Indoor air pollution is one of the world's largest environmental problems. Based on figures from the Institute for Health Metrics and Evaluation (IHME), 2.6 million people died prematurely in 2016 from illness attributable to household air pollution1. Indoor air pollution is caused by the inefficient use of solid fuels for cooking and cleaning.
It is predominantly women and young children who are killed by indoor air pollution.
According to the World Health Organization (WHO) these deaths are attributable to the following diseases2:
- ischaemic heart disease
- chronic obstructive pulmonary disease (COPD), and
- lung cancer.
The burning of solid fuels fills the houses and huts in poorer countries with smoke that kills the world’s poor by causing pneumonia, stroke, heart disease, chronic obstructive pulmonary disease, and lung cancer. The solid fuels responsible for this include wood, crop residues, dung, charcoal, and coal. The solution for this problem is straightforward: shift from solid fuels to modern energy sources.
And the following chart shows that the world is making progress in this direction. In 1980 almost two thirds of the world’s population used solid fuels for their cooking. 30 years later this is down to 41%. The chart also shows that it is a problem associated with poverty: In richer Europe and North America the share is much lower than in the rest of the world; and in the high income countries of the world the use of solid fuels is entirely a thing of the past.
The use of solid fuels is going down in all of the world’s regions. But the success rapidly developing South East Asia is particularly impressive: Here the share fell from 95% to 61%.
Share of population relying on different types of solid fuels for cooking by developing regions, 2007 – UNDP & WHO (2009)3
The obvious way to avoid indoor air pollution from solid fuel burning is for households to transition from traditional ways of cooking and heating towards more modern, cleaner methods. This can, for example, be in the form of transitioning towards non-solid fuels such as natural gas, ethanol or even electric technologies.
The chart below shows the percentage of households with access to clean fuels and technologies for cooking by countries and regions since 2000. This share has been increasing for most countries at low-to-middle incomes, however, rates of increase vary by country and region. Access to clean fuels are lowest in Sub-Saharan Africa where only 14 percent of households in 2016 had access. Progress has been much more significant in South Asia and East Asia over the last decade, with 18 percent and 16 percent of additional households gaining access, respectively.
In the visualisation below we see the absolute number of global deaths attributed to indoor air pollution from 1990 onwards. These deaths have been aggregated by region. Overall we see a decline in the number of pollution-related deaths since 1990, falling from 2.7 million to 1.6 million in 2017.
When broken down by region we see that this decline since 1990 is true across the world. Deaths from air pollution are largely concentrated in Asia and Africa. nearly 70% of all deaths in 2017 were in Asia, with more than a quarter in Africa & the Middle East, and only a couple of percent across the Americas and Europe (with most of these originating in Latin America & the Caribbean).
Absolute number of deaths can also be assessed and compared by country in the visualisation below. At the country level, India followed by China had the highest mortality figures in 2016 with 482,000 and 271,000 respectively. These numbers have, however, shown a significant decline in recent years. In the last decade alone the number of deaths from household air pollution in China has approximately halved.
This decline is also true for countries in Sub-Saharan Africa (SSA) with high mortality figures. Ethiopia and Nigeria – who have the two highest death tolls in SSA – have both seen a inverse-U trend of increase-peak-decline since 1990.
In the chart below we see that, at the global level, the death rate (per 100,000 people) from diseases related to indoor air pollution has declined significantly since 1990. There are large differences in the risk of indoor air pollution globally: in high-income nations, the likelihood is close to zero; in low- to middle-income nations, it can be the dominant form of air-pollution related deaths. This risk is, however, continuing to fall at the global level and in most low- to middle-income economies.
Income is a strong determinant of energy access and types of fuel sources.
In the first chart below we see the World Health Organization (WHO)'s depiction of the 'energy ladder'. This energy ladder is shown with income groups along the x-axis (getting richer from left to right) and types of energy sources on the y-axis (transitioning from solid fuels to non-solid fuels). As shown, very low and low income countries tend to mostly rely on traditional solid fuel sources such as crop waste, dung and firewood. As incomes rise, this energy mix tends to transition towards other solid fuels such as charcoal and coal. It is typically only when households shift within the middle income bracket that fuel sources transition from solid fuels into non-solid fuels (which would be considered cleaner fuels and technologies) such as ethanol, natural gas and finally electricity.
This income-dependent transition is seen in the second chart below which shows the percentage of households in any given country with access to clean fuels and technologies for cooking (on the y-axis) versus gross domestic product (GDP) per capita. The share of households with access to clean energy in countries below a GDP per capita level of $2,000 per year is typically less than 10 percent. As countries begin to bridge that gap between low and middle incomes, this share begins to increase until a final transition towards high-income where the majority of households have clean fuels and technologies for cooking.
The energy ladder - the link between household energy and development – WHO (2006)4
As shown above, lower-income countries tend to have lower access to clean fuels and technologies, and thereby a stronger reliance on solid fuels for cooking and heating. However, there are also differences in clean energy access within countries.
In the chart below we see the percentage of rural households who rely on solid fuels for cooking (y-axis) versus the percentage of urban households (x-axis). Also shown is a y=x comparison line; if the percentage of urban and rural households relying on solid fuels for cooking were equal, points would lie along this line. As we see - for all regions, the point lies above this line, meaning that the share of rural households relying on solid fuels is higher than the dependence in urban households. This difference is most significant in Southern Africa where 88 percent of rural households use solid fuels, whereas this share is only 12 percent for urban households.
This result correlates strongly with the finding that the share of households with electricity access is lower for nearly all countries in rural areas versus urban areas. Rural households are therefore less likely to have access to electricity grid connections and more modern energy sources, making their reliance on solid fuels more prevalent.
Indoor air pollution has a wide range of negative health impacts, which can lead to morbidity but also in many cases, mortality. The table below features summary data from the World Health Organization (WHO) on the extent of proven links between indoor air pollution and potential health outcomes. These health outcomes range from respiratory infections to chronic obstruction pulmonary disease (COPD) to lung cancer and have varying effects on the population depending on factors such as age and sex.
Health impacts vary in terms of the strength of evidence linking outcomes with indoor air pollution. The WHO define 'strong evidence' based on results from a range of studies on solid fuel using in developing countries with biochemical and laboratory evidence of health impacts; 'moderate I' has at least three studies showing strong evidence for specific age and sex groups; and 'moderate II' has at least three studies showing potential links but with more limited evidence.
Health impacts of indoor air pollution – WHO (2006)5
|Health outcome||Evidence||Population||Relative risk||Relative risk (95% confidence interval)||Sufficient or insufficient evidence?|
|Acute infections of the lower respiratory tract||Strong||Children aged 0-4 years||2.3||1.9-2.7||Sufficient|
|Chronic obstructive pulmonary disease||Strong||Women aged ≥ 30 years||3.2||2.3-4.8||Sufficient|
|Moderate I||Men aged ≥ 30 years||1.8||1.0-3.2||Sufficient|
|Lung cancer (coal)||Strong||Women aged ≥ 30 years||1.9||1.1-3.5||Sufficient|
|Moderate I||Men aged ≥ 30 years||1.5||1.0-2.5||Sufficient|
|Lung cancer (biomass)||Moderate II||Women aged ≥ 30 years||1.5||1.0-2.1||Insufficient|
|Asthma||Moderate II||Children aged 5-14 years||1.6||1.0-2.5||Insufficient|
|Moderate II||Adults aged ≥ 15 years||1.2||1.0-1.5||Insufficient|
|Cataracts||Moderate II||Adults aged ≥ 15 years||1.3||1.0-1.7||Insufficient|
|Tuberculosis||Moderate II||Adults aged ≥ 15 years||1.5||1.0-2.4||Insufficient|
This link between exposure to indoor air pollution and mortality rates is further shown in the chart below. Here, this plots the age-standardized death rate from indoor air pollution (measured per 100,000 individuals) on the y-axis, against the share of the population with access to clean fuels for cooking. Overall we see that countries with the highest death rate from indoor air pollution are those with very low access to clean fuels (i.e. have a high dependence on solid fuels instead). As access to clean fuels and technologies increases, death rates from household air pollution begin and continue to fall.
There are two potential factors which can merge to influence how deaths from household air pollution are linked to income. The first is that - as we have explored earlier in this entry - poorer households have a stronger reliance on solid fuels for cooking. This would suggest that poorer households would therefore be at higher risk of negative health impacts from household air pollution. The second is that overall health outcomes and life expectancy in poorer countries is typically lower than at middle and high incomes. As a result, exposure to air pollution overall may have a greater health impact on low-income households. Note that this second factor may combine with the decline in the solid fuel use to explain the falling death rates and absolute number of deaths from indoor pollution - underlying health may be improving just as the prevalence of pollution is also falling.
This relationship between pollution-related deaths and income are shown in the two charts below. The first shows the relationship between the age-standardized death rate from indoor air pollution (measured per 100,000 individuals) versus gross domestic product (GDP) per capita. Overall we see that death rates are highest at the lowest incomes, decline as average incomes transition towards $20,000. Death rates are low in most countries once GDP per capita rises above $20,000 per year.
The second chart below shows the same metric: age-standardized death rate per 100,000 versus the share of the population below the $1.90 per day extreme poverty line. This data tells a similar story: countries with a high share of the population in extreme poverty also tend to have the highest death rates from indoor air pollution. As the incidence of absolute poverty declines, the mortality rate also begins to fall.
The chart below shows death rates from indoor air pollution plotted against rates from outdoor particulate air pollution. Countries which lie above this line have higher death rates from indoor pollution relative to outdoor. Here we see that for a large number of countries, indoor air pollution is still dominant.
When we view countries trends over time we see a general shift towards the bottom left-hand corner (reducing both indoor and outdoor pollution death rates). Nonetheless, for many countries the dominant trend has been downward (i.e. a reduction in indoor pollution rates most specifically).
The US Environmental Protection Agency publishes a glossary related to indoor air here.
Institute of Health Metrics and Evaluation (IHME) Global Burden of Disease (GBDx) Data Tool
- Data: Number of deaths and death rates from household air pollution
- Geographical coverage: Global- by country
- Time span: 1990 to 2016
- Available at: http://www.healthdata.org/results/data-visualizations
World Health Organization, Global Health Observatory (GHO)
- Data: Use of solid fuels and burden of disease
- Geographical coverage: Global- by country and region
- Time span: 2012-2013
- Available at: http://apps.who.int/gho/data/node.main.133?lang=en
World Health Organization (WHO)
- Data: Household (indoor) air pollution factsheet
- Available at: http://www.who.int/features/qa/indoor-air-pollution/en/
Food and Agricultural Organization (FAO)
- Data: Forestry data on wood fuel for countries and world regions since 1961.
- Geographical coverage: Global- by country
- Time span: 1990-2015
- Available at: http://www.fao.org/faostat/en/#data/FO