Officials warned on Sunday that air pollution in certain areas of Hong Kong will reach "serious" levels, and urged vulnerable groups of people to reduce outdoor exposure.

The Environmental Protection Department (EPD) explained that the level of suspended particulates has gone up since Saturday night due to a "dusty air stream associated with the northeast monsoon".

As of 10am Sunday, eight of the department's 18 monitoring stations recorded "very high" pollution levels, just one tier below the most severe "serious" classification in the Air Quality Health Index.

The eight districts are Eastern, Central and Western, Tseung Kwan O, Kwun Tong, Tuen Mun, Yuen Long, Tai Po and North.

The EPD noted that with the monsoon moderating gradually, winds were expected to weaken, hindering the dispersion of pollutants. The pollutant level is likely to remain "relatively high in the following one or two days", it added.

Children, the elderly and people with heart or respiratory illnesses were advised to cut down on time spent outside, while employers of outdoor workers were told to assess the risks and take preventive measures to protect the health of their staff.

The Education Bureau said schools should take appropriate measures to safeguard students' health.

The northeast monsoon has also brought very dry conditions to the city.

At one point on Sunday morning, the relative humidity fell to 21 percent at the Observatory's Tsim Sha Tsui headquarters. The forecaster said that is the lowest mark ever recorded in April since the relevant monitoring equipment was set up in 1984.