A cloud of smog has enveloped the Philippines capital, Manila, with the country’s health authorities issuing a warning to the residents about the dangerous phenomenon.

The Taal volcano spewed above-average sulfur dioxide and volcanic smog on Friday, prompting authorities to close schools in dozens of cities and towns and to urge citizens to stay indoors.

The state institute of volcanology and seismology said it observed a surge of hot volcanic fluids in the crater lake, resulting in the emission of volcanic gases. Heavy air pollution spread throughout the capital region.

The alert remained at level 1 on the five-level scale, thus indicating a “slight increase in volcanic earthquake and steam or gas activity’.

Located on a scenic lake in Batangas province near Manila, Taal Volcano is 311 meters (1,020 feet) tall and is one of the most active of the Philippines’ total of 24 volcanoes.

In January 2020, the Taal volcano erupted, spewing a plume of ash and steam up to 15 km (9.32 mi) high, forcing more than 100,000 people to evacuate the area and dozens of flight cancellations as heavy ash reached the Manila.