Watch documentary on China Air Under the Dome by Chai Jing on youtube
No, actually not. It is proven cleaner and clearer both anecdotally and statistically.
Yes, Ap Lei Choi is not too bad, Repulse Bay, etc. is ok, but offshore is statistically the only place with a major difference. One has to trade the inconvenience for the cleaner lifestyle.
Not sure if you consider Lantau as "offshore" but the Tung Chung station frequently has worse pollution than other stations around Hong Kong.
Tung Chung, yes is offshore. Unfortunately, "contrails" from jets produce a fair amount of suspended particulates that irritate asthmatics and those with allergies. Lantau residents suffer due to these emissions. The government doesn't have a reporting station on Lamma at the present time but statistical data has been obtained in the recent past. Until bunker is eliminated in the ferries there will continue to be issues with residual emissions even on Lamma. Fortunately, it is still much better than repeated contrail emissions from jets.