Poor air quality in Hong Kong is likely to persist until the weekend as low winds fail to disperse pollutants. "Very high" concentrations of air pollution engulfed Causeway Bay, Central and Mong Kok yesterday morning. A dry continental airstream affecting the south China coast was to blame for the deterioration in air quality as low winds made pollution harder to disperse, leading to poor air quality in dense urban areas, according to the Environmental Protection Department. The high pollution is likely to persist until the weekend when winds switch from moderate northeasterly to easterly, the department said, citing Hong Kong Observatory forecasts. "Higher than normal levels of particulates and nitrogen dioxide have been recorded in the territory since Tuesday night," the department said in a statement yesterday. "It is expected that a higher than normal level of pollution will linger until dispersion improves." A reading of 10 was recorded at the Causeway Bay roadside monitoring station yesterday morning, while the station at Central hit nine and Mong Kok reached eight on the 11-tier Air Quality Health Index. The readings prompted the department to issue a warning that readings may again hit "serious", a black 10+, the highest level on the index. Pollution in Causeway Bay hit this level on Wednesday evening. A reading of "very high" or above on the index, levels eight, nine, 10 and 10+, prompts a warning by the department to children, the elderly and people with heart or respiratory illnesses to reduce physical exertion and outdoor activities. The general public are also advised to reduce time outdoors, especially in areas with heavy traffic. Yesterday, the Education Bureau urged all schools to take note of changes in the index. The hourly concentration of roadside-dominant nitrogen dioxide (NO2) was at 267 micrograms per cubic metre of air at 10am yesterday while concentrations of fine suspended particulates (PM2.5) - considered the most hazardous to health - hit 172. The World Health Organisation sets 200 as the maximum hourly concentration for NO2 and 25 as the maximum 24-hour average concentration for PM2.5.