Indoor air pollution (IAP) from solid fuels, mainly biomass and coal, ranked as one of top ten environmental risk factors of global burden of disease by the World Health Organization [1]. Epidemiologic studies have shown that indoor pollution from the use of solid fuels is associated with acute respiratory infections (ARIs), chronic obstructive pulmonary disease (COPD), and lung cancer [2-5], but limited studies are available on the cardiovascular disease (CVDs), which remains the leading cause of death worldwide. Potential biological mechanisms include oxidative stress, promotion of inflammation with a systemic release of cytokines, and blood coagulation [6].

The combustion of solid fuels in the home release substantial pollutants such as respirable particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), heavy metals, and many other organic pollutants [7] which have been linked to CVDs. According to recent update to the scientific statement from the American Heart Association (AHA), PM exposure can trigger acute cardiovascular events and accelerate chronic CVDs [8]. In China, CVDs is expected to increase considerably, and the future trends in blood pressure, diabetes, total cholesterol, and body mass index may drive the CVD epidemic during the next 20 years [9].

The present study aimed to examine whether in-home solid fuel exposure from cooking and heating is associated with adverse cardiovascular outcomes, focusing in particular on hypertension, coronary heart disease (CHD), stroke, and diabetes in China. The findings of this study has important public health implications because more than 70% of Chinese homes, mostly is rural areas, still use solid fuels [4], which may lead to continuing potential health problems.

In this large-scale population-based cross-sectional study, we found that solid-fuel user, particularly when exposed to cooking and heating, experienced higher risk of hypertension, coronary heart disease and diabetes, compared to non-solid fuel users, especially among the general population 40 years of age and older in China. There was a trend towards increased risk for hypertension, stroke, and diabetes for longer duration and greater amount of in-home solid fuel exposure throughout lifetime.

Epidemiologic data on the cardiovascular effects of solid fuel use in home are very limited. Though lack of epidemiologic data makes direct comparison with our results difficult, numerous cardiovascular epidemiologic studies showed associations with indoor air pollutants such as PM, PAHs, and heavy metals [7,12,13], which contribute to indoor high particulate level from solid fuel emissions [14]. A 10 μg/m³ increase in total suspended particle (TSP) generated from burning sugar cane plantations led to a 12.5% increase in hypertension-related hospital admissions in São Paulo State, Brazil [15]. In Australia, exposure to particles with a mean aerodynamic diameter less than 10 μm (PM₁₀) derived from ambient biomass smoke was associated with an elevated risk of ischemic heart disease, whereas no association was observed with cardiovascular admissions in total [16]. Indian women who cooked with biomass fuel had higher prevalence of hypertension than women who performing the cooking with cleaner fuel LPG [17]. Chronic exposure to fine particulate matter (PM_2.5) from household use of open fires was associated with an elevated blood pressure among Guatemalan women [18]. More recent epidemiologic data showed that personal exposure to PM_2.5 from biomass combustion was positively associated with systolic and diastolic blood pressure (BP) in women in rural China [19].

Epidemiologic studies regarding cardiovascular effects of solid fuel use have conducted mainly among women and found positive associations with hypertension and blood pressure [17-19], as our finding is consistent. For hypertension, we observed stronger association among women, with an OR of 2.00 for women and 1.54 for men, but the associations of solid fuel use with other outcomes were inconsistent by gender, as previous studies for air pollution related cardiovascular effects by gender are not consistent. It is assumed that women are more at risk for in-home solid fuel exposure because they generally spend more time in the home and do more of the cooking [20], but lack of information on amount of time spent of the cooking did not allow us to resolve it. However, we found the association with an increased risk of hypertension in both genders, suggesting household solid fuel use may also be an important risk factor for men. Our analysis stratified by smoking showed that the association for household solid fuel use and hypertension and CHD was stronger among never smokers and small or null among ever smokers. Epidemiologic studies suggest that a stronger environmental pollutants-CVD association is seen among never smokers compared with current smokers suggested that this may indicate "pressure" effects [21]. Our finding stratified by cigarette smoking indicates that residual confounding by smoking effects does not seem to explain our findings, but we cannot completely rule out the fact that it may partly mask it.

The mechanisms involved in the potential toxic effects of in-home solid fuel exposure-related cardiovascular system are unclear, but the main mechanisms may be linked to inflammation through the generation of reactive oxygen species (ROS) and oxidative stress. An experimental study showed that increased oxidative stress is responsible for activation of apoptosis in cardiac cells in the heart [22]. A recent study in China showed that the use of biomass for cooking greatly elevates PM exposures, particularly for those performing the cooking [14]. These PM contains pro-oxidative organic hydrocarbons, such as PAHs [7], particularly in particle phase, that may cause oxidative DNA damage and secretion of pro-inflammatory cytokines and chemokines that can lead to cardiovascular effects [6,13]. PAH exposure from the cooking is associated with oxidative DNA damage, assessed by 8-hydroxy-2'-deoxyguanosine (8-OHdG), among Chinese restaurant workers [23]. Chronic exposure to biomass smoke increased the number of leukocyte-platelet aggregates among Indian women [24], and is considered a risk factor for thrombotic disease such as myocardial infarction [25], stroke [26], and unstable angina [27].

We acknowledge several limitations to our study. First, given the cross-sectional design, reverse causation may be possible if an individual with hypertension, CHD, stroke, or diabetes is more likely to use a solid fuel for cooking or heating. However, although this is unlikely, further prospective investigation is warranted. Second, the exposure assessment of solid fuel was based on questionnaire data, not on observed measurements, and therefore might not represent an individual's actual exposure. Third, possible selection bias due to lack of data on lifetime average amount of solid fuel use (24% of 14,068) may raise concern. To check this, we analyzed the association between each CVD outcome and missing in lifetime average amount of solid fuel use among ever users. No association was found with CHD, stroke, and diabetes among the subjects with missing data in lifetime average amount of solid fuel use compared with the subjects without missing, whereas an increased risk of hypertension was found. Our findings are consistent when we considered various household solid fuel exposure indexes such as use of solid fuel, duration, total amount, and lifetime average amount. Therefore, it is unlikely that our observed findings are due to possible selection bias by lack of data in lifetime average amount of solid fuel use. Although qualitative assessment used in our study could suggest the source of exposure in this large population, we cannot provide information on which constituents of in-home fuel exposure had detrimental cardiovascular effects. Further longitudinal research using biomarkers of exposure are needed to investigate the specific chemicals of in-home solid fuel emissions and therefore better understand cardiovascular impact of in-home fuel exposure.

Our large-scale population-based study suggests that the use of solid fuel in the home maybe associated with an increased risk of hypertension, CHD, and diabetes in the general urban Chinese adult population. The longer the exposures to solid fuel in home, the greater their risk for cardiovascular events, implying evidence of chronic toxicity of solid fuel use in home environment. The results of this study, however, need be interpreted with caution due to the cross-sectional nature of the design. Additional longitudinal study is needed to confirm our findings.

BMI: body mass index; CI: confidence interval; CHD: coronary heart disease; CVD: cardiovascular disease; DBP: diastolic blood pressure; OR: odds ratio; PM: particulate matter; PM₁₀: particles with a mean aerodynamic diameter less than 10 micrometers; PM_2.5: particles with a mean aerodynamic diameter less than 2.5 micrometers; PAHs: polycyclic aromatic hydrocarbons; ROS: reactive oxygen species; SBP: systolic blood pressure; SHS: second-hand smoke; TSP: total suspended particle.

The authors declare that they have no competing interests.

MSL and JQH participated in study design, data collection, statistical analysis and interpretation of the results, and drafted the manuscript. DCC originated the study concept, raised funds for the study, participated in study design, statistical analyses, and critically revised the manuscript. FYZ, HLD, and LS collected data, analysis, and interpretation of data. All authors read and approved the final manuscript.