I have been reading about air quality and I keep seeing different numbers and reports. I want to understand what is considered good air quality and how to know if the air is safe to breathe. Some reports talk about the Air Quality Index but I do not understand the scale fully. I also read about pollutants like PM2.5 PM10 and ozone but I am not sure what levels are acceptable. It seems like there are many standards depending on the country. I want to know what people should aim for daily and how it affects health. If anyone can explain in simple terms what is considered good air quality it would help me plan outdoor activities 
Good air quality means the air has low levels of pollutants. The Air Quality Index usually shows a range from zero to fifty as good. This means that the air is clean and safe for all people including children and elderly. It allows outdoor activities without any concern for breathing problems.
When PM2.5 and PM10 levels are low it is considered safe. Low particles mean less risk of lung irritation or other health problems. People can exercise outdoors freely. Plants and animals also thrive better when the air is clean 
Good air quality also means gases like ozone nitrogen dioxide and carbon monoxide are below harmful levels. These gases can irritate lungs and reduce oxygen absorption. When they are low it is a sign that industrial and traffic pollution is under control.
This helps I did not know gases were part of it
Air looks clearer when quality is good. You can see distant buildings easily and the sky looks bright. Visibility is a simple clue that the air is healthy. People often notice the difference on calm days after rain.