自从1993年哈佛“六城”研究发表以来，科学家和公共卫生官员已经认识到死亡和微细颗粒物（或PM2.5，即直径小于2.5微米的空气颗粒物）之间的关系。
Ever since the publication of Harvard’s “Six Cities” study in 1993, scientists and public-health officials have been aware of the links between mortality and fine particulate matter, or PM2.5 (airborne particles with a diameter of less than 2.5 microns).