How does Singapore measure haze levels?

Singapore has 14 air monitoring stations across the country that continuously analyse key air pollutants, officials of the National Environment Agency said Saturday.

In a press tour of the agency’s facilities, environmental authorities explained how air pollution levels are derived from data collected from the stations.

Of the 14 stations, 11 are 'ambient' stations, chosen for their locations away from sources of inference such as pollution from motor vehicles.

In a briefing Saturday morning, Joseph Hui, deputy chief of technology and corporate development at NEA, said the agency’s equipment are carefully calibrated and regularly checked to ensure accuracy. They are also placed in rooms with controlled settings.

They measure the five key air pollutants internationally used to assess air quality: sulphur dioxide, oxides of nitrogen, carbon monoxide, ozone and particulate matter (PM).

The Pollutant Standards Index (PSI) is computed based on the concentrations of all five air pollutants measured over a 24-hour period.

During the haze, PM 10 records the highest concentration among the various pollutants.

Equipment for measuring PM 10 and PM 2.5 levels are the same, except for a filter which allows different-sized PM to go through.

The PM 10 filter allows all particles PM 10 and below to go through, thus it includes the PM 2.5 reading as well.

Real-time data is stored in the system at the stations. It is then transmitted to a data management system via wireless modems at regular intervals.

This data management system has customised software to derive average values and compute PSI automatically. From there, officers will manually upload the information onto the NEA website.

NEA is looking to make the uploading of information to the NEA website automatic from next year.

The three hour PSI reading is unique to Singapore, and was first introduced during the 1997 haze episode. It is calculated the same way the 24- hour PSI is, only with a three hour bracket instead.

Hui assured that the PSI readings being updated are "accurate" and there is no cause for panic.

Project director of the pollutant control department, Indrani Rajaram, added that the 24-hour PSI is a more accurate reflection of the total exposure to PM, and is used to determine health advisories.

The three hour PSI is simply "additional information" for the public, she added.

At 8am on Saturday, the 24-hour PSI reached as high as 246, which Indrani said was the highest-ever in Singapore. It falls under the "very unhealthy" range.

Hui advised the public to follow the health advisory. Those feeling unwell should consult a doctor immediately, and if you need to be outdoors for a long period, wear a mask, he said.

For the general population who need to make short walks such as to MRT stations, short periods of exposure should not be a problem, he said.

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