[mmooss]

> If you do something that causes air in one place to get 1 ppm dirtier and air in another place to get 1 ppm cleaner, and the populations are the same, the net impact on health cancels out.

The average ppm cancel out but the outcomes don't, because you have more sick people: Let's say 12 ppm is the maximum safe ratio for some pollution [edit: assuming it is a pollutant that has local impact, not like CO2 which has global impact]:

1) If place A has 10 ppm and place B has 10 ppm, then nobody is sick

2) If place A has 15 ppm and place B has 5 ppm, then people in place A are sick

Public policy generally doesn't work well with averages and similar analyses, because outcomes are usually discrete for each individual; it's not a stew where you can mix outcomes together and get something good.

As another example, if the economy results in one person making $1 billion and 999,999 making nothing, the average is $1 million per person - but what does that mean? 999,999 people are still in dire poverty. (It does have some significance, for things society does collectively - fund police and fire, and even help for poor people.)

[tlb]

Health effects of particulates are approximately linear. So the more particulates the more mortality. There's no threshold below which people are healthy and above which they're sick.

It's actually slightly sub-linear, meaning that an increase for one group and an equal decrease for another group produces slightly less total mortality. See https://pmc.ncbi.nlm.nih.gov/articles/PMC9109601/#gh2329-bib... for some numerically fitted curves (search for GEMM).