and unprotected killer among rural women and children who spend much of their time in the kitchen 6. The burden of disease due to IAP is high in developing nations that use biomass fuels. Smith estimated that IAP in India accounted for 4.2-6.1% of the total national burden of disease, which is considered as a major public health concern. This proportion is tantamount to about half million premature deaths annually 7.

The health risk of direct exposure to biomass combustion is evident in part due to cooking practices in poorly ventilated and crowded single roomed houses in developing countries 489. Studies have shown that proxy factors related to socio-economic status like income education, and use of biomass fuel to be highly associated with the level of IAP 741011. Despite the prevailing knowledge that exposure to biomass combustion products is high 491213141516171819, quantitative studies on factors affecting the level of IAP are limited in developing countries 13.

Data on the measurement of indoor NO₂ is limited in developed countries. Research has shown that the mean concentration of NO₂ in kitchens with a gas stove varied between 26 parts per billion (ppb) and 112 ppb 2021, while this was 18 ppb in kitchens equipped with electric stoves 20. Median NO₂ concentrations in living rooms were 5.8 ppb in Ashford (United Kingdom), 6.1 ppb in Minorca (Spain), 23.8 in Barcelona (Spain) 22 and 11 μg/m³ (5.8 ppb) in Umea (Sweden) 23. In low income homes of USA, mean concentrations of NO₂ were found to be 43 ppb and 36 ppb in kitchens and living rooms, respectively 24 and 30 ppb in children's bedroom 25. Increased level of NO₂ was related with type of fuel and ventilation efficiency 26. In addition, these studies consistently demonstrated that the type of fuel, purpose of fuel use, and location of indoor activities are associated with increased NO₂. The use of gas stoves and heaters were often accompanied with exceeding the current World Health organization (WHO) Air Quality Guideline (AQG) of 21 ppb for 8 hours 27.

A particulate matter (PM) of various aerodynamic sizes is also used to measure the level of IAP. The mean concentration of respirable suspended particles (RSP) in homes that use biomass fuels had variation depending on the type of kitchen and fuel used: 500-2000 μg/m³ in kitchens of India 28, 1200 μg/m³ in Mozambique 29, 850-1560 μg/m³ in Guatemala 16, and 1400 μg/m³ in Kenya 30. In Bangladesh, the average concentration of particulate matter (PM) of aerodynamic diameter of 10 microns (PM₁₀) in kitchens that use biomass fuels ranged between 237 and 291 μg/m³ 10. Smith showed that the concentration of PM₁₀ commonly varied between 200-5000 μg/m³ 4. In Ghana and Nicaragua, the average concentrations of PM_2.5 varied between 320-650 g/m³ in Ghana and Nicaragua 3132. These studies indicated that the type of kitchen (whether inside or outside), the type of stove (traditional or improved), ventilation condition, place of measurement (kitchen or sleeping room), and type of biomass fuel (wood, dung, or residues) were major factors affecting the concentrations of indoor air pollutants. The above cited aerial concentrations related to RSP, PM₁₀ and PM_2.5 exceeded by a factor of 2 to 40 times of the standards set by US Environmental Protection Agency, (US-EPA) for 24-hr and annual standards 33 and by a factor of 10 to 80 times of the present WHO 8 hr of air quality guideline (AQG) 27.

Biomass fuel in the form of firewood, agricultural residues and animal dung is the primary source of household energy in Ethiopia 3435. The majority of rural homes have only 1 room serving all types of household activities 36. Cooking takes place in the same room using traditional unvented stoves. Such rooms do not have functional ventilation outlets 337. Pocket studies in Ethiopia have shown high level of exposure to indoor air pollution that exceeded the WHO 1-hour and 8-hours AQG 27383940. However, the results of these studies could not be generalized to a larger population due to their methodological limitations.

A recent study conducted by the authors revealed increased indoor NO₂ concentration levels 41. It also has shown that ecology and season were factors that affect NO₂ concentrations. The present study was undertaken with an effort to further explore other factors associated with indoor air pollution in the rural Ethiopia. Given the high level of exposure to IAP in poorly ventilated housing units, increased morbidity and mortality due to acute respiratory infection (ARI) in the general population, especially among children under-five years old, are expected 342. Therefore, this study will have great operational relevance in achieving the Millennium Development Goal 7 (MDG 7, target 9, indicators 27 & 29) by generating important information on feasible interventions for the reduction of IAP in the Ethiopian rural homes.

Selected household characteristics affecting the level of indoor air pollution have their own role in changing the level of NO₂ in the context of our study area. Nearly all households in the study area used biomass fuel in the form of firewood, crop residues, and animal dung among which the first two predominated. The population in the study area is known for using wood most of the time throughout the year, crop residues such as stocks of maize and barley during harvest times, and animal dung during summer 46. While biomass fuel sources are relatively cheap and easily available locally, fuels of fossil origin such as kerosene was only used to light local lamps for the interior of housing units at night. Kerosene is less affordable for rural residents to use it for cooking purposes compared to residents of urban areas such as Addis Ababa 47. The use of biomass fuel as a primary source of household energy is consistent with findings of studies in other developing countries 11019283848.

Biomass fuel was extensively used for cooking traditional foods compared to other purposes of having fire events. Cooking was the most frequent household fuel use activity reported during all times of the day. Heating was the second most frequent activity; whereas only about a tenth of the households used biomass fuel to repeal mosquitoes at night. The use of heating indoor space predominated in colder villages, mainly in the highlands, while repealing mosquitoes prevailed in the lowlands. It is evident from the data that home heating in the highlands might have caused an additional fuel use burden, which possibly contributed to the increased concentration of indoor NO₂ compared to the relatively low fuel use burden required for repelling insects in the lowlands. The extreme temperature difference 49 might have contributed to the variation in the use of fuel for heating between the two ecological settings. Low temperature in early mornings and nights is common in the highlands of Ethiopia. Villages in the lowlands inherently possess a risk to malaria caused by mosquito bites and, therefore, there is a cultural practice in the study area for using indoor firing events to repel mosquitoes 4250.

Traditional foods that do not require a stock for more than a day were routinely cooked. The cooking time was equally important in all cases of cooking which involved commonly the mornings, mid-days and evenings. Traditional coffee and bread making are also common daily practices in the study area. Coffee in each household was served for a group of neighborhoods nearly on daily basis, which is a cultural heritage of Ethiopia. The relative time and cost of preparing these food items are a bit less than that for "injera" and "wat" which are widely used in other parts of Ethiopia, especially in the temperate and highland areas. The practice of "injera" and "wat" is expensive and the raw material, locally called "teff" is considered as a cash crop for the rural residents in the study area. The linear relationship between the number of food items cooked and frequency of cooking with the level of IAP is obvious given the increased respective amount of biomass fuels and the corresponding higher emission of other pollutants including nitrogen dioxide. This is in part a reflection of "dose-response" relationship.

Cooking and heating are the main household activities that lead to the excess NO₂ concentration due to solid biomass fuel use in general, and in the highland areas in particular. In other studies, given the range of the purpose of biomass fuel use, cooking has been implicated as the main factor for the greater proportion of exposure to IAP in developing countries 451. Biomass fuel emits about 50 times more pollution during cooking compared to cleaner fuels 4, while the exposure magnitude of breathing in pollutants could be twice more for the same population 7. Therefore, the magnitude of health risk due to biomass combustion can reach as much as 2-3 times greater than the risk among clean fuel users 49. Indoor smoke from biomass fuel is attributed to loss of healthy life in poor countries due to known health outcomes such as ARI, acute lower respiratory infections, and chronic obstructive lung diseases 52. It is possible to speculate based on our findings that higher degree of exposure to indoor air smoke goes to mothers and children who often spend most of their time indoors. This implies that the attainment of Child Health MDGs would be a challenge in developing countries, like Ethiopia.

Assumption of the within-subjects (within a household over time) variation in indoor NO₂ concentration by time was certainly important given the possible differences in the exposure to various fuel characteristics within households. This determined the presence of differences in exposure factors. The repeated between subject difference in NO₂ demonstrated by ecological factors and seasonality is an important effect that requires closer attention for designing an appropriate intervention, as well. The study revealed only an interaction between ecology and seasonality affecting the indoor NO₂ concentration by time. This was supported by our data that indicated variations of NO₂ to be dependent on the type of fuel in the bivaraite analysis, although this association remained significant for a crop residue in a linear mixed model.

Ecology, season, type of biomass fuel, the purpose and time of having fire events, and the frequency of cooked food items were all found to affect the level of indoor air in a bivariate analysis, while this association was consistent in a linear mixed model except for the type of biomass fuel for wood. In western countries, the type of fuel (gas or electricity), occupancy density, the number of cooked meals, frequency of cooking, season and income were found to significantly impact indoor NO₂ concentrations 2024252653. Those studies have indicated the presence of strong link among factors responsible for the increased level of indoor NO₂ both in the developed and developing countries. The new finding in our study is the presence of ecology as a factor that predominately affected the level of NO₂. The high level of NO₂ in the highland areas can also be explained by high proportion of wood fuel use. Wood is at least better than crop residues and animal dung in the energy ladder 54 and provides relatively better energy efficiency. When wood is used, it oxidizes relatively more indoor air nitrogen because of the relatively high combustion temperature than others.

The effect of housing volume was found to show little importance in affecting IAP as measured by NO₂, which was against our hypothesis. Together with the presence of window and kitchen in the multiple linear models, there was only very small proportion of explainable variance (less than 1%) in NO₂ concentration despite the statistical significance. The computed model was not able to indicate a practical relevance in explaining the direction and strength of the association between the magnitudes of indoor air pollution and the physical housing characteristics in our study area. The significant difference, however, could only be explained due to large sample size that could have picked up small differences for calculating p-value. Rural housing units, due to their nature of construction, allow the easily passage of indoor smoke through their thatched roof, open eves and unplastered or partially plastered wall, restricting the continued built up of indoor air pollutants. It is quite common to observe visually the penetration of intense smoke through such structures during active cooking times in early mornings when there is good visual contrast (personal observation). Windows in the majority of housing units in the area are represented by just small circular holes (usually < 5% of the floor area, which is often closed due to the fear of wind drafts). Furthermore, opening of windows is culturally believed to affect resident's heath in our study settings.

Lack of assessment for additional air pollutants such as PM and the absence of real time measurement for the indoor air pollution were major limitations of this study. Nevertheless, the present study has shown that ecology, season, purpose of fire events, the frequency of cooked food items and the frequency of fire events as being predictors of indoor NO₂ in a rural setting.

Agro-ecology, season of the year, fire use characteristics and the physical structure of the housing were found to be important determinants of indoor air pollution in this study.

Further study on personal exposure assessment using NO₂ and PM is highly recommended. In addition, relating indoor NO₂ levels with commonly seen childhood diseases, such as respiratory symptoms, is another area of significant relevance for a research given the high level of IAP in our study settings. Finally, the provision of separate kitchen, improved stoves with hood, and presence of window in kitchens are highly advised to manage low level of IAP.

AQG: ambient air quality; DSS: Demographic Surveillance System; EPA: Environmental Protection Agency; GM: geometric mean; GSD: geometric standard deviation; g/m³: gram per cubic meter; IAP: indoor air pollution; MDG: Millennium Development Goal; μg/m³: micro grams per cubic meter; mlmn^-: millilitre per minute; NEDA: N-(1-naphtyl) ethylenediammonium; NO₂: nitrogen dioxide; PM: particulate matter; PM₁₀: aerodynamic diameter of 10 microns PM; ppb: parts per billion; ppm: parts per million; RSP: respirable suspended matter; WHO: World Health Organization.

The authors declare that they have no competing interests.

AK and AE were involved in the study protocol design development, data collection, data quality monitoring, data analysis and preparation of the manuscript. YB and AA were involved in analysis and editing draft manuscripts. SW and DB designed, trained, and supervised NO₂ data collection and laboratory analysis. EM was involved in supervising and monitoring laboratory data analysis. AW was involved mainly in data analysis and its data quality management. All authors contributed to revising the final manuscript.