The 7.3 million figure is like the “12 years left” figure, that is to say it requires a little work to understand where it came from and what it means.
“Ambient air pollution was responsible for 4.3 million deaths” and “3.8 million
deaths every year as a result of household exposure to smoke from dirty cookstoves and fuels”.
I haven’t dug into the indoor figures but a quick reading suggests that attributing the death toll to fossil fuels in general is misreading the report.
For example, ”91% of those premature deaths occurred in low- and middle-income countries” so the bigger problem is how the fuel is used - since richer countries actually use more fuel per capita.
Cooking with coal on an open stove in an unventilated room will slowly kill you from the carbon particulate. Apparently this is a large percentage of the indoor pollution death statistic. The problem here is coal stoves specifically not fossil fuels in general.
The outdoor statistic is a bit more interesting. The Model they use to calculate the number [1] basically takes a curve of PM exposure level across populations around the world, multiplied by an integrated exposure-response (IER) function.
Over the last few years they have lowered the counterfactual concentration (the point below which PM has no efffect) and steepened the IER.
Through this methodology they can arrive at a death rate of nearly 5 million without a single death certificate ever actually stating “air pollution”.
They do this by trying to tease out the damage done by PM by observing places where PM has changed and then looking at how mortality rate due to cancers and such also changed.
It’s an interesting figure, but in a sense misses the forest for the trees.
Particulate matter definitely has adverse heath effects. One study said an average shortening of life expectancy in Europe of 9 months. But the overall “industry” that creates PM (everything from cars kicking up dust on the road to smokestacks) also is responsible for the modern world where ~8 billion people can survive. Is PM shortening lifespans? Or is PM drastically extending lifespans (e.g. preventing mass starvation) while simultaneously also somewhat shortening an idealized life that could have magically gotten everything it needed to survive except without any PM.
If you are going to publish a number of the harm of PM by extrapolating from an IER and PM levels, it would also be useful to consider the net effect, which would be staggeringly positive in terms of lives saved and lengthened not shortened.
[1] - https://www.who.int/airpollution/data/AAP_BoD_methods_March2...