Haze blanketed metropolitan Cebu in central Philippines for a seventh day yesterday, prompting the city government to issue health advisories and airport officials to ground small planes.
Weather forecasters said the haze could have been carried by monsoon winds blowing north-east from Indonesia's Sumatra island, 2,700km from Cebu, but the Environment Ministry said it was still trying to verify this.
The likely source, though, is Kalimantan, the worst-hit Indonesian island that is located 1,800km from Cebu and where thousands of hot spots have been registered.
The haze, caused by illegal forest fires, has affected millions of people across Indonesia, Singapore, Malaysia and now, the Philippines.
This year's crisis is shaping up to be the worst since 1997, when wildfires across Indonesia caused nearly all of South-east Asia to be blanketed in haze, including parts of the Philippines and Thailand, causing US$9 billion in damage.
Haze from local pollution is common in Cebu, which is home to four million people, but it usually disappears within a day.
The current cloud of pollution blanketing the city is blueish-grey and "unusually thick", state weather forecaster Romeo Aguirre told Agence France-Presse. It has also lingered for a week, although it has since thinned considerably.
The Civil Aviation Authority of the Philippines has grounded small turboprop aircraft without instruments that assist pilots during landing or take-off.
So far, no commercial flights have been cancelled, although pilots have reported difficulty landing at Cebu's Mactan airport.
The Environment Ministry said air quality in Cebu remains within the fair-to-good range, but city officials have advised the public to get face masks ready in case the situation worsens.
Meanwhile, Malaysia yesterday issued a warning of low visibility of less than 5km over the waters off nine states as a result of the haze.
The Meteorological Department said in a statement that Kedah, Penang, Perak, Selangor, Negeri Sembilan, Malacca, Johor, Pahang and Sarawak were affected by the low visibility, Bernama news reported.
The poorest air quality was recorded in Nilai, Negeri Sembilan, with an Air Pollutant Index (API) reading of 142, while Port Klang in Selangor and Malacca City also saw high API readings of 133 and 127 respectively.
A reading of 100 to 200 indicates unhealthy air quality, while 201 to 300 is considered very unhealthy and a reading above 300 is deemed hazardous. A reading of 51 to 100 is considered moderate.
Malaysia's Natural Resources and Environment Ministry has also said the country's overall weather was improving with fewer areas recording unhealthy API readings.
However, the Standard Chartered KL Marathon has been cancelled because of the haze.