SINGAPORE—Smoke from an increasing number of fires in Indonesia is pushing air quality to unhealthy levels in parts of peninsular Malaysia, but, thanks to winds blowing in its favor, Singapore has so far escaped the noxious haze that reached hazardous levels last month.

The likelihood of smog in any given part of the region around Indonesia’s Sumatra island–where most of the fires that lead to hazardous haze are located–is influenced by three main factors: rainfall, wind direction and human activity.

Rainfall
Satellite images show that the number of hotspots–areas of intense heat that are indicative of fires–in Sumatra totaled 292 as of Tuesday night compared with just 16 on July 12, according to Singapore’s National Environment Agency, after hotspot activity declined from peak levels reached in late June, when the number of fires was also in the high 200s.

Haze levels usually peak between June and September, the driest months in the region, also called the Southwest Monsoon–referring to the direction from which the wind generally comes. When rain does fall, it helps to control fires, thus reducing haze.

Wind Direction

Even if hundreds of fires are raging in Sumatra, Singapore can avert most of the harmful effects if the wind isn’t moving eastward across the Strait of Malacca. At this time of year, monsoon winds tend to dictate wind direction, in this case, mainly toward the northeast.

Anthony Tan, executive director at Kuala Lumpur-based consultancy Center for Environment Technology and Development Malaysia, says it is getting harder to predict when the monsoons will be active because of changes in global climatic patterns.

Winds have been generally blowing from south to north during the latest bout of hotspot activity, and Singapore lies almost due east of Riau in Sumatra, where most of the hotspots are.

“The low-level winds over Singapore will continue to vary between south-southeasterly and south-southwesterly,” the NEA said. Singapore may experience hazy conditions should the winds change to blow from the west or southwest,” the Agency said in itslatest haze forecast.

On the flip side, parts of Indonesia and peninsular Malaysia that lie north of Riau have seen pollution levels rise. Unhealthy air quality levels have been reported in some parts of Selangor, Malacca and Johor states in Malaysia. Further north, Kuala Lumpur has been covered in a light haze over the past couple of days.

Human Activity
Cross-border haze has been a recurring problem for Singapore and Malaysia since the 1980s, largely attributed to palm oil plantation owners and farmers deliberately, and illegally, burning vegetation for the purpose of quick and cheap land clearance.

This is the worst haze Malaysia has seen since 2005, Mr. Tan said by telephone.

Last month Singapore’s and Malaysia’s air-pollution indexes reached hazardous levels when a lack of rainfall and ill-fated winds combined to blanket large areas with heavy smog. Singapore’s three-hour Pollutant Standards Index peaked at 401 on June 21, the worst recorded level. The NEA describes a PSI reading above 300 as “hazardous.”

On Wednesday, the skies were blue in Singapore. The NEA said the three-hour PSI was at 24 at 3 p.m., local time, well below the threshold of 50 that divides “good” and “moderate.”