MANILA, Sep. 22 (CNA) - A small but restive volcano near the Philippine capital Manila spewed above-average sulfur dioxide and volcanic smog on Friday (Sep 22), prompting authorities to close schools in five cities and dozens of towns and urge people to stay indoors.

The state volcanology and seismology institute said it observed upwelling of hot volcanic fluids in the Taal volcano's crater lake, resulting in the emission of volcanic gases.

The alert remained at level 1 on a five-level scale, denoting a "slight increase in volcanic earthquake, and steam or gas activity".

Located in a scenic lake in Batangas province near Manila, the 311m Taal is among the most active of 24 volcanoes in the Philippines.

In January 2020, it spewed a column of ash and steam as high as 15km, forcing more than 100,000 people to evacuate and dozens of flight cancellations as heavy ash fell as far away as Manila.

"We have reports of respiratory illnesses in Batangas province due to intoxication from the volcanic smog," Randy Dela Paz, operations section chief at the civil defence's southern Manila office, told DWPM radio.

Volcanic smog, or vog, consists of fine droplets containing volcanic gas like sulfur that can cause irritation of the eyes, throat and respiratory tract.

Authorities on Friday suspended classes in dozens of towns and cities - Cavite, Laguna, and Batangas provinces adjacent to Taal volcano, and in five cities in the capital region.

The aviation authority on Friday told pilots to avoid flying close to the volcano's summit "as airborne ash and ballistic fragments from sudden explosions may pose hazards to aircrafts".