These kinds of measures aren’t objective. It’s like asking what is the safe amount of lead to consume. There is no safe amount. The science says you should have 0. The reality is you have to pick an arbitrary cutoff point that’s good enough.
There isn’t a single threshold either, there are several graded tiers they recommend cities and countries aim for, depending on how bad their pollution currently is. Ultimately, you’re right, 0 is the ideal. That said, one small nit: the cutoffs aren’t arbitrary, the WHO’s lowest level (“AQG”) is set to the “lowest levels of exposure for which there is evidence of adverse health effects.” This value was adjusted downward last year because we’ve accumulated new and compelling evidence of adverse health effects since the last time they set AQG guidance in 2005. Chances are high they’ll adjust downward again the future as we gather more evidence.
While "no safe amount" may technically be true from a scientific perspective in a world where we can measure infinitesimal amounts of things a policy line needs to be drawn. At some point the average net effect of the poison is going to be so far below the noise threshold that caring further is a wash or of negative benefit.
People that are rolling coal and clinging to their cars are also virtue signalling, just in the other direction
The reason this is stated is because lead is cumulative, that is not really the same thing as breathing particulates. Presumably, particulates do not accumulate over a lifetime.
It’s not like something like salt where some amount is fine or even good but too much could cause issues.
We also understand the mechanisms in which lead acts on the body. We know it's harmful in any amount. There is no safe level of lead.
Take your correlation, straight line it as a linear effect, and find the point where the effect size is less than we can observe. Ta dah.
As already mentioned elsewhere too, the Sun will eventually grow fat with helium and nom the planet along with a couple more.
And just because the sun expands in a few billion years doesn't mean I shouldn't be nice to my neighbor today.
The Earth will continue to orbit and spin and life will go on, until one day the Sun decides it's time to get fat and eat a couple planets.
How do you figure the PM2.5 level is tied to the sustainability of the planet? Or even the ecosystem we enjoy?