Indoor Air Pollution: The Facts

We look at the causes, types and health effects of household air pollution from the burning of wood and solid fossil fuels like coal. The main pollutants include particulate matter, black carbon and toxic gases including carbon monoxide. Driven by poverty, indoor air pollution is mostly confined to the poorest sections of developing countries in Africa and Asia. We explain all you need to know.
Woman cooking on open fire, indoor air pollution
Poverty precludes the use of cleaner fuels for some, forcing women to cook on open fires with wood. Image: © DailyFamily

Indoor air pollution kills close to 4 million people every year, mostly in developing countries. 1 The most vulnerable are poor people who do not have access to well-ventilated accommodation and clean energy (like electricity), and who use outdated cooking and heating equipment. Of all forms of air pollution, nothing harms the poor and disadvantaged as much as indoor air pollutants. Women and children are especially vulnerable, since they spend more time around the hearth. See also: the ethics of climate change for more about the unequal impacts of our climate crisis.

What Do We Mean by Indoor Air Pollution?

Indoor air pollution (IAP) – also known as household air pollution (HAP) – refers to the contamination of indoor air – most commonly in domestic households – as a result of physical, chemical or biological contaminants. 2

In developing countries, the main pollutant is smoke from the burning of wood and other biomass. This contains inhalable microscopic particles of black carbon and ash, tiny aerosols of liquid or solid matter, as well as gases like carbon monoxide (CO), sulphur dioxide (SO2), nitrogen dioxide (NO2) and polycyclic aromatic hydrocarbons (PAHs).

In more affluent developed countries, where most homes rely on cleaner forms of energy, including smoke-free coal, as well as superior cookers and heating systems, indoor pollution has been significantly reduced. Nevertheless, air quality in many households continues to be contaminated by tobacco smoke, radon gas seepage, accidental carbon monoxide poisoning, asbestos from old building materials, and volatile organic compounds from household products.

Indoor Pollution Levels Worse Than Outdoor Levels

Contained areas enable pollutants to accumulate faster and more easily more than open spaces do. Studies show that indoor pollutants cause between 100 and 1,000 times more human inhalation or exposure, per unit of mass emitted, than pollutants released outdoors 3. This higher intake fraction for indoor emissions shows that even small-scale combustion processes can significantly reduce indoor air quality (IAQ) and lead to damaging health effects.

In Latin America, for example, someone using firewood to cook typically inhales air containing up to 26 times more pollution than the safe limit laid down by the World Health Organization (WHO). 4

Even in developed countries, some indoor pollutants can be up to five times more concentrated than those outside. And this effect is aggravated – in Europe, at least – by the fact that people spend up to 90 percent of their lives indoors. 5

The main point, here, is that pollutant levels are closely linked to lack of air flow. In the outside air, pollutants are usually rapidly diluted and dispersed. But indoors, where there is little infiltration of air around windows and doors, or little natural or mechanical ventilation, the air exchange rate is low and pollutant levels can increase quite rapidly.

Of course, where unusual weather conditions such as ‘air inversion’ traps urban smog or ground-level-ozone inside a city, outdoor pollution levels, too, may skyrocket. But this simply reinforces the point that air flow disperses pollution, whether outdoors or indoors.

Woman cooking indoors, air pollution
Indoor cooking over a solid fuel fire in Guatemala. Traditional methods of cooking like these are a major source of household air pollution and ill-health for many women and children in the developing world. Image: © Global Alliance for Clean Cookstoves.

What Are the Main Causes of Indoor Air Pollution?

Solid Fuel/Biomass Burning in Developing Countries

As of 2020, nearly 3 billion people in developing countries across the world still rely on solid fuels – such as wood (charcoal, animal dung and other biomass) and coal – or kerosene, for cooking and other household energy needs. 6 Cooking is typically done indoors without proper ventilation, using smoky open fires or crude stoves. 1

According to the WHO, a household with an old coal-fuelled cookstove can produce 2,000 or 3,000 micrograms of particles per cubic meter. That’s roughly 200 to 300 times the recommended maximum. 7

As a result, roughly 4 million people die prematurely from illnesses attributable to indoor air pollution from inefficient cooking practices using fuels and technologies that generate a range of damaging pollutants, including tiny particles of soot that penetrate deep into the lungs. As we have seen, in this type of indoor environment, smoke concentrations can be 100 times higher than the safe level for fine particles. Women and young children are especially at risk as they tend to spend the most time around the domestic hearth. 1

Using open fires for cooking and heating also adds to global warming, due to the amount of greenhouse gases that are commonly emitted by solid fuels. (For details, see final section below.)

NOTE: The basic energy ladder in poorer countries has three rungs. On the lowest rung are those who use primitive fuels such as wood, animal dung or crop residues. On the next rung, people use transition fuels like charcoal, coal or kerosene. Finally, on the top level they use cleaner-burning and more efficient fuels such as electricity, liquid petroleum gas (LPG) or biofuels.

Asian Brown Cloud: Toxic Haze
Environmental Effects of Fossil Fuels

Smoke Pollution From Cooking

Smoke emissions from the burning of biomass fuels typically contains a mix of fine and coarse particulate matter (PM2.5, PM10), as well as gases such as carbon monoxide (CO), sulfur dioxide (SO2), nitrogen dioxide (NO2), plus a variety of organic air pollutants. The latter can include (benzene, 1,3-butadiene, acetaldehyde, acrolein, phenols, pyrene, benzopyrene, cresols, dibenzo pyrenes and dibenzo carbazoles). 8 Where coal is burned, then in addition to the above pollutants, tiny particles of heavy metals (e.g. arsenic and fluorine) are also emitted.

In a traditional solid fuel stove, around 6–20 percent of the fuel is discharged as toxic emissions into the household space. The exact quantity depends on the type of fuel, its moisture content, and the type of stove used. 8

PM air pollution size comparison
Particulate matter: size comparisons. Image credit: United States Environmental Protection Agency (EPA)

Scientists are especially concerned about particulate matter emitted in smoke emitted during solid fuel combustion. Particulate matter (or PM) – the scientific term for tiny particles of air pollution – is a mixture of solid and/or liquid particles suspended in air.

These particles can vary in size and composition but health authorities are principally concerned about particles that are 2.5 microns in diameter or smaller. (About 28 times smaller than the diameter of a human hair.) This is because these super-tiny particles are inhalable, and can penetrate the lungs, heart and brain. A small puff of cooking smoke can contain tens of thousands of these damaging particles. 9

Household air pollution from the combustion of solid fuel is now seen as a major contributor to chronic respiratory diseases such as chronic obstructive pulmonary disease (COPD), bronchitis and asthma. Exposure to HAP also causes respiratory infections and lung damage, notably in children. Exposure to indoor coal combustion emissions is associated with lung cancer.

Overall, emissions from these solid fuels are estimated to cause more than 3.5 million premature deaths annually. 10

Animal Dung

Residents in rural areas of Africa and Asia who find that the fossil fuels are priced beyond their means, resort to burning animal dung for household energy purposes. In Tibet, for example, rural Tibetan households consume large quantities of biomass, notably yak dung, for cooking and heating. In fact, energy obtained from yak dung alone accounts for more than half of all domestic energy consumed in pastoral areas of Tibet. 11

Wood Burning

Some people think wood is a ‘natural’ therefore ‘clean’ fuel. But this is not true. Wood burning emits the three main greenhouse gases – carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) – plus numerous toxic air pollutants. These include: carbon monoxide, acrolein, benzene, formaldehyde, and polycyclic aromatic hydrocarbons (PAHs).

But the biggest health threat from wood smoke comes from its emissions of PM2.5. As we have seen, these particles – in this case mostly ash and black carbon – can damage the respiratory system, and other major organs.

These days, there is less reliance on wood burning for cooking in developing countries, although charcoal remains the main cooking fuel in sub-Saharan Africa, South Asia, Latin America and the Caribbean, especially in urban areas. Broadly speaking, charcoal burning causes the same pollution as wood burning. Less nitrous oxide is emitted, but more carbon dioxide and carbon monoxide. Particulates are about the same. 12

Infographic, dangers of household air pollution
Image shows the potential dangers of household air pollution in Europe. Image: The European Commission Joint Research Centre.

What Causes Indoor Air Pollution in Developed Countries?

With the development of more climate-proof and thermally efficient houses and offices, in order to conserve heat, air leakage has been drastically reduced. As a consequence, indoor air is not refreshed very often. This lack of air flow allows pollutants introduced into a household, to accumulate within the building, causing discomfort, breathing difficulties and worse.

So what are the main household air pollutants in the West? According to the European Environment Agency, the main indoor air pollutants in Europe include: radon emissions, tobacco smoke, gases or particles from burning fuels, volatile organic compounds, and asbestos. 5

Radon

Radon is a radioactive gas produced by the radioactive decay of uranium in soil which came from uranium-bearing rocks. Usually it diffuses naturally through cracks and openings in the soil, and when it enters the atmosphere, it is rapidly dispersed. Problems occur, however, if radon in the soil finds its way – via cracks, pipes or loose fittings – into the foundations of buildings. Alternatively, it can enter a household through emissions from radon-containing gravels used in concrete foundations, or via tap water from underground sources.

Once inside a house, the radon is trapped and gradually accumulates. Unfortunately, you can’t see it or smell it. The only way to discover whether a house contains radon is to test for it.

If radon is inhaled it can cause serious health problems. For example, after completing a combined analysis of 13 studies that collectively involved 7,148 lung-cancer cases and 14,208 controls, one study concluded that residential radon accounts for around 2 percent of all annual deaths from cancer in Europe. 13 In the United States, the EPA has calculated that radon is responsible for 20,000 lung-cancer deaths a year. 14

Tobacco Smoke

One of the most hazardous and common of all household air pollutants is cigarette smoke. Both smoking and second-hand smoke inhalation cause lung cancer, heart disease and emphysema, and may contribute to the development of other cancers as well. Among the toxic constituents contained in cigarette smoke, are cyanide, carbon monoxide, carbon dioxide, benzene and particulate matter.

Smoking is a dangerous form of self-inflicted air pollution. In fact, severe urban pollution in Asian cities is often measured in numbers of cigarettes smoked, underlining the dangerous causal connection between tobacco and serious disease. The inhalation of second-hand smoke indoors can be equally harmful.

Carbon Monoxide Poisoning

Carbon monoxide is a colorless, odorless, tasteless, but highly toxic air pollutant. It is generated by the incomplete combustion of coal, gasoline (petrol), natural gas, wood or charcoal. The largest anthropogenic source of CO in the developed world is vehicle exhaust emissions.

Accidental carbon monoxide poisoning occurs when the gases emitted by the burning fuel are not properly vented into the atmosphere, and consequently are allowed to build up in an enclosed space. One of the most common situations where the build-up of carbon monoxide occurs, is when an unvented heating stove is used to heat a room whose doors and windows are firmly shut against the cold. 15

That said, almost every form of fuel combustion emits carbon monoxide. So, no matter what the situation is, if you are burning any sort of fuel in an enclosed room or space, make sure that the heater is properly vented – that is, connected to the outdoors by means of a chimney, flue or pipe – thus allowing all combustion fumes and particulates to escape into the atmosphere.

After carbon monoxide is inhaled, it enters the bloodstream and replaces the oxygen in the body’s red blood cells with carbon monoxide. This may lead to serious tissue damage, or even death. The worldwide incidence and mortality of carbon monoxide poisoning is approximately 137 cases and 4.6 deaths per million, respectively. These statistics have hardly changed over the last 25 years.

Global map of population with access to clean fuels and cooking technologies
Global map showing the percentage share of the population with access to clean fuels and cooking technologies. According to the International Energy Agency (IEA), two-thirds of Sub­Saharan Africa has no access to modern energy. This is why indoor open fire cooking takes place across hundreds of thousands of homes in villages all over Africa. Image: © OurWorldinData. (CC BY 3.0)

Nitrogen Dioxide Pollution

Indoor air pollution from nitrogen dioxide arises in a similar way to CO pollution: namely, from sources such as wood-, oil-, gas-, coal-, and kerosene-burning stoves, heaters, ovens and fireplaces, especially unvented or poorly maintained appliances.

Volatile Organic Compounds (VOCs)

VOCs include a wide range of chemical compounds, with both short and long-term health effects.

An analytical review of 77 surveys of household VOCs in the United States found that the most harmful indoor VOCs were acetaldehyde, acrolein, benzene, formaldehyde, hexachlorobutadiene, 1,3-butadiene, benzyl chloride, 1,4-dichlorobenzene, acrylonitrile, carbon tetrachloride, and vinyl chloride. These compounds exceeded health standards in most homes. 16

VOCs are emitted by thousands of different products. Examples include: office equipment such as copiers and printers, correction fluids and photographic solutions, paint strippers, paints and lacquers, cleaning supplies, furniture wax, and detergents, pesticides, glues and adhesives and permanent markers. Benzene is also released in petrol fumes from fuel stored in attached garages.

House dust has been shown to be a repository for polychlorinated biphenyls (PCBs), PAHs, plasticizers, flame retardants, and other VOCs.

Asbestos Contamination

Asbestos, the name used to describe a group of minerals (e.g. chrysotile, amosite, and crocidolite) with fire-retardant and insulating characteristics. It was used, in particular, in the manufacture of ceiling and floor tiles. Asbestos consists of fine, tough fibers that – if disturbed – can become airborne. And if inhaled, they can become lodged in the lining of the lung.

After studies revealed that some of these fibers are carcinogenic, it led to the banning of asbestos, along with the widespread removal of all asbestos materials and coverings from buildings. Paradoxically, current evidence indicates that chrysotile – the most common form of asbestos – may not be especially cancer-causing, whereas some of the less common forms are much more dangerous.

Unfortunately, inexpert attempts to remove household asbestos materials can release carcinogenic fibers into the air, increasing asbestos levels and endangering residents. Sometimes, therefore removal is not feasible unless the building in question is empty.

Common sources of asbestos include: roofing shingles (bitumen strip slates), ceiling and floor tiles, and heat-resistant fabrics.

Lead Pollution

The most common source of indoor lead exposure in developed countries is old lead-based paint. But this only happens when lead-based paint is inappropriately removed from surfaces by sanding, scraping or burning. More rarely, airborne lead particles in homes can come from outdoor lead dust blown in by the wind, or contaminated earth tracked inside.

Biological Contaminants

Biological pollutants come from tiny living things in the air. They include: house dust, mites, animal dander (skin flakes), cat saliva, mold and pollen. There are numerous sources of these pollutants, many of which can be reduced by regulating the relative humidity level of the household. A relative humidity of 30-50 percent, for example, is usually the recommended range for houses. Wet or damp materials or surfaces may act as a breeding ground for mold and mildew, as well as bacteria. So these should be thoroughly dried out whenever possible.

Damp, warm conditions are also the favored habitat of house dust mites – the trigger for dust mite allergy. House dust mites are microscopic bugs that live on waste like the dead skin cells that humans and pets shed. Although mostly harmless, their feces – if inhaled or ingested – can trigger allergic reactions such as asthma, eczema and rhinitis.

Child difficulty breathing, indoor pollution Africa
A young boy watches the pot cook over a solid fuel fire in the smoky interior of a household in sub-Saharan Africa, while unknowingly breathing in thousands of tiny particles of soot and ash. Image: © Global Alliance for Clean Cookstoves.

What Are the Health Effects of Indoor Air Pollution?

The emissions of dangerous particulate matter and toxic gases generated by inefficient cooking fires, causes a raft of fatal diseases, including coronary heart disease, strokes, chronic obstructive pulmonary disease (COPD) and lung cancer. Furthermore, health impacts from indoor air pollutants may be experienced immediately after exposure, or years later.

In total, the World Health organization (WHO) states that 3.8 million people die prematurely from illness caused by household air pollution caused by the inefficient use of kerosene and solid fuels for cooking. 1 For more, please see: Health Effects of Air Pollution.

Breakdown of Deaths from Household Air Pollution

Of the total of 3.8 million deaths from indoor air pollution:

– 27 percent are due to pneumonia
– 27 percent – coronary heart disease
– 20 percent – chronic obstructive pulmonary disease (COPD)
– 18 percent – stroke
– 8 percent – lung cancer

Pneumonia

The effects of household air pollution nearly double the risk for childhood pneumonia and accounts for 45 percent of all pneumonia fatalities in children under 5 years old. Household air pollution is also risk for pneumonia in adults, and contributes to 28 percent of all adult deaths from pneumonia.

Coronary Heart Disease

Around 11 percent of all deaths due to coronary heart disease, or roughly a million premature deaths per year, is caused by exposure to indoor air pollution.

Chronic obstructive pulmonary disease

One in four deaths from COPD in adults in developing countries is caused by indoor air pollution. Women exposed to high levels of household smoke from cooking are more than twice as likely to develop COPD than women who cook with cleaner fuels and more modern stoves.

Stroke

Around 12 percent of all fatalities in adults due to strokes are caused by household air pollution arising from inefficient cooking practices.

Lung Cancer

Carcinogens emitted during the combustion of cooking fuels like wood, charcoal, coal or kerosene account for about 17 percent of lung cancer deaths in adults. The risk is even higher among women, because of the time they spend preparing food.

Other Health Problems from Indoor Air Pollution

Microscopic particles and other pollutants in cooking smoke irritate and inflame the throat and lungs, undermining immunity and reducing the oxygen-content of the blood. The same pollution is also linked to tuberculosis, mouth, nose and throat cancers, and other problems, including low birth weight.

Traditional cooking processes like those mentioned here, may also involve women and children in making a considerable physical effort to source and gather solid fuel for cooking and heating. This restricts other productive activities (earning money, school attendance, craft work) and may expose women and children to the risk of injury and violence.

For example, many of the old-fashioned fuels and technologies used in poorer households for cooking, heating and lighting, carry increased safety risks (especially burns), when compared to more modern ones. The introduction of electricity, for instance, instantly eliminates several of these problems, and liberates women and children in the process.

Map of global population with access to electricity
Global map showing the percentage share of the population with access to electricity. © OurWorldinData (CC BY 3.0) 17

Indoor Air Pollution and Climate Change

Right now, fossil fuels (such as coal, charcoal, heating oil, kerosene and LPG) and biomass (such as wood and crop residues) continue to be burned every day, in millions of households in the developing world. This releases more and more greenhouse gas emissions that fill the troposphere and add to the greenhouse effect that is destabilizing our climate system.

On average, 3 billion kilos of solid fuels (including coal) are burned on indoor fires in developing countries every day. This burning fuel emits around 6 billion kilos of CO2 into the air. That’s three times more than the amount of greenhouse gas emitted each day by all the private cars in the U.S.

Indoor air pollution threats from climate change include an increase in microbial and mold growth following increased levels of flooding. Also, greater airborne pollution from the indoor burning of mosquito repellents, in more regions, due to the increasing range of malaria mosquitoes, is almost certain to occur.

But what can be done? How can we help people to free themselves from relying on unhealthy cooking methods, and also prevent climate change from getting worse?

The only answer, is to decarbonize our power plants so that they produce low-emission electricity, and then make sure that this electricity is made available to all communities. The increased wealth that this will bring to communities will help offset some of the costs associated with climate change mitigation in the developing world.

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