PSI or PM2.5? What air quality measures should I refer to when there is haze?

SINGAPORE - The spectre of haze loomed over Singapore on Aug 24, as the Pollutant Standards Index (PSI) in the central region inched ever closer to unhealthy levels.

The 24-hour PSI reading was 98 at 9pm, just shy of 101, which indicates unhealthy air.

For weeks, Sumatra and Kalimantan have been ablaze with forest and land fires fuelled by a super El Nino.

How badly Singapore is affected hinges on wind direction.

The Straits Times explains how local air quality readings should be used to plan outdoor activities.

How should residents gauge air quality?

Haze and worsening air quality are shown through two readings on the National Environment Agency’s (NEA) website: the 24-hour Pollutant Standards Index (PSI) and the one-hour PM2.5 readings. Both measures serve different functions.

During hazy periods, the one-hour PM2.5 – an indicator of fine micro-pollutants in the air – is what you should be watching because it points to current air quality. PM2.5 levels should be checked immediately before outdoor activities, such as going jogging or walking your dog.

PM2.5 refers to micro-pollutants found in smoke haze from Indonesian forest fires. These particles – 30 times smaller than the width of a strand of hair – can get lodged deep in the lungs and enter the bloodstream.

On the afternoon of Aug 24, the one-hour PM2.5 readings reached elevated levels in the central and east regions, at 56ug/m3 and above. PM2.5 values can be volatile and tend to fluctuate over the day, especially during haze.

PM2.5 is the dominant pollutant during periods of transboundary haze, and has the most influence over the 24-hour PSI readings.

The 24-hour PSI is computed based on the daily average concentration of six pollutants, including PM2.5, sulphur dioxide, carbon monoxide and ozone.

The calculations are done based on readings from the past 24 hours.

It is an index to provide easily understandable information about daily levels of air quality.

Haze has descended when the PSI crosses 100, reaching unhealthy levels.

What should the 24-hour PSI forecast be used for?

During a haze episode, the National Environment Agency will issue a third air quality measure called the 24-hour PSI forecast.

Not to be confused with the 24-hour PSI, the PSI forecast indicates what the air quality is expected to be in the next 24 hours and can be used to plan activities ahead of time.

In a nutshell, during haze, people should refer to the PM2.5 values if they want to go outdoors within the hour, and refer to the PSI forecast if they are planning an outdoor activity the next day.

How should people protect themselves during haze?

During haze, if the PM2.5 readings are high – between 151ug/m3 and 250ug/m3 – people should avoid strenuous outdoor activities for the next hour. Vulnerable people – who include seniors, pregnant women, children and patients with lung and heart disease – should avoid all outdoor activities.

These are outlined in the personal guide published on NEA’s website.

N95 masks are not needed for short periods of exposure, such as commuting to work.

These masks are recommended under certain conditions, such as when a healthy person must be outdoors for several hours when the air quality is in the hazardous range, with a PSI reading of over 300.

The authorities have said that national N95 mask stockpiles are sufficient.

When the outdoor air quality appears to be worsening, close doors and windows.

If the air-conditioner draws in unfiltered air from outside, close the outdoor air intake opening. Portable air purifiers can also be used to further improve indoor air.

When planning for outdoor activities the next day, refer to the 24-hour PSI forecast and accompanying health advisory. When air quality is forecast to remain unhealthy at 101 and above, healthy individuals should reduce prolonged and strenuous outdoor activities.

Seniors, pregnant women and children should minimise prolonged outdoor exertion, and those with lung and heart disease should avoid such activities.

For healthy folk, continuous exposure to unhealthy air over a few days could cause eye, nose and throat irritation. They will resolve on their own in most cases.

Lee Joon Loong, medical director of Paddington Medical Clinic, reminds patients with asthma to keep their reliever inhalers readily available and ensure that their inhalers are not expired and are being used correctly. 

Similarly, patients with hay fever should make sure that their antihistamines or nasal sprays are readily available and use them if their symptoms flare up.

“Patients should seek medical review early if their symptoms are worsening rather than waiting until they become severely breathless,” said Lee.