The nation's most active volcano rumbled and emitted clouds of choking soot that left a layer of ash on villages near Mayon, about 330 kilometres (200 miles) southeast of Manila, officials and witnesses said.
The fine particles are being produced as fragments of rock collapse in the volcano's lava flows, chief government volcanologist Renato Solidum said.
"It's not very thick, just a few millimetres of ash, but that is the most dangerous part because it is very fine ash," he said in a television interview.
Health officials warned the tiny particles could cause respiratory problems or skin diseases, and could affect the thousands of people crammed into evacuation centres beyond the eight-kilometre danger zone.
Witnesses reported ashfall a considerable distance from the restive volcano and civil defence officials said it was hard to predict where the ash would settle because of shifting winds.
Fine white powder drifted across Guinobatan village, some 14 kilometres from Mayon.
Vehicles driving along the village's roads kicked up clouds of volcanic dust as residents, some wearing face masks, complained of stinging eyes and said they could feel the particles irritating their skin.
The government's chief epidemiologist Eric Tayag said residents should stay indoors and keep windows closed if necessary to avoid exposure to the ash.
He said anyone who had to venture outside should wear long-sleeved clothes and cover their nose and mouth with a damp cloth.
Volcanic ash has proved deadly in the past. During Mayon's last eruption in 2006, the volcano oozed lava and vented steam and ash for two months.
No one was killed by the eruption itself, but three months later, a powerful typhoon dislodged tons of volcanic ash and debris that had collected on Mayon's slopes. The fast-moving avalanche of mud and boulders crushed entire villages, leaving more than 1,000 people dead.
Mayon, which has been spewing lava and ash for a week, remains at alert level four, meaning it could violently erupt any day.
The elevated risk has prompted authorities to evacuate more than 9,440 families at least 45,336 people -- from the danger zone.
Provincial governor Joey Salceda said there were only about 500 people still in the zone.
"We are doing all sorts of contortions just to bring them in," he told AFP, while adding he was reluctant to resort to force.
Salceda said "definitely (evacuated villagers) will spend Christmas and even New Year's Day in evacuation centres".
Government medical teams have been sent to the centres to guard against any outbreaks of infectious diseases or illnesses spawned by ashfall, Salceda said.
He expressed concern that bored evacuees may try to sneak out and return home so they can celebrate Christmas in familiar surroundings.
To avert this, various agencies are carrying out activities in the evacuation centres such as concerts, painting lessons and children's parties, the governor said.
The 2,460-metre (8,070-foot) volcano, which is famed for its near-perfect cone, has erupted 48 times in recorded history. In 1814, more than 1,200 people were killed as lava buried the town of Cagsawa.
