Why are the air quality grades displayed by China officials and the US Embassy different?

the world health organization (WHO) believes that PM2.5 less than 1 is a safe value. Who has put forward strict PM2.5 standards for all countries, but most cities in the world have failed to meet the standards, especially developing countries. Therefore, in 25, the World Health Organization put forward the guideline limit value of PM2.5, and suggested that three transition periods and the final target value should be set up for reference by all countries in the world. In the first transition period, it is suggested that the annual average value of PM2.5 is 35 μ g/m3 and the daily average value is 75 μ g/m3; In the second transition period, it is suggested that the annual average value of PM2.5 is 25 μ g/m3 and the daily average value is 5 μ g/m3; In the third transition period, it is suggested that the annual average value of PM2.5 is 15 μ g/m3 and the daily average value is 37.5 μ g/m3. For the final target value, it is suggested that the annual average value of PM2.5 is 1 μ g/m3 and the daily average value is 25 μ g/m3.

at present, China adopts the standard of the first transition period, with an annual average of 35 μ g/m3 and a daily average of 75 μ g/m3. The PM2.5 standards of developed countries such as Europe and America refer to the standards of the third transition period. For example, according to China's air quality standards, the air quality is good when the daily average concentration of PM2.5 is 75 μ g/m3, while it is moderately polluted according to American standards.