The local air quality index hit the “serious” level in some districts on Tuesday, with the Environmental Protection Department (EPD) warning that the situation is expected to last for a few more days.

The EPD said the influence of Typhoon Mawar raised the inflow of pollutants, and the air was most filthy in areas like Yuen Long and Tuen Mun.

"At the 'serious' level, employers of all outdoor workers are advised to assess the risk of outdoor work and take appropriate preventive measures to protect the health of their employees," the department wrote in a statement.

It also advised children, the elderly, and those with heart or respiratory illnesses to cut down on outdoor activities.

Meanwhile, the Observatory issued its new prolonged heat alert for the first time as the mercury hit 35 degrees Celsius in the afternoon.

The Labour Department also issued the amber heat stress at work warning at one point.

Under the government’s guidelines, workers involving moderate, heavy and very heavy physical workload should rest for 15, 30 and 45 minutes per hour respectively when the amber warning is in force.