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– 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.

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 usually use for cooking, 64.5% reported LPG and 32.3% charcoal. Among those who use charcoal, 38.6% use it often. About half have a kitchen located inside a house with good ventilation. Almost all reported having TWC either sometimes or often. Most of them cook every day.

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).

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 ( Table 4). As compared to those who never use it, cooks who sometimes use charcoal have twice the risk of HBP (OR=2.04; 95%CI 1.32–3.15) and those who use charcoal over twenty years have 1.38 times the risk of HBP (OR=1.38; 95%CI 1.01–1.89). In the literature, there is increasing evidence to link biomass smoke and HBP ^{24, 25} . A study in Peru found that biomass users had an increased risk of both prehypertension (OR=5.0; 95%CI 2.6–9.9), and hypertension (OR=3.5; 95%CI 1.7–7.0) ¹² . In Bangladesh, it was found that among rural women, each additional year of biomass smoke exposure could increase the risk of HBP by 61% (OR=1.61; 95%CI 1.16–2.22) ²⁶ . In Bangladesh, it was found that among rural women, one additional year of biomass smoke exposure to increase risk of HBP by 61% (OR=1.61; 95%CI 1.16–2.22) ¹³ . 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 ( Table 3) with a significant OR of 2.74 (95%CI 1.66–4.53) for family members ( Table 4). The result showed a difference in the risk of HC among those who use wood, charcoal, and LPG. This risk also varied particularly according to the frequency of charcoal use. Compared to nonusers, an elevated risk of HC was found among cooks who sometimes use charcoal (OR=2.04; 95%CI 1.32–3.15), and among those who use charcoal over 20 years (OR=1.73; 95%CI 1.22–2.44). Among cooks, every year of using charcoal will increase risk of HC by about 1% (OR=1.010; 95%CI 1.002–1.017). Risk of HC was also increased among family members of cooks who often use charcoal (OR=1.52; 95%CI 1.02–2.27). Though the evidence was limited, other studies have found an association between cholesterol and COPD, a disease often found among biomass users ²⁶ . A study in Ghana also found a strong association between wood smoke exposure and several hematological and biochemical indices, including HC (OR=20.44; 95%CI 2.610–160.2) ²⁷ . 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 ( Table 3). Logistic regression analysis revealed a significant risk of diabetes among cooks using charcoal sometimes (OR=2.09; 95%CI 1.17–3.73) as compared to the never user group ( Table 4). Among family members of cooks, risk of diabetes was elevated by using biomass fuel (OR=1.88; 95%CI 1.03–3.46), and years of using charcoal (OR=1.013; 95%CI 1.001–1.024). Similar results have also been reported by several studies on the effect of particulate matter or traffic-related air pollutants on diabetes ²⁸ . In addition, experimental studies may provide potential mechanisms, including glucose homeostasis, systemic inflammation, stress in the liver and endoplasmic reticulum, and alterations of mitochondrial and other adipose tissue ²⁹ . Currently, epidemiological studies on the effect of indoor air pollution on diabetes are rare. A study of women in Honduras reported an association between the prevalence of prediabetes/diabetes and PM2.5 in kitchen biomass cooking stoves ³⁰ . This was consistent with the results from a previous study from Shanghai Putuo, which also found an elevated risk of several cardiovascular diseases including diabetes (OR=2.48; 95%CI 1.59–3.86) among people using solid fuel at home ¹⁴ .

Those who use biomass for cooking had a risk of CHD 4.35 times (95%CI 0.10–18.97) of LPG users; and those using charcoal sometimes had risk of CHD 4.11 times (95%CI 1.40–12.11) of never user group. These results are consistent with evidence from cigarette smoke and ambient air pollution. 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.

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.