It is that time of year again - when N95 masks fly off the shelves, air purifiers get dusted off, and complaints about sore throats and dry eyes soar.
Some days, people in Singapore wake up to an acrid smell, while smog-like shrouds block out the horizon and private planes have to be diverted to Johor.
Experts say that people should not draw conclusions on air quality based on just visibility levels, or how bad the haze smells.
The haze could be more visible because there is more water vapour in the air, according to the Ministry of the Environment and Water Resources. And the smell may be caused by other compounds generated when vegetation and peat are burned.
Air pollution researcher Erik Velasco of the Singapore-MIT Alliance for Research and Technology's Centre for Environmental Sensing and Modelling said: "The low visibility and acrid smell associated with the haze... are subjective and cannot be used to measure air quality."
Instead, one of the best ways to determine how unhealthy the air is in real time is to look at the one-hour PM2.5 levels published by the National Environment Agency (NEA), say experts.
Understanding the haze
Typically, two types of particles make up the haze.
There are the coarser ones, which the human body is mostly equipped to filter out. These particles are large enough to be trapped by the nasal passages or end up being passed directly through the body.
The bigger worry is the PM2.5 pollutants - so called because they are no larger than 2.5 microns, or one-thirtieth the diameter of a human hair.
These can become trapped deep in the lungs and are tiny enough to pass through linings into the bloodstream.
Long-term exposure to these particles on a regular basis has been linked to increased risk of death from heart and lung complications such as lung cancer or heart disease.
On a regular, non-hazy day, the maximum concentration of PM2.5 is usually between 20 and 35 micrograms (mcg) per cubic m, said Dr Velasco. They start becoming a serious problem when the numbers hit 100, and dangerous when they exceed 200.
Yesterday, the one-hour PM2.5 levels surged past 200 in many parts of the island by midday, with southern Singapore hitting 319 at 6pm.
Starting in April last year, the Government incorporated PM2.5 levels in the calculation of the overall PSI for the first time, to give a more accurate measurement of air quality.
The short-term effect of high PSI levels is apparent to anyone who stays outdoors for too long - headaches, irritation of the eyes and nose and aggravation of conditions such as asthma or bronchitis.
But the long-term health impact of exposure to haze for a couple of months a year - as in Singapore's case - has rarely been studied or fully understood.
Most people would agree that it is bad for one's health, but just how bad is it?
The effect of pollutants
Said atmospheric chemist Mikinori Kuwata, who is with Nanyang Technological University: "To understand the health impact, you have to first understand the haze itself."
While major pollutants such as ozone are present, researchers are still trying to figure out all the other chemicals which make up the smoke - especially particulate matter - that has blown hundreds of kilometres from Sumatra, and how they interact with each other on the way here.
Dr Kuwata said that researchers also refer to air pollution studies elsewhere, for instance the United States, which has its own problems with peat fires. These studies have linked the fires with more hospital visits for heart failure and asthma-related complications.
But the findings cannot be extrapolated to this region because of differences in the chemical composition of pollutants.
For example, temperature and humidity, which change the way chemicals in the air interact with one another, are very different in the tropics compared with the US.
The peat fires in Indonesia also occur in soil of a different composition.
In a study conducted in 2013 - when the three-hour PSI level hit a record 401 during the haze - environmental engineer Rajasekhar Balasubramanian looked at the health risks for a person who has been exposed to haze for a prolonged period of time.
He found that if Singapore were to be exposed for 10 days every year for 70 years to air pollution levels similar to those recorded during the June 20 to June 28 haze period, an average of 18 people in a million could get cancer in their lifetimes.
He also collected air samples between Sept 12 and Oct 2 when there was no haze. The risk of developing cancer fell to 12 in a million people for the non-haze period.
The study was published last year in the journal Environmental Science and Technology.
To put things in perspective, on a day when PM2.5 levels hit 100mcg per cubic m, a person will take in around 1,100mcg of these pollutants if he stays outdoors throughout the day.
For comparison, a smoker will inhale between 10,000mcg and 40,000mcg of PM2.5 pollutants for every cigarette consumed.
The average healthy person should not be unduly worried about the levels of haze experienced this year, say experts.
"We should be objective," said respiratory specialist Yap Wee See of Mount Elizabeth Novena Hospital.
"When the haze is at a lower level, slight modifications to your lifestyle to reduce exposure are probably enough," said Dr Yap.
If PM2.5 readings rise above 100, for example, it might be a good idea to move that evening run to an indoor gym.
Children, pregnant women, the elderly and those with chronic conditions should also take greater care to avoid the haze.
Symptoms like eye or throat irritation are caused by short-term exposure to the haze and are usually self-limiting, said Liew Woei Kang of SBCC Baby and Child Clinic.
His advice for coping with the haze?
"Drink lots of water to flush out the inhaled toxins, and take a healthy balanced diet rich in antioxidants."
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