Victoria Harbour with serious air pollution Photo: Wikimedia Commons

Hong Kong was plagued by the worst, 10+ level of air pollution on Wednesday, the third time in the past six days it has hit the highest mark on the measurement system, sparking government warnings against outdoor exercise.

The area quality index gave an average reading of eight, with six monitoring stations hitting over 10, which is the maximum rating of the indicator. The western part of the city was said to be affected by the pollutants blown from the River Delta region the most.

The Environment Protection Department explained that the weather was hot but with little wind, which favored the accumulation of air pollutants that were brought from the River Delta region. Meanwhile, the sunlight also helped intensify the formation of photochemical smog, resulting high levels of ozone and fine suspended particulates (PM2.5) in the atmosphere.

Dr Cheng Luk-ki, Division Head of Scientific Research & Conservation Department Green Power, said it was rather abnormal for Hong Kong to have such poor quality amid the humid and cloudy weather.

Since Tap Mun, the eastern side of Hong Kong, also had record poor air quality, sources of pollutants could have come from not only the River Delta region, but also from somewhere farther, Cheng added.

Air quality is expected to improve as a strong easterly wind picks up, according to the Hong Kong Observatory.