I don't doubt that you had a wonderful childhood in HKG. However, when was the last time you were back to the Fragrant Harbour? The pollution levels have increased significantly and are off the charts used in the western world, not that they don't have their own pollution problems! What needs to be look at is the levels and types of pollution that are most harmful. The most common pollutant in the USA, CAnada, and Europe is Ozone, this is produced mostly as a by product of vehicular exhaust through a complicated process involving warm weather and sunlight. In high levels it is unhealthy to humans and plant live.
Hong Kong has Ozone, but more importatntly the leading pollutant is respirable suspended particulate. (particles smaller that 10 micrometers) Respirable suspended particulate at high levels can cause chronic and acute effects on your health, particularly the pulmonary function, as the particles penetrate deep into the lungs and cause respiratory problems.
Sai Kung is beautiful, but it's not immune to the pollution, have a look at Tap Mun station to see that rural areas of H.K. are heavily polluted. They are simply not far enough away from the source! So why live in this type of air if it's not absolutely necessary?