Kirk R. Smith is Professor of Environmental Health Sciences at the University of California, Berkeley, United States.
Incomplete combustion of household woodfuel releases smoke that is damaging to
human health; efficient stoves and adequate ventilation are essential.
A large part of the world’s population uses fuelwood for household cooking and space heating, mostly in developing countries. Energy from traditional biomass fuel is thought to account for nearly one-tenth of all human energy demand today (more than hydro and nuclear power together), and wood-based fuels probably make up some two-thirds of household use.
In poor developing-country households, wood, charcoal and other solid fuels (mainly agricultural residues and coal) are often burned in open fires or poorly functioning stoves. Incomplete combustion leads to the release of small particles and other constituents that have been shown to be damaging to human health in the household environment. Too little is known, however, to distinguish any differences in health effects of smoke from different kinds of biomass.
Given that levels of household solid fuel use are expected to remain high, efforts to improve household air quality are concentrated on improving stove efficiency and venting the smoke away from the home.
With proper stoves and good fuel burning practices, fuelwood and charcoal as well as other biomass can be burned cleanly, producing mostly carbon dioxide and water. Such conditions are difficult to achieve in poor rural and urban areas where small-scale inexpensive wood-burning stoves are used, however. Woodfuel that is not properly burned to carbon dioxide is diverted into products of incomplete combustion – primarily carbon monoxide, but also benzene, butadiene, formaldehyde, polyaromatic hydrocarbons and many other compounds posing health hazards. The best single indicator of the health hazard of combustion smoke is thought to be small particles, which contain many chemicals.
Figure 1 shows the emissions of major toxic pollutants from fuelwood per meal in typical cookstoves in India, relative to the most common clean fuel available, liquefied petroleum gas (LPG). The two wood species measured (Acacia spp. and Eucalyptus spp.) produced some 25 times more small particles than LPG, with other biomass fuels producing even more. Studies comparing emissions from hardwood and softwood species are not available from developing countries, although anecdotal accounts suggest some differences. Studies in the United States and other developed countries (e.g. Fine, Cass and Simoneit, 2002; Environment Australia, 2002), however, have suggested that hardwood species generally have somewhat lower emissions than softwood species for combustion in fireplaces, which may provide the closest comparison with typical stoves in developing countries. Differences by species, however, are unlikely to be significant compared to those resulting from other parameters affecting human exposure, such as fuel moisture, burning rate, ventilation and cooking behaviour.
It should be noted that use of wood charcoal, a relatively clean-burning fuel, may be increasing in some developing countries, especially in urban Africa, while the use of household fuelwood and other solid biomass is slowly decreasing. Charcoal fuel, however, can pose other kinds of health risks as well as forest impacts.
The energy ladder: pollutant emissions per meal by cooking fuel, relative to LPG (1.0 on the scale) (measured in India; note log scale)
Many developing-country households use woodfuel stoves that lack working chimneys or hoods for venting the smoke outdoors. Although there have been no large-scale statistically representative surveys, hundreds of small studies around the world in typical local situations have shown that such stoves produce substantial indoor concentrations of small particles – typically 10 to 100 times the long-term levels recommended by the World Health Organization in its recently revised global air quality guidelines for protecting health (WHO, 2005). Even stoves with working chimneys, however, do not completely eliminate indoor pollution, as there is often substantial leakage into the room and some smoke returns into the house from outside.
The significant emissions of health-damaging pollutants per unit activity, combined with daily use in close proximity to large human populations, means that household biomass fuel use produces substantial total population exposure to important pollutants – probably more exposure, in fact, than is caused by global fossil fuel use (Smith, 1993). Exposure is highest among poor women and young children in developing countries, both rural and urban, as these are the groups most often present during cooking.
Estimated burden of disease for major risk factors, measures as percentage of total healthy life years lost in the world in 2000
Since the mid-1980s and more frequently since the mid-1990s, many dozens of published epidemiological studies have examined a range of health effects from indoor air pollution due to solid fuel. Because of the difficulty and expense of assessing exposure in households, however, most have used a surrogate for true exposure – often simply whether the household was using biomass fuels or not. Moreover, most studies do not distinguish fuelwood from charcoal or from other biomass fuels or sometimes even coal. Although it is not possible to distinguish the health effects of different biomass fuels with current information, emission studies show wood to be generally somewhat cleaner than crop residues and animal dung, the other major biomass fuel types (Figure 1).
Despite the imprecision of the measure, health effects of several sorts have repeatedly been found for households that use biomass fuels, which in most cases include or consist entirely of woodfuel.
The effects include:
WHO, in a risk assessment that combined the results of many published studies (Ezzati et al., 2002), compared the burden of illness and premature death from solid fuel use with other major risk factors, including outdoor air pollution, tobacco smoking and hypertension. The results indicate that solid fuel use may be responsible for 800 000 to 2.4 million premature deaths each year (Smith, Mehta and Maeusezahl-Feuz, 2004). A comparison of the central (“best”) estimates for the risk factors examined (Figure 2) places solid fuel use approximately tenth among major health risks in the world in terms of potentially preventable lost life years. Biomass fuel is responsible for about 95 percent of this total – but the risk due specifically to fuelwood and charcoal is not known.
Biomass fuel use has been found to be associated with tuberculosis, cataracts, low birth weight in babies of exposed expectant mothers, and other health conditions in a number of other studies. The evidence is not yet considered as definitive as that for the diseases above, however.
In 2006 the International Agency for Research on Cancer reviewed the global evidence and classified household biomass fuel smoke as a probable human carcinogen, while coal smoke was classified as a proven human carcinogen (Straif and IARC Monograph Working Group, 2006). This could be interpreted to mean that biomass smoke is only weakly carcinogenic. Most of the biomass fuel evidence was from wood smoke.
Given what has been seen in studies of outdoor air pollution and active and passive tobacco smoking, heart disease could also be expected from biomass smoke indoors, but no studies seem to have been done in developing-country households. Similarly, asthma might also be expected as an outcome, and this premise is currently being investigated.
|Women cooking on a traditional stove (three large stones) with high fuel consumption and uncontrolled fire (above) and on an improved smokeless stove (below), Ghana|
It is one thing to determine that ill health is associated with a particular risk factor, but sometimes quite another to show that reduction in the risk factor will actually produce an improvement in health. One study of this type is currently under way, a randomized trial of improved wood-burning stoves in highland Guatemala. The study focuses on childhood pneumonia but is also examining heart and lung effects in women. Preliminary results have already been reported indicating a reduction in serious pneumonia among infants when households switch from an open wood fire to an improved stove with chimney (Smith, Bruce and Arana, 2006), as well as a significant drop in blood pressure among women (McCracken et al., 2005). Blood pressure is highly predictive of heart disease in all populations where it has been studied.
Although the risk estimates will continue to be refined and new health effects will probably be recognized, the challenge in a development context is to find a viable intervention that can be relied on to reduce exposure and improve health cost effectively. Alternative fuels, such as LPG, are easier to use, produce fewer emissions and cause less exposure to pollutants. However they are expensive, not accessible everywhere and culturally unfamiliar, and they may not be feasible in developing countries, especially in poor rural areas (Smith, Rogers and Cowlin, 2005).
Properly cut and dried fuelwood and well-designed, well-built and well-used improved stoves with chimneys and hoods reduce kitchen pollution substantially. Successful dissemination of well-operating and durable stoves in large populations, however, has not been easy. In some areas, cultural constraints to the adoption of improved stoves are important. Cooking traditions are deeply rooted, and in many cultures the fire is the centre of the home and has much cultural and spiritual significance. Some improved stove designs fail to give adequate attention to the cultural and social significance of how fire is used in households. That such stoves may also have social (e.g. time-saving), ecological (e.g. tree-conserving) and economic (e.g. fuel-saving) benefits, however, encourages further work to find ways to disseminate them widely.
A national programme for disseminating improved stoves is currently under way in Nepal, but no air pollution or health assessments of the results have been done as yet. Since better standard methods and new equipment for assessing the pollution and health implications of improved stove programmes are now being developed and field tested, there should be reliable information soon about the actual changes produced by this and other improved stove and fuel programmes around the world.
A national competition is under way in China to find the best of a new generation of biomass “gasifier” stoves which are now starting to be sold in the country. These stoves, which can be used to burn fuelwood as well as other types of biomass, promote internal secondary combustion of partially combusted smoke and also have chimneys; they are designed to produce extremely low emissions. Laboratory tests indicate that when such stoves are operating well they have emission levels rivalling those of LPG. Designing them to be reliable in household use as well as inexpensive is a challenge, but this second generation of improved stoves shows promise not only for high energy efficiency but also for the potential to reduce air pollution exposure substantially. This would imply substantial reduction in global warming impact as well. Measurements in households over time will be needed to verify these benefits.
It could be said that the smell of wood smoke from the hearth is as old as humanity itself, since many anthropologists define the beginning of humanity as the moment when our ancestors learned to control fire. With such a long association, the risks from wood smoke may be difficult to recognize. And wood, of course, dominated human fuel demand for hundreds of thousands of years in most parts of the world. Even today, it is probably true to say that biomass fuels provide most of the energy for most of humanity.
Nostalgia triggered by the sight and smell of a fire in the hearth has fostered complacency about the risk of an open wood fire and continues to do so today. Inefficient conversion of woodfuel – and indeed of all fuels – to energy has negative economic, health and environmental impacts. Household use of woodfuel in devices that do not burn the fuel completely is not compatible with a long-term strategy of sustainable development. Cooking and heating processes are also important for the correct use of fuels and stoves, to ensure that less energy is used and less fuel consumed.
Wood and other biomass fuels can be burned cleanly with the right technology and thus can have a long-term role in sustainable development where they are renewably harvested. Thus programmes for the modernization of woodfuel use for household and cottage industries in the poorest areas of developing countries should be part of the development agenda.
If any population in developed countries experienced the high pollution levels found in hundreds of millions of poor village households from biomass fuel, no additional evidence would be needed to trigger massive intervention efforts – another sign of the extreme disparities in the world. In poor countries, however, reliable evidence and careful assessments are required to determine the most cost-effective means and priorities for addressing the wide range of health and other problems due to poverty.
Environment Australia. 2002. Emissions from domestic solid fuel burning appliances. Technical Report No. 5. Parkes, Australia. Available at: www.deh.gov.au/atmosphere/airquality/publications/report5
Ezzati, M., Lopez, A.D., Rodgers, A., Vander Hoorn, S., Murray C.J.L & Comparative Risk Assessment Collaborative Group. 2002. Selected major risk factors and global and regional burden of disease. Lancet, 360: 1347–1360.
Fine, P.M., Cass, G.R. & Simoneit, B.R.T. 2002. Chemical characterization of fine particle emissions from the fireplace combustion of woods grown in the southern United States. Environmental Science and Technology, 36: 1442–1451.
McCracken, J.P., Díaz, A., Arana, B., Smith, K.R. & Schwartz, J. 2005. Improved biomass stove intervention reduces blood pressure among rural Guatemalan women. Presented at the 17th Annual Conference of the International Society for Environmental Epidemiology, Johannesburg, South Africa, 13–16 September.
Smith, K.R. 1993. Fuel combustion, air pollution exposure, and health: the situation in developing countries. Annual Review of Energy and Environment, 18: 529–566.
Smith, K.R., Bruce, N. & Arana, B. 2006. The Guatemala air pollution intervention trial (RESPIRE). Presented at the Annual Conference of the International Society for Environmental Epidemiology, Paris, France, 2–6 September.
Smith, K.R., Mehta, S. & Maeusezahl-Feuz, M. 2004. Indoor smoke from household solid fuels. In M. Ezzati, A. Lopez, A. Rodgers, S. Vander Hoorn & C. Murray, eds. Comparative quantification of health risks: global and regional burden of disease due to selected major risk factors, pp. 1435–1493. Geneva, Switzerland, WHO.
Smith, K.R., Rogers, J. & Cowlin, S.C. 2005. Household fuels and ill-health in developing countries: what improvements can be brought by LP gas (LPG)? Paris, France, World LP Gas Association & Intermediate Technology Development Group.
Straif, K. & IARC Monograph Working Group. 2006. Carcinogenicity of some indoor pollutants: emissions from household combustion of coal, household combustion of biomass fuel, and high-temperature frying. Lancet Oncology (In press).
World Health Organization (WHO). 2005. WHO air quality guidelines global update 2005. Copenhagen, Denmark, WHO Regional Office for Europe. Available at: www.euro.who.int/Document/E87950.pdf