A tropical cyclone caused Hong Kong’s air quality to drop on Wednesday, with the city’s Observatory issuing a typhoon signal No 1 at 2.40pm. Tropical Storm Podul was centred about 630km southeast of the city at 5pm and forecast to move at about 25km/h toward Hainan Island. It brought hot and hazy weather to the city. The Observatory said the chance of the signal being upgraded to No 3 on Thursday was not high, as Podul would maintain a distance of more than 500km from Hong Kong. By 4pm, seven of the city’s 13 air quality monitoring stations – located in Central and Western district Kwun Tong, Tsuen Wan, Yuen Long, Tuen Mun, Tai Po and Tung Chung – had flagged a health risk of “very high” to “serious”. Tung Chung recorded the worst threat, with ozone levels reaching above 300g micrograms per cubic metre and respirable suspended particles PM10 was above 70mcg/m3. “Higher than normal levels of ozone and particulates have been recorded since noon,” an Environmental Protection Department spokesman said. He added more stations may report serious risks later in the day. “The high ozone levels have also led to formation of nitrogen dioxide.” The department expected the pollution levels to go down as Podul moved away on Thursday. The public was advised to consider changes in the weather when planning activities. When the health risk is “very high” or above, children, the elderly and those with heart or respiratory illnesses are advised to reduce physical exertion and outdoor activities. If the air quality health index reaches a “serious” level, employers of outdoor workers are advised to take appropriate preventive measures to protect employees’ health. Some heavy showers and squally thunderstorms are expected in the next few days.