Air quality has always been the focus of everyone’s attention. The World Health Organization (WHO) 2021 edition of the “Global Air Quality Guidelines” (the latest version) recommends that the PM2.5 safety standard is an annual (long-term exposure) average concentration of 5 micrograms per cubic meter, and a 24-hour (short-term exposure) average concentration of 15 micrograms per cubic meter. 　　Recently, scientists from Monash University in Australia published the results of a study showing that in today’s world, almost no piece of land and space is “pure land”. A study of fine particulate matter (mainly PM2.5) in the daily environment conducted by researchers shows that only 0.18% of the world’s land area…