If you google pollution in Hong Kong you get a very different story. I would have to research more thoroughly than to just take the word of anyone who cites heritage.org.
After a quick google search:
http://www.numbeo.com/pollution/rankings.jsp (higher is worst)
Hong Kong - 57.41
New York - 52.82
Los Angeles - 60.92
Houston - 60.37
Pollution Index is an estimation of the overall pollution in the city. The biggest weight is given to air pollution, than to water pollution/accessibility, two main pollution factors. Small weight is given to other pollution types.
Again higher is worse.
Hong Kong 16, 58, 68, 35 depending on location.
Los Angeles 82, 93, 63, 59 depending on location.
New York 52-80 depending on location, too many to list.
Here's Hong Kong's full report.
Most of it is in green, except for the one area with yellow. Green = Good, Yellow = Moderate.
Meanwhile Beinjing is at around 212. Here is a comparison of Chinese cities.
Hong kong is obviously the cleanest.