6 natural gas, and LPG. Households were believed to move up the energy ladder with rising income, abandoning energy in the lower rung of the ladder as doing so became affordable. This linear fuel switching model would make it possible to link primary energy use directly to the benefits of modern clean energy, especially better health outcome. Instead, the authors found that the dynamics of household fuel use were not so linear or unidirectional. Households were found to move to multiple-fuel cooking with rising income in a bidirectional process—meaning that separate, coexisting factors simultaneously pushed households away from biomass and pulled them back. Even when households had been using LPG for many years, they rarely abandoned fuelwood use. Fuelwood savings from using LPG ranged from 35 percent on average in one village to as little as zero percent in another. Expenditures on fuelwood were higher in some cases in households using both fuelwood and LPG than in fuelwood-only households. The authors concluded that economic development, which would enable use of technologies higher on the energy ladder, did not correlate with pollution reduction.

Fuel stacking is especially problematic for health outcomes. Johnson and Chiang (2015) concluded from running a model that that using a three-stone fire for a few minutes a day would elevate PM2.5 concentrations above any of the targets set by the World Health Organization. Traditional charcoal stoves could be used longer but still to less than 10 minutes a day.

The International Energy Agency (IEA) projects that, given current trends, 2.1 billion people will still lack access to clean cooking in 2030. The three most likely and widely available options for clean cooking are electricity, natural gas, and LPG. Across the world natural gas is potentially available primarily in urban areas; in Sub-Saharan Africa, which has the largest access gap, natural gas is not available even to urban households. The IEA hence estimates that in the scenario in which only policies already announced are implemented, about two-thirds of those gaining access will be adopting LPG as the primary cooking fuel. Although LPG is a fossil fuel, because its combustion largely does not produce black carbon and other emissions with high global warming potential, LPG would play an important role as a household fuel even in the scenario achieving net-zero greenhouse gas emissions by 2050. In the latter scenario, more than one-third of those gaining access (under universal access by 2030) would do so by adopting LPG, requiring an annual investment of US$1.2 billion in LPG infrastructure alone (IEA 2021).

There is evidence that subsidies have boosted household use of LPG and led to its adoption as the primary cooking fuel among some better-off households, but many poor households have used subsidies to start using LPG, only to discontinue its use later. Even among those who continue to use LPG, traditional use of solid biomass continues. There are numerous reasons for not abandoning it even as households cite gas or electricity as their primary sources of energy for cooking. Aside from higher costs of exclusive use of modern energy, occasional or frequent shortages of electricity and gas, fuel price volatility (especially LPG, although natural gas prices can also rise markedly), and cooking preferences are among other reasons (Kojima 2021a and 2021b). Of these, households have no control over a lack of reliable supply. Many developing countries suffer from unreliable electricity and some experience frequent power outages lasting hours. A lack of LPG, either altogether or occasional, is closely correlated with a lack of sufficient demand, creating a vicious cycle. If the supply is unreliable or non-existent, households do not adopt LPG and use it regularly, limiting consumption. Because demand is low, marketers cannot achieve the requisite economies of scale to develop commercially viable markets for LPG and choose not to devote efforts to supplying these new markets, perpetuating the lack of supply reliability.

There is only a limited literature documenting _abandonment_ of polluting fuels and stoves by households. For example, there is a large literature on household use of LPG based on data from national household surveys or other surveys to study primary household energy sources, including adoption of LPG as the