Defining clean fuels and technologies
The WHO definition of clean, transitional, and polluting fuels and technologies used for cooking, heating, and lighting is described below. This information will be updated to reflect refinements in categorization. WHO is fuel and technology neutral, meaning the WHO does not advocate certain types of fuels or technologies over others, except to support the use of the cleanest possible options in each setting.
Cooking
Clean fuels and technologies are those that attain the fine particulate matter (PM2.5) and carbon monoxide (CO) levels recommended in the WHO global air quality guidelines (2021). The WHO Guidelines for indoor air quality: household fuel combustion (2014) provide PM2.5 and CO emission rate targets for devices, which are linked to the levels from the Air Quality Guidelines.
Fuel and technology combinations will be classified as clean if they achieve:
- either the annual average air quality guideline level (AQG, 5 µg/m3) or the Interim Target- 1 level (IT1, 35 µg/m3) for PM2.5; and
- either the 24-hour average air quality guideline level (AQG, 4 mg/m3) or the Interim Target-1 level (IT-1, 7 mg/m3) for CO
WHO’s official reporting on progress toward Sustainable Development Goal 7 to ensure access to affordable, reliable, sustainable and modern energy for all will use this categorization.
Clean
The following fuels and technologies are known to be clean for health at point of use and are categorized as clean for PM and CO household emissions: solar, electric, biogas, natural gas, liquefied petroleum gas (LPG), and alcohol fuels including ethanol.
For other fuel/technology combinations including biomass, the cooking system is classified as clean if it meets the emission rate targets in the WHO Guidelines (2014), as confirmed in laboratory testing following an international laboratory testing protocol with tests conducted by a third party. The International Organization for Standardization (ISO) released a set of voluntary performance targets that are constructed such that certain tier levels align with the WHO Guideline levels which confer minimal health risk. Using these ISO 19867-3 Voluntary performance targets (VPTs) for cookstoves based on laboratory testing, a stove that achieves Tier 4 or Tier 5 for PM2.5 emissions based on the voluntary performance targets (VPTs) is classified as clean for PM2.5 emissions. Stoves must also be classified as Tier 5 for CO emissions to be considered clean for health. The Clean Cooking Alliance Clean Cooking Catalog contains results from laboratory tests of stoves and fuels, which can be consulted to determine Tier levels using the ISO 19867-3 targets.
| ISO VPT Tier | WHO Category for CO | WHO Category for PM2.5 |
| 5 | Clean | Clean |
| 4 | Transitional | Clean |
| 3 | Transitional | Transitional |
| 2 | Polluting | Polluting |
| 1 | Polluting | Polluting |
| 0 | Polluting | Polluting |
Clean biomass fuels/technologies for cooking include:
Mimi Moto forced air gasifier stove
Transitional
Transitional fuels and technologies are those that provide some health benefit, although the fuel and technology combination does not achieve WHO recommended levels. Using the ISO VPT tiers, a stove that achieves Tier 3 for PM2.5 emissions is classified as transitional for PM2.5 , indicating that it provides substantial reductions in health risk, but cannot be considered clean for health. A stove that achieves Tier 3 or Tier 4 for CO emissions is classified as transitional for CO emissions.
Polluting fuels and technologies are those that do not provide a health benefit. The WHO Guidelines (2014) recommend against the use of unprocessed coal and discourage the use of kerosene. Thus, these fuels are considered polluting. Other fuels and technologies including biomass are classified based on the ISO VPT tiers. A stove that achieves Tier 0, 1 or 2 for PM2.5 emissions is classified as polluting for PM2.5 emissions. A stove that achieves Tier 0, 1, or 2 for CO emissions is classified as polluting for CO emissions.
Additional technologies are emerging that could be used with transitional or polluting stoves to improve their performance (i.e. Jet-Flame). However, stoves must be tested in combination with the device to determine whether their combined use meets the PM2.5 and CO emissions levels that are considered “clean”.
Heating
Space heaters are defined as clean following the WHO Guidelines for indoor air quality: household fuel combustion (2014).
Clean
The following heating fuels and technologies are known to be clean for health at the point of use and are categorized as clean for PM and CO household emissions:
- Central heating (regardless of the energy source used)
- Heat pumps
- Manufactured space heaters, traditional space heaters, manufactured cookstoves, or traditional cookstoves if they use electricity, piped natural gas, LPG, biogas, or alcohol/ethanol. Solid fuel manufactured or traditional stoves or space heaters are clean if they meet the emission rate targets in the air quality guidelines as described for cooking fuels and technologies (i.e. ISO VPT Tiers 4 or 5 for PM2.5 emissions and Tier 5 for CO emissions).
Transitional heating devices are manufactured space heaters, traditional space heaters, manufactured cookstoves, or traditional cookstoves that meet ISO VPT Tiers 3 or 4 for CO emissions and Tier 3 for PM2.5 emissions.
Polluting heating devices include moveable heating pans and open fires or three-stone stoves used for heating.
Lighting
Lighting technologies are defined as clean based on the energy source. Clean fuels/technologies for lighting include:
- Electricity (including solar panels)
- Solar-powered lanterns or flashlights
- Rechargeable flashlights, mobiles, torches, or lanterns
- Battery powered flashlights, torches, or lanterns
- Biogas lamps
- LPG lamps
Polluting sources of light include gasoline lamps, kerosene/paraffin lamps, oil lamps, candles, or open fires.
Related resources
Contact and updates
Please contact householdenergy@who.int with any questions.
Please contact catalog@cleancookingalliance.org and householdenergy@who.int with third-party test results for verification and consideration for other clean or transitional household energy technologies that are not listed.