The low air quality that Metro Manilans breath the past couple of days is not coming from the smog or so-called "vog" due to sulfur dioxide emitted by the Taal Volcano but rather from the pollution brought by emissions from vehicles caught in heavy traffic, according to Department of Environment and Natural Resources – Environmental Management Bureau.

DENR-EMB's latest assessment and monitoring report on air quality factoring in wind direction, among others, noted that "real-time air quality measurements indicated heightened alert in some part of Metro Manila, due primarily attributed to emissions from heavy vehicular traffic, specifically during rush hours."

"Air quality varies in time and places and can change anytime, depending on pollution sources and meteorological factors," it added.

From EMB's initial assessment, hazy skies on 21 and 22 September may be attributed to ground level and suspended particulate matter (PM) concentration.

"Spikes particulate matter due to heavy traffic and thermal inversion. The latter is a climate phenomenon," the DENR-EMB reported.

It added that air quality in Parañaque City is '"acutely unhealthy" while Makati and Pateros areas are "unhealthy". However, there were times when residents of Makati, Caloocan, Parañaque, and San Juan enjoyed "fair" air quality.

EMB also noted that one of the sources of PM emissions comes from mobile (transport), area and stationary. "This occurrence may be primarily attributed to emissions from heavy vehicular traffic, especially during rush hours," it added.

Quezon City and Makati City have already alerted their residents through advisories on localized air quality, with Makati suspending afternoon classes at all levels for both public and private schools.