In our field study in rural Nepal, the use of gas stove was associated with a decrease in watery eye symptoms in housewives. The risk estimates for chronic cough, wheezing, phlegm, sputum, and previous diagnosis of eye disease decreased in housewives using gas stoves compared to that in traditional stove users, but the results were not statistically significant. The risk of eye discomfort as well as 24-hour PM2.5 levels were significantly lower in gas stove-using households than in traditional stove-using households. We believe that the decreased household air pollution levels including PM2.5 concentrations in gas stove kitchens may be the reason for the better respiratory and eye health symptoms. However, the cross-sectional nature of our study, small number of study participants, and non-significant findings regarding respiratory symptoms highlight the need for cautious interpretation as well as future longitudinal and intervention studies.
According to a recent estimation of the World Health Organization, household air pollution caused 3.8 million deaths in year 2016, globally . Although the proportion of the population using solid fuel is decreasing, still 41% of the global population are using coal, wood, charcoal, dung, crop residues, and kerosene as their primary fuels . Due to accumulating levels of evidence on adverse health effects of household air pollution, transition from solid fuels to cleaner fuels such as gas and electricity may avert the deaths of millions of lives.
Nepal is one of the poorest countries in the world showing rapid increase of non-communicable disease, which accounts for more than 44% of mortality and 80% of outpatient visits . Routine health examination and early treatment of the non-communicable disease are crucial for the disease management, but the primary prevention, by avoiding modifiable risk factors, would be a more realistic solution in view of the poor medical infrastructure and socioeconomic status of Nepal. Because particulate matter air pollution is second leading health risk factor in Nepalese , decreasing household air pollution levels by transition of solid fuels to cleaner fuels may be an important public health intervention. In our study, we found decreased household PM2.5 concentrations and improved watery eye symptoms in rural housewives using gas stoves compared to those in housewives using traditional stoves. Although not significant, a marked decrease was observed in the risk of chronic respiratory symptoms in gas stove users. The recent national sample survey of Nepal showed that about 88% of households uses solid fuel (wood: 77.3%; straw/shrubs/grass: 3.6%; animal dung: 6.9%) and 12% uses clean fuel (electricity: 0.6%; LPG/natural gas/biogas: 11.5%) for cooking in rural Nepal . With the view of high prevalence rate of solid fuel use in rural Nepal and findings from our study, energy transition from using solid fuels to cleaner fuel may improve respiratory and eye health symptoms of rural Nepalese housewives.
COPD is a chronic respiratory disease characterized by airflow obstruction and lung parenchyma damage, which are not fully reversible [7, 24]. Exposure to cigarette smoke, occupational toxins, and household and ambient air pollution are known to be the major risk factors for COPD development . Household air pollution, originates from burning of solid fuels, may cause inflammation and oxidative stress of the respiratory epithelial cells, leading to bronchiole obstruction and disruption of the lung functions . Therefore, it is biologically plausible to explain the direction of risk estimates for chronic respiratory symptoms in gas stove users observed in our study, although the results were not significant.
Several studies reported adverse health effects of household air pollution on eye health including ocular irritation . Because biomass gas contains diverse toxic chemicals which may irritate the eye, reduced risk of watery eye symptoms during cooking among gas stove users in our study is also an expected result. In an intervention study in Guatemala, use of chimney stove was associated with reduced level of exhaled carbon monoxide levels and eye irritation symptoms among the study participants . In a cross-sectional study in Pakistan, use of smoke-free stove with chimney was associated with reduced carbon monoxide concentration in the kitchen and of watery eye symptoms in housewives during cooking .
Studies in Nepal and other developing countries have suggested the possible association between eye disease and household air pollution. Not only irritation of the eye itself but also oxidative stress induced by toxic chemicals in the biomass gas may change the lens opacity . In a cross-sectional study of 143 Nepalese women, use of biomass fuel was associated with nuclear cataract and change in nuclear color in the lens, compared to that in gas fuel users . In a population-based study in India, the use of biomass fuel was associated with an increased risk of cataract in women . In our study, we observed a decreased risk of eye disease history among gas stove users compared to that among traditional stove users. However, cautious interpretation is needed due to the statistically insignificant results and the absence of detailed information regarding eye disease history.
Intervention studies in developing countries showed respiratory health benefits after reducing household air pollution levels . By decreasing household air pollution levels through chimney wood stove installation, reduced respiratory symptoms were observed in Mexico and Guatemala [32, 33]. In a non-randomized intervention study in Nepal, improved cough, phlegm, headaches, chest wheezing, and eye irritation symptoms were observed in 36 housewives who changed from the use of traditional to improved cook stove .
Other observational studies in developing countries including Nepal showed association between respiratory health and types of stove and fuel use. In a cross-sectional study of 1,392 rural Nepalese participants, use of biomass fuels was associated with increased risk of air flow obstruction compared to gas fuel users . In a hospital-based case-control study with 606 lung cancer patients and 606 healthy controls in Nepal, household air pollution exposure from burning wood, charcoal, agricultural waste, and dung was associated with increased lung cancer risk . In a cross-sectional study of 841 rural Mexican non-smoking women, use of biomass fuel was associated with increased risk of phlegm, coughing with phlegm, wheezing in the past, and FEV1/FVC ratio below 70%, compared to gas fuel users . By analyzing two waves of China family panel study and nine waves of China health and nutrition survey data, use of LPG was associated with lower risk of recent (1 month) physical discomfort (disease not specified) and better self-rated health scores .
Although statistically insignificant, we found clear reduction in risk estimates of chronic respiratory symptoms in gas stove users compared to that in traditional stove users. However, the risk of mild to severe obstruction on spirometry test did not show any difference between the types of stove use. Similar results were also observed in several studies , and further studies are needed to evaluate whether the exposure to smokes from burning solid fuels are more likely to affect chronic respiratory symptoms instead of airflow obstruction state in spirometer test.
One of the possible explanations for this phenomenon may be the existence of secondary and tertiary stoves in studied households (shown in Additional file 6). Among the 90 study participants, about 60% had a secondary stove and 9% had a tertiary stove in their kitchen. People using traditional stoves as the primary stove tend to have a gas stove as their secondary stove, and people with gas stoves tend to have a traditional stove as their secondary stove. Because we only focused on primary stoves in our study, the existence of secondary and tertiary stoves may lead to misclassification bias, which may shift our effect estimates toward the null in this case.
Reasons for the high prevalence rate of COPD in rural Nepalese encompass poverty, high reliance on solid fuels, use of traditional stove, illiteracy, and poor use of medical facilities . However, evidences remain limited regarding the health benefits of use of gas fuels instead of solid fuels . Therefore, well-designed longitudinal and intervention studies are needed in the future. In addition, we also observed low household income and high illiteracy rate in our study participants (Table 1). Although LPG/LNG and electricity were widely available in our study site, study participants were using solid fuels as their primary fuel due to the economic cost and convenience (shown in Additional file 5). Therefore, future intervention study should focus on not only the significant health benefits of changing traditional stoves to gas stoves, but also on cost-effective intervention methods, which may lead to sustainable use of clean energy fuels in the local community of rural Nepal. In addition, effective educational materials regarding adverse health effects of household air pollution as well as gas stove manuals for illiterate people are needed.
Our study has several limitations. First, because of the study design, we were only able to evaluate cross-sectional association. Further longitudinal and intervention studies are crucial to confirming the causal association between types of stove used in households, and respiratory and watery eye symptoms in housewives residing in rural Nepal. Second, adverse health effects of traditional stove may be over estimated by the higher smoking rates of traditional stove users. Although we adjusted for the current smoking status, second-hand smoking status, and average years of smoking in the analysis model, cautious interpretation is needed. In addition, we did not have detailed information regarding the amount of cigarettes smoked per day. However, in sensitivity analysis, by restricting our study population to non-smokers, we found similar results with our main analysis results. Third, because our study was conducted in only one rural village of Nepal with a relatively small number of study participants, similar studies in different regions of Nepal with larger numbers of study participants are needed to generalize our study findings. The small sample size in our study seems to have led to statistically insignificant results, although effect sizes were strong and directions are biologically plausible. Fourth, because our study was conducted during the summer season, we were unable to capture seasonal changes in household PM2.5 concentrations, and other seasonal factors associated with household PM2.5 concentrations. In addition, because we did not measure the outdoor PM2.5 concentration, we were unable to adjust for the effects of ambient PM2.5 on household PM2.5 concentration levels. However, most of the study participants had never changed their cooking stoves in the past, and the study participants’ houses were concentrated in a single mountain village, located far away from main roads or factories. Therefore, the effects of ambient PM2.5 concentration on kitchen PM2.5 concentration may be minimal and may not differ by the types of cooking stoves. In addition, the 24-hour average PM2.5 concentrations of each household were relatively low (mean 22 µg/m3) compared to the ambient PM2.5 concentration in Nepal, estimated by the Global Burden of Disease research group (100 µg/m3 in year 2017) , which suggests minimal influence of the ambient PM2.5 concentration. Furthermore, distinctively increasing patterns of PM2.5 concentration have been observed between 5 and 9 AM and between 5 and 8 PM, which suggests the existence of particular factors other than ambient air pollution. Lastly, although we measured the kitchen size, window size, door size, and time activity patterns of the study participants, we were unable to use the data due to the coding errors regarding the units of measures.