This formula is also described somewhere on the website of NEA. But to me, this is only half the answer.
Main question is (not to you): Why is the PM2.5 reading not included into the PMI, although this is already a standard in many countries, not just since yesterday. It is well known and established that the particles with a size smaller than 3 micrometers do have a negative effect on the health of people.
It seems, only after a mounting pressure and questions from the public the PM2.5 readings were published, but without any scale or interpretation as to which levels of particle concentration refer to which air quality levels. Neither NEA nor anybody else has clarified the reasons for this (imho questionable) approach.
Using the absolute readings and linking them to the scale used in other countries it reveals the unhealthy levels of PM2.5 concentration the public was exposed to in the past weeks when the official PMI was only published as 'Moderate'. Why?