Why what you see is not what you get

SINGAPORE - "Why does the Pollutant Standards Index or PSI reading not match what I see and smell?"

The National Environment Agency (NEA) acted on Saturday morning to address that oftheard sentiment that had been spreading on the ground.

Defending their air quality readings, the government agency said on Saturday that they followed international scientific specifications for collecting air samples.

It explained that the real-time readings were culled from 11 sophisticated monitoring stations located in schools, reservoirs and army camps around the island.

These stations detect and analyse five different types of pollutants in the air. The data from these stations is relayed to a smart system, which then converts the data to averages and calculates the PSI. This gives the overall haze picture Singapore faces, rather than the situation at a specific spot.

So the PSI may also not correspond to what people see as visibility varies at different locations, explained Mr Joseph Hui, NEA's deputy chief executive, who oversees technology and corporate development.

"Our readings are based on where our air monitoring stations are sited. There will be some apparent difference."

Over the past week, many online comments had questioned the accuracy of the NEA readings, and yet others had posted readings of particulate matter from devices they had bought.

Particulate matter refers to the bits of dust, which can be as small as a fraction of the width of a human hair, suspended in the air.

The NEA cautioned that the small handheld devices can be subject to interference, such as moisture or mist in the air.

The devices can mistake the small liquid droplets in the air for particulates and provide an inflated reading.

Mr Hui added that smell was also too subjective. "There are certain (pollutants) that one individual can smell, but another individual may not be able to smell (them). I'm very sensitive to cigarette smoke so to me, it seems as if there is a lot of cigarette smoke around. Another individual might not be able to detect it."

The NEA also took pains to emphasise how scientifically rigorous the Singapore's air monitoring and P METICULOUS: An NEA officer looking at readings from an PM2.5 SI system had been set up to be.

Developed by the US Environmental Protection Agency (USEPA), the PSI and its associated health advisories are backed by scientific and medical research studies, said Mr Hui.

Each of the 11 stations also has analysers certified by the USEPA. These analysers continuously sample air.

To increase the accuracy of the readings, the sample is kept at least eight degrees above ambient temperature to reduce its humidity.

A total of 12 officers operate NEA's air quality monitoring network.

Calculating the PSI involves a complex computation using the analysis of the concentrations of pollutants in the air.

On Saturday, the NEA also pointed out that the 3-hour PSI reading was "unique to Singapore" and was only "indicative" and "not accurate" in estimating the actual impact of the haze on health.

Rather, the 24-hour index would be a more accurate reflection of the total exposure one would have had to the pollutants, it said.


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