It is possible for a place to lose air cleanliness completely: leading to a situation where the pollution levels are said to have become hazardous. There are two key ways of telling if a given place has lost air cleanliness completely.
The first way of telling if a given place has lost air cleanliness completely would be through direct observation using the senses. There are, for instance, cities that are so polluted that it is possible to see some sort of fog (even during the clearest of weather), and that is a manifestation of pollution. There are also places that are so polluted that you can smell the pollution, or to an extent that the effect of the pollution can be felt through the skin.
The second way of telling if a given place has lost air cleanliness completely would be through the use of scientific instruments. It emerges that there are instruments through which levels of pollution can be measured. The way the whole thing works is such that there is a calibration scheme, and once air pollution levels go beyond numbers, they are deemed to be hazardous.
In the final analysis, if a place is so polluted, it is definitely not a good place to be. True, the pollution may be brought about by industrialization that leads to social development and reduction of poverty, but if that is achieved at the cost of environmental wellbeing, then it doesn’t make much sense.