Q&A to clear the air about haze

It looks clear outside and there is no smoke smell, but the PSI levels on the National Environmental Agency (NEA) website are above 300. What should I trust?

The public should not depend on the visibility and smell of the air because there are other contributing factors in the air besides the PM2.5 concentration, said an NEA spokesman.

Members of public should instead look at the three-hour PSI reading and the one-hour PM2.5 concentration level as a guide to adjust immediate activities like going outside for a jog.

"Don't trust your eyes and nose because they are not what detect harmful substances in the air," said air pollution researcher Erik Velasco of the Singapore-MIT Alliance for Research and Technology's Centre for Environmental Sensing and Modelling.

Is the online site www.aqicn.org reliable?

On Sept 18, the Beijing-based environmental non-profit group Air Quality Index China released a real-time, interactive map displaying air pollution levels around the world.

The AQICN map, which automatically updates every 15 minutes, compiles verified data from official environmental protection agencies such as NEA.

But its values may differ from those on the NEA website because of a difference in the formula that it uses to calculate the PSI.

Dr Velasco said the NEA website is based on instruments that are approved by international standards and it would be safer to refer to it.

Which is a better gauge, the PSI or PM2.5 readings?

The 24-hour PSI forecast reflects the PSI trend. It is also a gauge for an individual's total exposure to PM2.5 concentrations and should be used to plan ahead for activities, such as whether one should go to work or school. During haze episodes, health advice will be based on the 24-hour PSI.

The three-hour PSI reading and the one-hour PM2.5 concentration reading are based on more recent PM2.5 readings than the 24-hour PSI forecast. So these readings are better guides as to what one should do at a particular point of the day.

What is expected of the haze for the new few days?

The hazy conditions will continue over the next few days, but will gradually improve over the weekend, said NEA.

Wind patterns are expected to change - coming from the South-east instead of the South-west, where the fires and smoke are, Dr Velasco said.

This article was first published on September 26, 2015.
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