At 20 per cent, that would be still one-fifth of the time. In actual number of days in a year, this is rather a lot for Hong Kong residents to be breathing bad air. You might have already suspected that the air quality was actually worse than the API said it was. Weather conditions can exacerbate air pollution. For example, very hot conditions in the summer increases pollution.
Windless conditions can aggravate matters.
Hot and still days are potentially diabolical. Furthermore, air circulation is often poor in our dense, built-up environment. If your job requires you to be on the streets frequently, you are likely to be breathing in unhealthy air much of tile time. Anyone and everyone who works at street level suffers the most. For example, police officers on the beat, traffic wardens, tens of thousands of shopkeepers with street shop fronts, hawkers, road workers of all sorts, street newspaper vendors, postal delivery officers and other people in the delivery business.