Air quality in five areas within four cities in Metro Manila was "very unhealthy" due to "thermal inversion", the Environmental Management Bureau (EMB) reported on Wednesday.

As of 12 pm, the EMB's National Capitol Region office said air quality in Caloocan City, Las Pinas, Muntinlupa and two areas in Paranaque had become "very unhealthy".

A total of 13 areas in 12 other cities were also found to have air quality that is "unhealthy for sensitive groups" of people.

Two areas were recorded in Marikina while other areas are from Makati, Mandaluyong, Manila, Muntinlupa, Navotas, Pasay, Pateros, Quezon City, San Juan, Taguig, and Valenzuela.

According to EMB, Metro Manila might be experiencing "thermal inversion", where warm air is positioned on top of the cold air which serves as a cover that prevents pollution from moving up.

The EMB advises the public in the affected areas to wear face masks, stay indoors and monitor the official update on the air quality in their area. Robina Asido/PHS