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close this book Boiling Point No. 02 - Special Edition April 1991
View the document Smoke Pollution
View the document Dialectics of Improved Stoves by Kirk R Smith, East-West Centre, Hawaii, USA
View the document White Rabbits !
View the document The Chimney Approach to Smoke Pollution
View the document Research Needs- Biofuel Stove Technology
View the document Woodsmoke - who will put it out?
View the document Cookstove Smoke - The Other Side of the Coin
View the document Domestic Air Pollution in Rural Kenya
View the document A Chimney is Not Enough!
View the document Indoor Air Pollution in Rural Malaysia
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Domestic Air Pollution in Rural Kenya

Tests with traditional and Maendeleo woodburning stoves for emissions of PAHs, TSPs and CO in

1988-90 (abridged). by Amelia Mutere, Assistant Science Editor, International Centre of Insect Physiology, Nairobi, KENYA


Air pollution has often been described as an urban problem in industrialized countries, where the high level of economic development has led to a high consumption of energy per capita. However, scanty research in developing counties shows that for some important pollutants significant, if not larger global populations are at risk. This occurs in the inverse situation ie. agricultural, rural, indoor locations where traditional biomass fuels are dominant. In Kenya it is estimated that 70% of the population use wood, charcoal and agricultural residues to meet its energy needs for cooking and heating.

The smoke from burning biomass is the most ancient and ubiquitious form of air pollution. As people are so familiar with such smoke it is often assumed to be benign. To many people the smell of wood or other biomass smoke in the air is pleasant, leading to nostalgia about the pastoral life. Indeed, smoke in the rural households can be very useful for mosquito control, thatch and food preservation, food and fuel drying.

Nevertheless, these biomass fuels yield high emissions of a number of pollutants. The most dangerous emissions actually occur during incomplete combustion of carbonaceous materials which releases a wide range of organic materials of entirely different character from those in the fuel itself. Seven combustion related categories of air pollutants had received the most attention: suspended particulates (SP), carbon monoxide (CO), hydrocarbons (HC's) and other organic emissions such as nitrogen oxides (NOX), sulphuroxides (SOX), ozone (O3) and lead (Pb).

All of these pollutants can be found along with CO2 in biomass smoke, but the relative amounts differ from those found in fossil fuel smoke. Many of the hydrocarbons in biomass smoke have been identified as being mutagenic compounds and some are carcinogenic. One polycyclic aromatic hydrocarbon of particular interest, benzo(a)pyrene, is the most studied of all the carcinogenic chemicals. Emissions, concentrations, exposures and doses are important in determining the health effects of biomass smoke.

Five major categories of ill health could be expected from high exposures to biomass combustion products:

1) Chronic obstructive lung disease (COLD)

2) Heart disease, particularly cor-pulmonal caused by pulmonary damage

3) Acute respiratory infections (ARI) particularly in children due to degradation of the respiratory de fence mechanisms.

4) Low birth weight due to maternal exposures and associated with a range of perinatal and infant ill-health.

5) Eye disorders, conjunctivitis, blindness.

6) Cancer

In addition to their interest in deforestation, many governments, NGOs and aid agencies have become concerned about these health effects and have fumed to research on improved stoves to reduce smoke and increase fuel efficiency. Lately however, it has become clear that earlier efforts in stove development and design suffered from lack of proper monitoring, evaluation and field testing. A new generation of stove designs and dissemination methods is being implemented in the 1990s and this will take advantage of the past lessons learnt.

In order to determine the success of improved stove programmes it is necessary to be able to compare quantitatively the performance of improved and traditional stoves in a variety of parameters, two are efficiency and smokiness. Field measurements are absolutely essential to understand the performance and acceptance of a particular stove.

In Kenya improved cookstoves are presently being designed and distributed by KENGO and Maendeleo among others. There is a need for epidemiological data pertaining to health effects and performance data on the various improved stoves being tested among the population so as to be able to recommend the design with not only the greatest thermal efficiency but also the best performance with respect to harmful emissions.

Carbon Monoxide Emissions

Carbon monoxide is a colourless, odourless gas. It is highly poisonous and has a high affinity to haemoglobin (HB). It causes asphyxia by depriving the tissues of the necessary supply of oxygen. CO interferes with the ability of the blood cells to carry oxygen. Heart and nerve tissues are particularly susceptible to oxygen deficiency so that CO can seriously impair coronary and cell nervous system junctions. Research studies done on the effects of pollutants from biomass combustion have proven clinically beyond reasonable doubt that they cause or aggravate diseases to human health.

The commonly reported health effects of exposure to CO and smoke during domestic cooking are headache, dizziness, external dysphnea, diarrhoea, urinary frequency, sweating, thirst, weight loss, loss of libido, irritability and insomnia.

The factors that lead to higher sensitivity to CO exposure are of special concern to women. There are basically three reasons for this concern. first, women generally have less haemoglobin in reserve than men. One consequence of this is that women are more prone to anemia than men. Secondly, during pregnancy there is an additional demand on women's haemoglobin. Lastly there is evidence from animal studies and women who smoke that CO can effect the unborn child by reducing birth weight and increasing prenatal death rate.

CO monitoring took place about 45 km outside Nairobi. In Nyachapa village more than 50% of houses have Maendeleo stoves. Firewood and agricultural residues are the only fuels used for cooking although charcoal is sometimes used for space heating.

The equipment used to measure CO was a Neutronics Portable Combustion Optimizer (PCO). It is used primarily to test combustion efficiency in stoves with flues. The PCO simultaneously measures and digitally displays the CO content in parts per million (ppm CO). In 18 houses, CO measurements were taken, each measurement was continuous, took about 10 minutes and was repeated 3 times. Twelve homes had the Maendeleo improved stove and 6 have the traditional 3-stone stove. For each measurement, 3 positions were sampled as follows:

1. Sampling the ambient air outside the kitchen.

2. Measurements at the spot where the cook sits while in the kitchen.

3. Measurements at the position of the cook while stirring the food.

The probe was always placed at the position of the cook's nose and mouth to represent the gas that the cook is breathing. Table I gives the CO concentration for 12 homes with the Maendeleo stoves, while Table 2 gives the CO concentration for 6 homes with the traditional 3-stone stoves.

Table 1 - CO Concentration, Maendeleo Stove

Ambient Air

Sitting Position

Stirring Position








































The results show the average readings for the cook's sitting position of 49 ppm for the 3-stone was nearly at the WHO maximum acceptable occupational concentration of 50 ppm over an 8 hour period. That is nearly 2.7 times higher than the average reading for the cook's sitting position for the Maendeleo stove (19 ppm).

Table 2 - CO Concentration, 3-stones

Ambient Air

Sitting Position

Stirring Position






















There were high average CO concentrations for both the Maendeleo (88 ppm) and the 3-stone stove (212 ppm) for the cook's stirring position. But again, the Maendeleo concentrations were 2.4 times lower.

The most notable factor to this researcher was the ventilation factor. When the kitchen door was opened and the window of the kitchen open, the cross ventilation would cause the CO concentration to go down to below 10 ppm. All of these measurements were taken with the door closed because unless someone was entering or exiting the door normally remained closed.

There needs to be more spot and continuous CO measurements taken to evaluate the exact exposure. Nonetheless, with good ventilation, use of dry biofuels and the use of the Maendeleo stove, it is possible to cook within the WHO occupational limits.

Particulates - TSP and PAH

Many countries consider particulate matter to be one of the most serious pollutants. The United States listed suspended particulates as the second most important primary pollutant only after SO2. Particulates aggravate respiratory and heart diseases, cause coughing, throat irritation and chest discomfort. They are known to have high concentrations of carcinogenic chemicals such as benzo(a)nyrene.

WHO has set recommended limit for TSP's for 8 hour public exposure at between 100-150 mg/m3. The Environmental Protection Agency (EPA) of USA has established 260 mg/m3 for TSP's over 24 hours and this concentration is not to be exceeded except once in a year. These levels will be useful for comparison with the TSP's observed in this study.

Collecting the smoke or sampling was done by the relatively new method of personal air samplers which measure the air that the individual is inhaling. The samplers were set to inhale 3.0 litres per minute. Once the smoke had been collected on paper filters, the total suspended particulates (TSP) were gravimetrically calculated (weighing the filter before testing and after the collection period) and then the PAM's were extracted and measured by high performance liquid chromatography.

TSP Measurements

The average cooking time in the homes is between 5.4 and 5.7 hours. TSP levels for the 3 stone stoves were 2.6 times higher on average than for the improved stoves. Nonetheless, the Maendeleo levels are still above the recommended level of 100-150 ug/m3 over 8 hours set by WHO and 260 ug/m3 recommended over 24 hours by the EPA. Taking the TSP average obtained for the Maendeleo of 1725 ug/m3 and figuring it over 8 and 24 hours, one will get 1300 ug/m3 and 410 ug/m3 respectively. It is more important to realize that these standards are for the general population and not occupational standards as with the CO testing. Research needs to be done to establish an occupational standard for developing countries.

Tests using different fuelwoods shows that Eucalyptus TSP value for 3-stone stove is 2.3 times higher than the Maendeleo and with Jacaranda it is 1.7, Zanthoxlum is 2.3 times and Cyprus is 2.6 times higher for the 3-stone stove. Given that all of the field variables like, house design, user, stove etc. were the exact same and measurements were taken in the same week to limit the weather variations, one can assume that the differences stated above are almost totally due to the design of the stove. In terms of exposure and doses this is extremely significant. The mean time spent using the stove per day was not significantly different between stoves.

There is a very significant difference between the personal exposure to TSP's with the different biofuels tested. Cyprus was by far the best biofuel and finger empharbice was the worst in terms of particulates.

One would need to know the moisture content of the wood and the size of the particulates (if they are respirable/inhalable) to categorically state how harmful the wood might be if used as a biofuel. In terms of the lowest TSP measurements to the highest (most harmful to health) the fuels used would be arranged as follows:

Cyprus < Eucalyptus saligna < Grevillea robusta < Yellow acacia < Cordia africana < Zanthoxlum gillettii < Pinus patual < Jacaranda mimosifolia < Juniperus procera < Acacia mearsii < Croton megalocarpus < Maize cobs < Schinus moue < Sisal leaves <

Euphorbia tirucallii.

Table 3. Comparison of wood type, stove type and benzo (a) pyrene emissions.

Wood Type

BaP maendeleo stove

BaP3 - stone stove

% reduction by using maendeleo

Eucalyptus saligna
















In conclusion, measuring the TSPs of different biofuels that contribute to the complex problem of rural, indoor air pollution helps one to understand the dangerous doses received by women and children in the kitchen while cooking. It can also help development workers determine which biofuels might be best to plant on woodfuel plantations or other agroforestry programmes. This researcher found that the cook's perceptions about biofuels had a lot to do with whether she would use them or not. As this researcher talked to the women in the kitchens oddly enough everyone liked using sisal very much and complained about Eucalyptus saligna; yet in terms of TSP's sisal emits 4.4 times more particulates. However, it may be that because sisal is just outside most homes and very easy to collect and light, the women find the benefits outweigh the high particulate levels. Again Eucalyptus was very strange to them, most had never seen it before and they were suspicious of it.

PAH Measurements

Polycyclic Aromatic Hydrocarbons are part of a larger group of compounds known as Polycyclic organic matter (POM). PAH is sometimes also referred to as polynuclear aromatics (PNA). They have attracted a lot of interest because many of the compounds are carcinogenic. Many PAH can exist in any or a mixture of physical states in aerosols. In addition to PAH in POM, the aza and imino arenes have been found to be potentially carcinogenic. Most other categories of POM are of less environmental interest or are not found in large amounts in organic combustion products.

Most PAM's in the atmosphere are found in small particles formed from incomplete combustion of fuels. There is lime natural PAH emitted into the air except from natural combustion sources such as forest fires.

Small-scale burning of biofuel has high enough emissions to be a significant source of PAHs on a global basis. In the atmosphere, PAH can be oxidized by ozone as well as photochemically decomposed. The PAH that has been studied most is benzo(a)pyrene (BaP) a five-ring hydrocarbon. Therefore, in the literature, there is a tendency to cite PAH emission concentrations, exposures and health impact data in terms of BaP concentrations. The principal health effect of concern is cancer and a number of PAHs have been shown to be mutagenic, carcinogenic in animal tests and constituents in mixtures of known human carcinogenicity.

PAH can be formed in any combustion process involving compounds containing carbon and hydrogen even when there is no PAH in the original fuel. The amount of POM and PAH formed will vary widely, although it can be said that efficient, controlled combustion favours low POM emissions, whereas inefficient burning favours high emissions.

Investigating the concentrations of PAH in the indoor air requires sophisticated equipment and has not been attempted by many researchers nor have emission standards or standard testing techniques been developed in less developed countries.

This study expects to find a variety of PAH in village woodfuel smoke and to compare the concentrations from improved and traditional stoves and also to compare the emissions from the different types of biofuels used. This study is unique in the fact that it is looking deeply at the PAH content of the test filters.

The measured concentrations of the PAM's are overall high. But it is not surprising to this researcher given the fact the other data, CO, TSP and hours spent in the kitchens per day in this study were also high by any international, industrial standard. It just emphasised how much more research into health effects needs to be done.

PAH comparisons between the woods is proportionally the same and CD be correlated with the TSP levels. In other words, the type of wood was not nearly as significant as the actual TSP level. Where there was a high TSP level, there were high PAH's. All the individual PAH levels rose or fell proportionally with the TSP levels. The results cannot actually choose one wood as being more carcinogenic without first seeing that the TSP is higher.

To this researcher, these results between the woods are not conclusive. Either the woods were so similar in chemical make up compared to, for example, coal or diesel extracts that no significant difference existed between the firewoods, or the technique and sensitivity of the equipment used was not enough to show the difference.

However, comparisons between the two stoves can certainly be made. The improved, Maendeleo stove again as with the CO and TSP results proved to be emitting at least 50% less of each type of PAH tested. Table 3 shows the Benzo (a) pyrene comparisons and the percent reduction obtained using the Maendeleo stove.

It can be seen that this work is not conclusive in some monitoring and evaluation areas. Nonetheless, it is a stepping stone for conducting further studies. The Maendeleo stove proved beyond reasonable doubt that it does reduce emissions. Still there are many unresolved questions. What kinds of contribution to global pollution does rural air pollution make ? What kind of diseases are conclusively linked to rural indoor air pollution and what can be done about it ? To what extent can improved stoves arrest the obvious yet unproven damage to health ?

Editorial Note.

Detailed data for the tests referred to are available from Boiling Point or from the author at GATE/GTZ.

A Case for Smokeless Stoves?

Sri Lanka Daily News, 6th April 1991

Beijing, (AFP) -A restaurant owner and several municipal officials have been punished for killing a 300 year old tree in the Chinese capital, the Xinhua News agency reported.

The Scholar Tree, which stood over 12 meters (40 feet) tall, died in 1988 from smoke and heat after a restaurant opened a year earlier set up six stoves near it, the agency said.

It did not say how many officials were held responsible and what punishment they received.

However, the Hong Kong China News service, a semiofficial Chinese News Agency, said three officials, the heads of the City and the Dongcheng District Garden Bureaux and the Dongcheng District vice-head, were reprimanded for negligence.

The owners of the restaurant and local park officials were fined, it said.

Xinhua quoted Beijing Mayor Chen Xitong as telling City officials to learn from the case and do a better job in protecting trees, which he said were part of this ancient city's treasures.

Senior leader Deng Xiaoping encouraged the Chinese people to continue his 10-year-old pet project