Thousands of people were warned to stay indoors and many schools were shut Friday as a thick haze blanketed the Philippine capital and surrounding provinces.
A spike in sulphur dioxide emissions from a volcano and a weather phenomenon that traps smog were blamed for the hazardous air enveloping the heavily-populated region.
Communities near Taal volcano, which sits in a lake about 50 kilometres (30 miles) south of Manila, were urged to stay indoors and close their doors and windows to protect themselves from toxic gases.
Taal has been belching sulphur dioxide for weeks. At least 58 students in Batangas province fell ill on Thursday after a surge in emissions, an official said.
"They complained of chest pains and difficulty breathing," local disaster agency officer Jacque De Taza told AFP.
Hospitals in the province were on alert to respond to emergency situations caused by the volcano's gas emissions.
Vog, or volcanic smog, should "not be taken lightly as sulfur dioxide poses a serious threat to human health", said regional health director Ariel Valencia in a statement.
A grey haze also hung over the sprawling capital and nearby provinces, prompting some local governments to cancel classes.
In parts of the city, concentrations of the most dangerous PM2.5 particles -- so tiny they can enter the bloodstream -- reached "acutely unhealthy" levels, the environment department said.
The weather agency said a "thermal inversion, high humidity, and calm wind conditions" had trapped pollution over the region.
"These floating minute particles in the air could be from smoke, pollutants, or volcanic aerosols," the agency added.