Worth keeping in mind that the targets the WHO is setting are likely temporary, and they’re just goals we should aim for, because the air pollution is currently higher than the goal. There is a historical pattern of these goals being adjusted toward zero.
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.
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.