The smell of something burning is often associated with worsening air quality and transboundary haze, but this may not always be the case, said the National Environment Agency (NEA).

Such smells could come from a variety of sources, including local and regional fires, it said in response to queries from The Straits Times.

On Feb 27, air quality in the northern parts of Singapore hit unhealthy levels for three hours. But this was due to elevated levels of ozone, an odourless pollutant.

On Tuesday, a burning smell was reported in many parts of Singapore after winds blowing from the north brought in smoke from a hot spot in Johor.

However, despite the lingering smell and hazy conditions, air quality remained in the "moderate" range.

But the hourly concentration of fine particulate matter (PM2.5) entered elevated levels between 6am and 7am in the east of Singapore on Tuesday, before returning to normal levels from 8am.

During haze caused by forest fires in the region, the dominant pollutant is PM2.5.

Q What are some possible causes of the burning smell and hazy landscapes?

A The ongoing dry conditions, due to the current phase of the north-east monsoon season, are causing lower rainfall and warm temperatures. These conditions are conducive to the development of land and forest fires in neighbouring countries as well as local vegetation fires, said NEA.

Since the second week of February this year, the Singapore Civil Defence Force (SCDF) responded to about 30 cases of local vegetation fires, said the SCDF spokesman, in response to queries from ST.

Most of these fires involved small pockets of vegetation around Singapore, but the largest fire on Feb 23 spanned the size of about a football field, the spokesman added.

Smoke haze from fires result in the occasional burning smell, slight haziness and reduced visibility.

The burning smell tends to be accentuated at night and in the early morning when lighter winds cause an accumulation of particulate matter in the air.

Q Should we go outdoors when there is a burning smell in the air?

A There is no good correlation between a burning smell in the air and the air quality in Singapore. Instead, refer to the hourly PM2.5 concentration readings and personal guide at www.haze. gov.sg when deciding whether to go ahead with immediate outdoor activities.

If PM2.5 concentrations reach elevated levels between 56 and 150 micrograms per cubic metre of air, people should reduce strenuous outdoor activities, while vulnerable people, including the elderly, pregnant women and children, should avoid such activities for the next hour.

Q Is the haze returning soon?

A Dr Erik Velasco, an expert on air quality, noted that in recent days, the number of hot spots in Riau and Sumatra in Indonesia, and the southern areas of Malaysia have increased a little due to the current dry conditions.

"We are in the tail of the north-east monsoon, thus plumes from Malaysia's wildfires may be triggering such burning smell," said Dr Velasco.

"If the dry conditions continue, we could expect haze events, but fortunately, rain is forecast for this month."

However, before the rain arrives, there could be some light haze in the following days, similar to the slight haziness seen over the past few days, but Dr Velasco does not foresee any severe episodes.

Correction note: This story has been edited for accuracy.