askST: Why is there a burning smell in Singapore even though there is no haze

SINGAPORE – Many people have reported a burning smell in the air in the past few days, with the odour becoming more noticeable at night, and in the east.

But measurements in Singapore have not indicated that air quality has deteriorated. The 24-hour Pollutant Standards Index (PSI) has remained in the good to moderate range, while one-hour PM2.5 readings have remained within the “normal” band.

The Straits Times looks at what is causing this smell.

1. Why does the air smell of smoke, even though the PSI and PM2.5 readings are normal?

In a Facebook post on the morning of March 26, the National Environment Agency (NEA) said transboundary haze affecting Singapore arises mainly from forest and peat fires in the region.

“Besides generating smoke particles, vegetation and peatland fires also generate a complex mixture of gases which cause an acrid burning smell.”

NEA said that these gases that are causing the smell are different from the air pollutants monitored for air quality.

Singapore monitors six key pollutants: particulate matter of less than 10 microns (PM10) in size, fine particulate matter of less than 2.5 microns (PM2.5) in size, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide. A strand of human hair is about 70 microns in diameter.

As at 5pm on March 26, the 24-hour PSI ranged from 41 in the north to 63 in the east, and the one-hour PM2.5 ranged from 2 micrograms per cubic m in the north to 12 mcg per cubic m in the east.

Air quality is considered good for PSI values of between 0 and 50, and moderate for PSI values of between 51 and 100. For PM2.5, air quality is considered normal for readings between 0 and 55 mcg per cubic m. 

Dr Steve Yim, a professor of environmental health at NTU, said that when vegetation and peat burn, they release something called volatile organic compounds (VOCs). Some VOCs are detectable by the human nose at extremely low concentrations, sometimes as low as a few parts per billion.

Furthermore, in the case of peat fires with insufficient oxygen, they can lead to smouldering, which produces a higher volume of the odorous gases compared with high-temperature flaming fires, Dr Yim added.

2. Why is the smell worse at night?

Many residents have said the smell of smoke becomes stronger at night, which NEA says on its website is due to weaker winds in the evening.

“When the winds are weaker, the air pollutants are not diluted and dispersed as quickly as in the day. Cooler temperatures at night also restrict air pollutants from dispersing upwards,” the agency said.

As a result, the smell of smoke is stronger at night and weakens again when the wind picks up speed in the morning.

3. Why does it seem worse in the east?

In the past week, residents in eastern Singapore, in particular, have reported a strong smell of smoke.

Moderate haze was detected by satellite remote sensing in the parts of Johor 20km to 40km east and north-east of Singapore on March 20 to 22 and March 25, said Dr Koh Tieh Yong, an adjunct associate professor from the NUS Department of Physics’ minor programme in meteorology and climate science.

“As the prevailing wind has been from the north-east in this period, some of the lighter VOCs may have been carried by the wind into Singapore,” Dr Koh said.

Given its proximity to the parts of Johor experiencing haze, the eastern part of Singapore was more affected by the smell.

4. What health precautions can people take?

Several doctors The Straits Times spoke to have noticed an increase in cases of eye, nose or throat irritation in the past month.

Dr Kenneth Tan, who runs clinics in Punggol and Joo Chiat, said he has noticed a recent rise in the number of patients complaining of a sore throat.

Similarly, Dr Elaine Chua of Bedok Medical Centre also noticed a rise in such cases throughout March, especially in the past week.

“There are some cases of patients who have not had asthma attacks in a long time having asthma attacks recently,” she said.

However, it is not clear if these cases are related to the burning smell in the air.

Dr Chua said that people who are prone to allergies should not rely on only PSI readings as an indicator of whether particles in the air can be a trigger.

“The amount of allergens in the air does not have to be high to trigger an allergic reaction. As long as there are allergens in the air, their conditions may become exacerbated,” she said.

“Specifically, if the current conditions are caused by the burning of organic materials, it may trigger a strong allergic reaction in those who are sensitive.”

She said those with asthma can consider using preventive inhalers, and those with allergic rhinitis can consider using nasal steroids to help with their symptoms.

For people who feel uncomfortable due to the smell of smoke, Dr Philip Eng, a senior consultant respiratory physician at Mount Elizabeth Hospital, recommends using an air purifier in their bedroom at night.

This article was originally published on The Straits Times. Its inclusion on this website is solely for education purposes.