16 October 2015

NEA's Stubborn Refusal To Publish 1-Hour PSI

National Environmental Agency ("NEA") has rejected calls to publish the 1-hour PSI, explaining as follows:

"Since April [2014], the 24-hour Pollutant Standards Index (PSI), three-hour PSI and one-hour PM2.5 concentrations have been published hourly, on the hour, on the NEA website ….

The scale used to convert PM2.5 concentrations to the PSI has been derived based on health studies of exposure over a 24-hour period. This is why the NEA uses the 24-hour average concentrations to compute the PSI.

Although there have been recent studies of sub-daily or shorter PM2.5 exposure, the evidence from these studies is insufficient for the development of one-hour PSI based on exposure to PM2.5 for a one-hour period.

During periods of trans-boundary haze, the primary pollutant determining the PSI level is fine particulate matter, or PM2.5. Only the 24-hour PSI value has a corresponding Ministry of Health advisory because scientific and epidemiological studies of the health effects of exposure to particulate matter have been based on this duration of exposure.

The forecast of 24-hour PSI levels, which the NEA issues every evening during haze periods, and the corresponding health advisory can be used to plan ahead, such as for activities for the next day. For a guide to more immediate activities, the three-hour PSI and one-hour PM2.5 concentration levels can be used as indicative measures to make adjustments to daily activities.

For example, if the three-hour PSI and one-hour PM2.5 concentration levels are high, people may wish to postpone strenuous outdoor activities such as jogging."

No Logic

NEA's explanation makes no sense.

▪ NEA already publishes the 3-hour PSI and the 1-hour PM2.5 concentration level, despite the fact that only the 24-hour PSI value has a corresponding Ministry of Health advisory:

▪ NEA advises the public not to use their eyes nor noses to determine the pollutant concentration levels.

▪ NEA advises the public to use the 1-hour PM2.5 concentration level as an indicative measure to make adjustments for more immediate activities.

Although everyone knows that the higher the 1-hour PM2.5 concentration level the more polluted the air is, many people would find it difficult to tell how bad a 1-hour PM2.5 concentration level of, say, 180 micrograms per cubic metre is and how they should adjust their immediate activities.


Why does NEA not publish an index based on 1-hour PM2.5 concentration level?

Here are some plausible reasons.

Firstly, it may result in disputes when the 1-hour PSI exceeds 300 and workers demand that outdoor work stop, because they think that the Ministry of Health's advisory to avoid outdoor activity when the 24-hour PSI exceeds 300 applies to the 1-hour PSI too.

Secondly, the 1-hour PSI is too volatile and even if management agrees to stop outdoor activities when the 1-hour PSI is, say, higher than 300, it may result in having to make decisions on outdoor work every hour.

Thirdly, it may lead to more people spending more time indoors, resulting in a reduction in economic activity.

Fourthly, inasmuch as the Government is ever so concerned about what foreign investors and visitors think about Singapore, the 1-hour PSI may give foreigners the impression that Singapore's air is more polluted than what is indicated by the 24-hour PSI.

These are important considerations. However, the health of everyone who works outdoors is not only important too but much more important.


Neither the 24-hour PSI nor the 3-hour PSI is useful for planning outdoor activity in the immediate timeframe. We need real-time pollution concentration levels in a form that is easy to comprehend.

By publishing 1-hour PM2.5 concentration levels, NEA already acknowledges that the public needs such information even though there is accompanying no Ministry of Health advisory. It should take the obvious next step and publish an index based on 1-hour PM2.5 concentration levels for each of the five regions. This is not difficult; it is a straightforward mathematical exercise. The question is whether or not NEA wants (or whether or not the Government wants NEA) to do it.

If NEA does not want to give the impression that it approves of an official 1-hour PSI reading, it can simply call the index by some other name, e.g., 1-hour PM2.5I.


1. Insufficient Evidence from Studies to Use an Hourly PSI TODAY 3 Oct 2015.

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