All air monitoring stations across the city recorded “high” to “serious” health risk levels from yesterday afternoon to the evening. Tuen Mun in the western New Territories was worst hit, with the air quality health index reaching “high” from 12 noon and peaking at “serious” with the highest reading of 10+ at 4pm. It dropped to “very high” at 7pm and dropped to "moderate" at 10pm. Yuen Long, also on the western side of the New Territories, was also among the most polluted zones, with the level reaching “very high” from 3pm to 5pm. From 9pm, the air quality improved to "moderate", Causeway Bay saw the worst air quality on Hong Kong Island. The index hit “high” at 4pm, worsening to “very high” from 5pm to 7pm. The index remained "high" from 8pm until midnight. Central recorded a “high” level from 1pm to 11pm when It dropped to "moderate". The Environmental Protection Department said the situation arose because the city was being affected by an airstream with higher background pollutant concentrations. It said sunshine enhanced photochemical smog activities and the formation of ozone during the day. The high level of ozone prompted the formation of nitrogen dioxide, particularly in parts of the urban area and at roadsides. The Observatory said the weather would become cloudier with a few showers towards the end of next week. Until then, pollution levels are expected to remain higher than normal. A “very high” reading means that children, the elderly and people with existing heart or respiratory illnesses should keep physical and outdoor activities to a minimum or avoid such activities altogether, according to the department. The general public is also advised to reduce to a minimum all outdoor physical exertion and cut time spent outdoors, especially in areas with heavy traffic. The index was introduced in December 2013 to replace the old air quality index. It is modelled on a Canadian index, tying concentration levels of various pollutants with health effects.