Cooking fuel and respiratory symptoms among people living with HIV in rural Uganda

Discussion

We identified an association between use of firewood for cooking (as compared to charcoal) and chronic cough among HIV-infected females in rural Uganda, adjusted for indices of HIV disease stage (CD4 count or viral load). To our knowledge, this is the first investigation of the association between cooking fuel and respiratory symptoms in an HIV-infected cohort. The results of this study are consistent with published literature on the respiratory effects of HAP in the general population, which has been associated with chronic respiratory symptoms, impaired pulmonary function, respiratory infections and lung cancer in HIV-uninfected cohorts, particularly among females [10]. Furthermore, prior work has demonstrated that chronic cough poses a greater burden on health-related quality of life for females than males [11]. Although it is plausible that they are related, the consequences of HAP on respiratory symptoms should not necessarily be assumed to be similar between HIV-infected and uninfected populations. Either synergistic or attenuated effects of HAP on the pulmonary health of HIV-infected individuals can be postulated based on formative data in the field. For example, HAP and HIV each cause systemic inflammation [7, 8], which may result in an additive or multiplicative effect of HAP on respiratory pathology among HIV-infected individuals, compared to HIV-uninfected populations. Similarly, HIV-infected individuals are more susceptible to respiratory infections due to immune compromise, resulting in higher incidences of pulmonary tuberculosis, pneumocystis and bacterial pneumonias [12]. Thus, any incremental increases in respiratory symptom burden caused by HAP exposure among HIV-infected individuals must also take into account underlying lung disease and whether it may attenuate the absolute effects of HAP on lung health. By demonstrating a relationship between HAP and respiratory symptoms, our data provide a rationale for further quantifying these complex associations among HIV-infected populations.

While there are no comparable studies among HIV-infected populations in sub-Saharan Africa, two recent studies measured the odds of cough among biomass-exposed populations in Malawi [13] and Nigeria [14]. Similar to our work, both studies identify increased odds of cough among females cooking with firewood. Urban and rural Malawian females who cooked with firewood as compared to charcoal had >300% increased odds of cough, and rural Nigerian females cooking with firewood as compared to kerosene gas had >400% increased odds of cough. Neither study reported data on HIV prevalence. Two notable differences might explain the increased odds of cough noted in those studies compared to our work. Firstly, the Malawian study population was 42% peri-urban and focused on females with primary cooking responsibilities, each of which may have led to increased pollutant exposure among study participants. Secondly, the Nigerian study compared firewood to a clean-burning fuel, whereas we compared firewood to the only other fuel prevalent regionally: charcoal, a biomass fuel.

The elevated risk in models restricted to females, but not males, may be due to the role of females in cooking in the region [10]. The increased chronic cough risk among those using firewood versus charcoal may be related to airborne endotoxin inhalation [15], which is associated with chronic respiratory symptoms and accelerated pulmonary function decline [16].

Chronic respiratory symptoms, including cough and dyspnoea, are prevalent self-reported conditions in established HIV cohorts in high-income settings, with comparable proportions to those reported here [2, 4]. Markers of inadequate HIV viral control were associated with chronic cough in our cohort, in keeping with published literature in several large US-based HIV-infected cohorts that have shown lower CD4 count and higher HIV-1 viral load to be independent risk factors for chronic respiratory symptoms [2, 4, 6]. In addition, respiratory symptoms in these cohorts are associated with impaired pulmonary function, postulated to be a result of prior pulmonary opportunistic infections and/or chronic inflammation-associated obstructive lung disease. Our study was limited to an HIV-infected population, and we are therefore unable to assess the relative contribution of HIV infection per se to the chronic cough burden. Importantly, HAP and HIV are each associated with chronic respiratory symptoms, so an essential unanswered question raised by our work is whether there are interactive effects between HIV infection and HAP that exacerbate chronic respiratory disease risk.

We also identified an association between chronic cough and lower socioeconomic status, as measured by the household asset index [9]. Our results are in accordance with published literature on associations with chronic cough, in which lower socioeconomic status portends increased risk of chronic productive cough [17, 18] and chronic bronchitis [19–21].

The main strength of this analysis is the reliance on a large, longitudinal study of HIV-infected persons in rural Uganda with rare missing data or losses from observation. The results of our work can be applied to HIV-infected people living in environments with high biomass exposure.

Our study has several limitations. Firstly, cooking fuel type and respiratory symptoms are self-reported and subject to misclassification. However, any misclassification is likely to be noninformative and bias towards the null. Secondly, we compare firewood to another biomass fuel source (charcoal), rather than a cleaner burning fuel. However, this probably underestimates any true association between biomass fuel exposure in general and chronic cough in this population. Furthermore, >80% of sub-Saharan Africa is exposed to biomass smoke [22]. Comparing the two regionally prevalent biomass sources more accurately reflects the local biomass exposure patterns, increases the generalisability of our results to the HIV-infected sub-Saharan African community and provides critical knowledge to inform risk-reduction practices while global access to cleaner energy sources is evolving. Thirdly, no data were collected on the quantity of cooking fuel used or the participant's role in cooking activities, and self-reported cooking fuel type is a blunt instrument for quantification of household air pollution exposure. We partially accounted for this with models stratified by sex to account for presumed increased exposure among females, but acknowledge that future studies that comprehensively quantify biomass and other environmental exposures are necessary. Lastly, pulmonary function testing was not performed in this cohort. This limits inferences between chronic respiratory symptoms and pulmonary function.