Cooking with wood or coal increase risk of major eye diseases


A study involving nearly half a million people in China reveals a clear link between cooking with wood or coal, and an increased risk of major eye diseases that can lead to blindness, according to a report published today in PLOS Medicine.

About half the world’s population – 3.8 billion individuals – are exposed to household air pollution from cooking using ‘dirty’ solid fuels, such as coal and wood.

While previous studies have reported a possible link between cooking with solid fuels and an increased risk of cataracts in women, it is unclear whether similar associations also exist with other major eye diseases, such as conjunctivitis, keratitis and glaucoma.

The researchers from the University of Oxford’s Nuffield Department of Population Health (NDPH) and the Chinese Academy of Medical Science and Peking University, Beijing, analyzed data from almost half a million Chinese adults in the China Kadoorie Biobank.

All the study participants were asked about their cooking habits by questionnaire, then tracked for hospital admissions of major eye diseases through linkage to health insurance records. Over the ten-year follow-up period, there were 4,877 cases of conjunctiva disorders, 13,408 cataracts, 1,583 disorders of the sclera, cornea, iris and ciliary body (DSCIC), and 1,534 cases of glaucoma among study participants.

Compared with those who cooked using clean fuels (electricity or gas), solid fuel users tended to be older, female, rural residents, less educated, agricultural workers and regular-smokers. After accounting for these factors properly, the results showed:

  • Long-term use of solid fuels for cooking was associated with 32%, 17%, and 35% higher risks of conjunctiva, cataracts, and DSCIC, respectively, compared with those who cooked using clean fuels;
  • There was little difference in risk between the different types of solid fuel used (for instance, coal versus wood);
  • There was no association between long-term use of solid fuels and an increased risk of glaucoma;
  • Individuals who switched from using solid to clean fuels for cooking had smaller elevated risks (over those who had always used clean fuels) compared to those who did not switch. People who switched had 21%, 5% and 21% higher risk for conjunctiva, cataracts, and DSCIC, respectively.

Dr Peter Ka Hung Chan, research fellow in the Nuffield Department of Population Health, University of Oxford, and lead author of the study, explained these findings: “The increased risks may be caused by exposure to high levels of fine particulate matter (PM2.5) and carbon monoxide, which can damage the eye surface and cause inflammation.”

Burning wood also increases the risk of eye injury from sparks or wood dust. The investigators propose that the reason there was no association between solid fuel use and risk of glaucoma was because this disorder affects internal eye structures, which are less exposed to pollutants in the air.

Among individuals who used solid fuels for cooking, the study did not find significant difference in excess risk between those with and those without cookstove ventilation (such as a chimney).

In China, despite recent success from government-led clean stove initiatives, around 400 million people still used solid fuels for domestic purposes in 2018. Worldwide, the percentage of the global population relying on solid fuels for cooking has only decreased modestly since 2010, by 11%. Most of these people live in low-income countries, particularly in Africa and Asia. This can make it difficult for those affected by eye disorders to access effective and affordable treatment.

Professor Zhengming Chen, Professor of Epidemiology and Director of China Programmes at the Nuffield Department of Population Health, University of Oxford, and a senior author for the study, said, “Among Chinese adults, long-term solid fuel use for cooking was associated with higher risks of not only conjunctiva disorders but also cataracts and other more severe eye diseases. Switching to clean fuels appeared to mitigate the risks, underscoring the global health importance of promoting universal access to clean fuels.”

Professor Liming Li from Peking University and a senior author for the study, said, “Our study adds yet another piece of evidence to support governmental efforts to facilitate fuel transition, and the general public should be informed about the potential risks of eye diseases, some of which are highly disabling, related to solid fuel use.”

Commenting on the research, Imran Khan, Director of Programme Strategy & Development at international development organization Sightsavers, commented that “Sightsavers’s vision is of a world where no one is blind from avoidable causes and the study highlights that eye problems result from a number of factors, not just the traditional causes we commonly think of.”

“Environmental factors and societal traditions, such as cooking with solid fuel, can have a big effect on eye health and show the need to work across all levels of health systems to improve outcomes. From government to communities, it’s important to raise awareness of eye conditions, reduce these avoidable causes, and provide accessible health services.”

Wood smoke is a complex mixture of gases, liquids and solid particles (aerosol) produced by incomplete combustion or pyrolysis of wood and other wood products such as charcoal, wood pellets, sawdust, and so on, at elevated temperatures and reduced oxygen [1].

While complete combustion of wood requires adequate supply of oxygen and produces carbon dioxide and water with no visible smoke, incomplete combustion results in the production of smoke.

Besides the major combustion products (carbon dioxide and water), wood smoke consists of more than 200 distinct organic compounds [2], many of which have been shown to induce acute or chronic health effects in exposed humans. Of these, particulate matter, especially the fine particulate matter (PM2.5), is of most concern. Other hazardous components of wood smoke are carbon monoxide, nitrous oxides, formaldehyde, and polycyclic aromatic hydrocarbons (PAHs), including carcinogens such as benzo(a)pyrene [2, 3].

Human exposure to wood smoke is as old as mankind. In prehistoric times, man used wood as the primary fuel for heating in the cold, lighting in the dark, and for cooking food [3] and for thousands of years, wood served as the sole source of energy for humankind [4].

Although increasing modernisation has led to the supplementation of wood by fossil fuels (such as coal and petroleum products) and electricity, it is still a major source of energy for the population in developing countries accounting for 50 to 90% of the fuel used for cooking and heating purposes in this population [4, 5].

The demand and use of wood fuels has also increased (especially among the poor) in many developed nations due to scarcity of fossil fuels coupled with an increasing interest in sustainable energy production [4, 6].

Although there has been a decline in the proportion of the world’s households relying mainly on solid fuels for cooking, with about 60 percent of the world’s population currently using modern fuels [7], the population in sub-Saharan Africa has not kept up with this global trend as reports show that this population still has the most widespread use of solid fuels [8]. About 83% of the population in WHO African region were estimated to be primarily reliant on polluting cooking options [9].

Of all the solid fuels (wood, coal, charcoal, dung, crop residues), wood fuels (firewood, charcoal and other crop residues) are predominantly used among the population in SSA, accounting for more than 90% of residential energy consumption in rural areas [10]. It has been reported that per capita consumption of wood fuel in SSA is 2–3 times higher than that in any other region [11]. Of the 2.770 billion people in developing countries projected to depend on wood fuel by 2030, sub-Saharan Africa alone accounts for 33.14% (918 million people) of this population [12].

Indoor air pollution from inefficient traditional wood burning stoves and open fires remains the major source of wood smoke exposure to humans, especially households in rural areas of developing countries who rely exclusively on woods for their cooking and heating needs [2]. Women, who do most of the household cooking, children under the age of five and the elderly who spend more time in the household are more exposed [13, 14]. Higher level of exposure may be seen in individuals with some occupations.

These include wild land fire fighters [15], charcoal producers [16, 17, 18], farmers involved in agricultural burnings [2] and individuals involved in commercial cooking and food processing using wood fuel [19, 20, 21]. Other significant sources of exposure may include bushfires, consumption of food items preserved or processed with wood smoke [19, 22, 23] and the ambient air [24, 25].

Wood fuel is cheap, has widespread availability and potential renewability [11] and as compared to fossil fuels, might help to reduce impacts of long-term carbon emissions on climate change [26]. However, exposure to smoke from its combustion has been of significant public health concern especially in developing countries. Household air pollution (HAP) from solid fuels globally accounted for 2.576 (2.216–2.969) million deaths and 77.16135 (66.08637–88.04887) million disability-adjusted life years (DALYs) in the year 2016 [27], amounting to 7.87% and 7.14% of total deaths and DALYs, respectively, attributable to all risk factors in 2016. With these figures, HAP was ranked as the 8th leading mortality risk factor and the 10th leading risk factor for disability-adjusted life-years (DALYs), globally in 2016 [27].

Low- and middle-income countries in SSA had the highest number (134) of age-standardized deaths (globally) per 100,000 capita from HAP in 2016 [9]. In 2017, 24% of global deaths and 34% of global DALYs attributable to household air pollution occurred in sub-Saharan Africa [28]. HAP from solid fuels accounted for about 35.64 % and 36.33% of total deaths caused by lower respiratory tract infections in sub-Saharan African children <5 years and women between aged 15–49 years, respectively [29].

Over the past decade, there has been a number of reviews focussing on household solid fuel use and its effect on human health [30, 31, 32]. However, there remains a lack of solid evidence on wood smoke exposure and the associated health effects in sub-Saharan African population. Hence, this review aims to systematically assess available evidence on exposure to pollutants in wood smoke and the health effects associated with this exposure in sub-Saharan African population. The findings of this review may help prioritize methods to control emissions from wood burning and reduce its associated diseases across sub-Saharan Africa.


More information: Ka Hung Chan et al, Long-term solid fuel use and risks of major eye diseases in China: A population-based cohort study of 486,532 adults, PLOS Medicine (2021). DOI: 10.1371/journal.pmed.1003716


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