It is probably because a billion? people still use open wood fires indoors with poor ventilation. Visualize a fire in the middle of a hut with a pot on it and a small hole in the ceiling. AQI in that hut is going to be very bad. Creating systems where people can afford to use propane or other gas stoves are a huge win for the health of humanity and ecosystems.

Die drei Plagen des Bauern: Hausrauch, ein undichtes Dach und eine untreue Frau

> The three plagues of the farmer: household smoke, a leaky roof and an unfaithful wife

Supposedly an old saying, but almost certainly retcon. Not necessarily wrong though, look at houses a few hundred years ago, that sort of thing is still ubiquitous in the developing and pre-developing world.