Cooking with biomass fuel and cardiovascular disease: a cross-sectional study among rural villagers in Phitsanulok,

Burning biomass fuel is a major source of indoor air pollution; Background: about 40% of Thai people still use biomass for cooking. There is increasing evidence of the association between biomass smoke exposure and serious health effects including cardiovascular disease. The object of this cross-sectional study was to investigate the association between biomass use for household cooking and cardiovascular outcome, including coronary heart disease, hypertension, high cholesterol, diabetes mellitus, and stroke among rural villagers in Phitsanulok, Thailand. Data from 1078 households were collected using a face-to-face Methods: interview questionnaire. After being adjusted for gender, age, cigarette smoke, Results: secondhand smoke, and exposure to other sources of air pollution, it was found that the family members of cooks using biomass fuel were at risk of coronary heart disease (CHD; OR=4.35; 95%CI 0.10–18.97), high blood pressure (HBP; OR=1.61; 95%CI 1.10–2.35), high cholesterol (HC; OR=2.74; 95%CI 1.66–4.53), and diabetes (OR= 1.88; 95%CI 1.03–3.46). Compared to LPG use, using wood was associated with stroke (OR=7.64; 95%CI 1.18–49.61), and using charcoal was associated with HC (OR=1.52; 95%CI 1.04–2.24). Compared to never user, household cooks who sometimes use charcoal had an increased risk of HBP (OR=2.04; 95%CI 1.32–3.15), HC (OR=2.61; 95%CI 1.63–4.18), and diabetes (OR=2.09; 95%CI 1.17–3.73); and cooks who often use charcoal had an elevated risk of stroke (OR=3.17; 95%CI 1.04–9.71), and HC (OR=1.52; 95%CI 1.02–2.27) to their family members. The study results were consistent with those found in studies Conclusions: from other parts of the world, and supports that exposure to biomass smoke increase cardiovascular diseases. The issue should receive more attention, and promotion of clean fuel use is a prominent action.


Introduction
Cooking smoke is a major source of household air pollution, which affects billions of people around the world, especially in developing countries. Globally, nearly 3 billion people still use solid fuels (wood, charcoal, crop residues, and dung) for cooking and heating 1 . Smoke from wood burning contains a large number of pollutants, including particulate matter, carbon monoxide, nitrogen dioxide, formaldehyde, and a number of highly toxic organic compounds, such as benzene, 1, 3 butadiene, benzo[a]pyrene and other toxic polycyclic aromatic hydrocarbons 2 .
The use of solid fuel for cooking and/or household energy sources increases respiratory and non-respiratory illnesses in both adults and children. Those effects that are well established are acute respiratory infections, chronic obstructive pulmonary disease (COPD), lung cancer, asthma, tuberculosis, and cataracts 3,4 . In children, biomass use is related to mortality, and acute lower respiratory tract infections, and some other non-respiratory illness, such as poor lung function, low birthweight, nutritional deficiency, and impairment of learning ability 5,6 .
Though with limited evidence, recent studies linked biomass smoke exposure and cardiovascular diseases (CVD), e.g. coronary heart disease (CHD), hypertension or high blood pressure (HBP), diabetes, and stroke [7][8][9][10] . In laboratory studies, chronic exposure to biomass smoke increased the thickness and plaque of blood vessels 11 . In epidemiological studies, Peruvians who live in high altitude environments and use biomass fuel had an elevated prevalence of HBP 12 . A study among villager women in Bangladesh reported an association between elevated cumulative exposure to biomass smoke and the prevalence of HBP 13 . A similar result was found in a study in Shanghai Putuo, which found using solid fuel increases the risk of HBP, CHD, and diabetes 14 ; and a study in Shanxi, China reported an increased risk of HBP, CHD, stroke, diabetes, and dyslipidemia 15 A recent study by Yu et al. 16 also linked solid fuel use to cardiovascular mortality.
On a global scale, CVD is the number one cause of death and is responsible for about 18 million deaths annually 17 . In Thailand, CVD accounts for 23% of the national mortality 18 . Currently, there is no study on the effect of biomass smoke on CVD in Thailand. It was reported that about 40% of Thai households still use biomass, mainly charcoal, wood, and agriculture residue, for cooking 19 . The objective of this study is to investigate a possible association between biomass use for cooking and cardiovascular diseases, including CHD, HBP, HC, diabetes, and stroke. The study uses data from a cross-sectional survey among rural villagers in Phitsanulok, Thailand. The result could be used for disease prevention and control, and to support the global literature.

Study design and setting
This is cross-sectional study. Participants are rural villagers living in Phitsanulok Province, Thailand. Phitsanulok is a midsize province located about 400 km north of Bangkok. There are 866,891 people in the area of 9 districts. Most of the people are rice farmers 20 .
Study participants and sampling procedure Participants were randomly selected using multistage sampling. Out of the 9 districts in Phitsanulok province, 5 were randomly selected. In each district, one sub-district and a local healthpromoting hospital were approached. In each sub-district with support from the local health-promoting hospital, a total of 1,150 households were approached and 1,134 (98.6%) people agreed to participate in the study. In each household, only one participant who was responsible for household cooking and aged over 20 years was selected. After data cleanup, 56 (4.9%) items of data were missing important information, such as age, gender, cooking practice. The final data from 1,078 people were used for statistical analysis.
The minimum sample size was calculated to be 1,034, using unmatched cross-sectional study with the following assumptions:two-sided significance level = 95%; power of detection = 80%; percent unexposed with outcome = 5%; and odds ratio = 2.0.

Study questionnaire
Data was collected using a face-to-face interview questionnaire, which was administered by 15 village health volunteers (provided as Extended data in English 21 ). The interviews took place in the house of participants. The data was collected during the period of May-June 2017. Health volunteers were all trained on how to properly carry out the interview and use the questionnaire. The questionnaire was designed to collect information on demographic data, fuel use for cooking, and other cooking practices. In addition to general demographic data, participants were also asked a history of tobacco use (ever, never), and working in factory environments using "yes" or "no" questions. Ever smoker referred to those who smoke more than 100 cigarettes in their lifetime. Data on pesticide use was also measured by "yes" or "no" questions: "Have you ever spray or mix pesticide?". For cooking fuel data, we asked about the types of fuel they used for cooking food (wood, charcoal, LPG, electricity), and the frequency of using each types of fuel. Data collected on cooking practices were types of cooking oil, the frequency of tears while cooking (TWC) (never, sometimes, often), kitchen location (inside a house, outside a house, both inside and outside a house), and the characteristics of kitchen ventilation (good or poor ventilation).
The presence of cardiovascular disease was determined by the participant response to the question: "Have you ever been diagnosed with the following diseases (coronary heart disease (CHD), high blood pressure (HBP), high cholesterol (HC), diabetes mellitus, stroke) by a medical doctor?". For diseases among their family members, we asked "Did you have a family member with the following diseases?".
The content validity of the questions was tested by three experts, and the Index of Item Objective Congruence (IOC) was between 0.7-1.0. The questionnaire was also tested for question sequencing and understanding using a group of 30 people with a similar background to the intended participants.

Statistical analysis
Demographic and prevalence of cardiovascular disease were descriptively analyzed. Comparison between groups were analyzed using chi-square test for categorical variables, and independent t-test for continuous variables. The association between cardiovascular disease was analyzed using logistic regression with odds ratios (OR) and 95 percent confidence interval (CI) adjusted for gender (male, female), age (continuous data), cigarette smoking (ever, never), living smoker (yes, no), working with smokers (yes, no), and exposure to air pollution (yes, no). All statistical analyses were performed using IBM SPSS version 19 and OpenEpi (online version 3.01). Statistical significance was set at a p-value of less than 0.05.

Ethical considerations
The study was approved by the Ethical Committee of Naresuan University (COA No. 485/2016), and written informed consent from the respondents was obtained before the interviews were conducted.

Results
Most of the respondents were women (84.2%) with a mean age of 53.04 ± 12.93 yr. The highest education levels were primary school or high school. Most were farmers (36.0%) and 20.2% were causal workers on farms. About 10% were smokers and 33% lived with a smoker. Additional information on the demographic data is shown in Table 1 and in Underlying data 22 .
About 70% of the respondents reported using biomass for cooking (Table 2). However, when asked for fuel types that they The study found HBP, HC, and diabetes to be the most common cardiovascular outcomes (Table 3). Compared to non-user group, biomass users had a significantly higher prevalence of HBP, and HC, and their family members also had more incidence of HBP, HC, diabetes, and heart disease.
Further analysis using logistic regression and control variables, revealed that compared to gas users, biomass users had family members with elevated CHD, HBP, HC, and diabetes (Table 4). Among different types of fuel, household cooks using wood had a significant elevated risk of CHD (OR=7.64, 95%CI 1.18-49.61), and their family members had an elevated risk of HC (OR=1.52, 95%CI 1.04-2.24). Comparing frequency of charcoal use, those who use charcoal sometimes or often are more likely to have CHD, HBP, HC, and diabetes as compared to those who never use charcoal. The family members of charcoal users also had a significant increase of HC and stroke. When using TWC as an indicator for smoke exposure, it was found that those who always had TWC had significantly increased risk of stroke (OR=2.16; 95%CI 1.08-4.32), and those with sometimes TWC had a CHD risk (OR=2.64; 95%CI 1.02-6.81). Regarding kitchen location, the family members of cooks having kitchens both inside and outside a house had an elevated risk of stroke (OR=4.60; 95%CI 1.14-18.54).

Discussion
This study presented an association between cardiovascular diseases and exposure to smoke from biomass, mainly charcoal, which is relatively cleaner when compared to wood, coal, or dung, a biomass which were often found in the literature. The study  also showed that biomass use not only affects household cooks but also their family members. It was found that biomass users have a higher prevalence of HBP and HC, and their family members had a higher prevalence of HBP, HC, diabetes, and CHD (Table 3). Further analysis using logistic regression and control for potential confounder showed a significant OR of biomass use and CHD(F), HBP(F), HC(F), and diabetes(F) ( Table 4). Compared to LPG, wood use also had a strong association with stroke (OR=7.64; 95%CI 1.18-49.61). Among charcoal users, those who use it sometimes or often had an elevated risk of CHD, HBP, HC, and diabetes for themselves, and risk of HC and stroke for their family members. The results are consistent with the literature. Previous research found biomass smoke contains a lot of pollutants, especially fine particulates, and carbon monoxide which are known to cause cardiovascular effects 2 . In laboratory studies, biomass smoke exposure was associated with endothelial inflammation 23 .
For hypertension, we found both cooks and their family members have a higher prevalence of HBP (Table 3). Further analysis indicated an elevated risk of HBP (OR= 1.61; 95%CI 1.10-2.35) among family members of cooks using biomass for cooking ( Recent studies in Honduras also linked PM2.5 and black carbon exposure and HBP among women using traditional and improved stoves 25 . The current study also found a higher prevalence of HC among cooks and their family members using biomass fuel ( The higher OR might be explained by the difference in biomass types, which was found to be wood in other studies, while most of respondents in this study use charcoal which is relatively cleaner.  We found about 10% of the respondents had type 2 diabetes and the prevalence of the disease was higher among biomass users ( In animal studies, biomass fuel smoke caused arteriosclerotic effects in animal blood vessels 11 . Studies found COPD as a risk factor of CHD 31 ; and our previous study found elevated chronic symptoms, such as chronic cough, dyspnea and runny nose which is a sign of COPD among cooks using biomass fuel for cooking 26 . Epidemiological studies also reported an association between solid fuel smoke exposure and CHD 32 . A study in Pakistan found that rural women who currently use solid fuel had an increased risk of acute coronary syndrome (OR=4.8; 95%CI 1.5-14.8) 33 . This is consistent with a study from Shanghai Putuo, which found solid fuel use in the home is associated with CHD (OR=2.58; 95%CI 1.53-4.32) 14 , and study from Shanxi, China found an elevated risk of CHD (OR=2.25) among solid fuel users 15 .
In this study, respondents who use wood (OR=7.64; 95%CI 1.18-49.61) and charcoal (OR=2.03; 95%CI 0.58-7.09) had an elevated risk of stroke as compared to clean fuel users (Table 4). Among charcoal users, those using charcoal sometimes (OR=1.66; 95%CI 0.44-6.29) and often (OR=2.76; 95%CI 0.56-13.50) seem to have a higher risk of stroke but a significant elevation was found only among the family members of cooks using charcoal often (OR=3.17; 95%CI 1.04-9.71). This was consistent with the literature. The association between household solid fuel use and stoke were also reported in a study from Shanghai Putuo (OR=1.87; 95% CI 1.03-3.38) 14 , and study from Shanxi, China (OR=1.64) 15 . In ambient settings, a long-term effect of PM exposure on cardiovascular disease, including stroke, was well established 34 . It was estimated that for each 10 µg/m 3 increment in PM10, risk of overall stroke events will increase by 1.06 times (95%CI 1.02-1.11), and the risk of stoke mortality by 1.08 times (95%CI 0.99-1.18) 35 .
One potential drawback of this study was the use of self-reported data of diseases. Without the confirmation of medical records, the survey diseases are subjected to information bias. However, the bias will be distributed equally to all comparison groups, and this tends to underestimate the result. The number of participants included in this study was also rather small to detect the actual association of a rare disease, e.g. stroke. By using cross-sectional design, the study result cannot explain the causal relationship, because it is not known whether exposure or the disease occurred first. However, the problem is minimal for rare diseases.

Conclusions
The results from this study support research findings in other part of the world that using biomass for cooking increases the risk of cardiovascular diseases. This study also confirms the negative effects of using charcoal, which is considered to be a relatively cleaner fuel as compared with wood, dung, coal, and other agricultural residues. Concerned organizations should pay more attention to the issue and promote clean fuel usage.