The numbers for haze and what they mean

The numbers for haze and what they mean

Three-hour average PSI, 24-hour PSI, PM2.5, PM10. The numbers can be quite hazy to many. Here, we explain what the readings mean.

Why don't you give us hourly PSI readings instead of three-hour averages?

The hourly reading would have too many spikes and fluctuations, and can be confusing to the public in terms of how to react, explained Mr Joseph Hui, Deputy CEO (Technology and Corporate Development Group) of the National Environment Agency (NEA).

The PSI level recorded a high of 371 at 1pm on Thursday. Does this mean the air was at a hazardous level for the whole day?

No. The health advisory is based on the 24-hour PSI, which is a better reflection of a person's total exposure to the particulate matter levels. The 24-hour PSI at 7pm was 237, which means that air quality was at an unhealthy level. It is the highest 24-hour PSI in Singapore's history.

The NEA will introduce hourly updates of 24-hour PSI today. How is this different from the hourly updates?

The 24-hour PSI is derived from the average of data collected in the past 24 hours. For instance, at 10pm today, the 24-hour PSI reading would be the average PSI level from 10pm yesterday to 10pm today. For the three-hour average, it would be the average PSI level from 7pm to 10pm today.

Why is the 24-hour PSI used?

It is more useful because health impacts are based on concentrations and duration of exposure. NEA's health advisories are based on the 24-hour PSI as scientific studies on the health effects of haze are based on these measurements.

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