Air-con alone won’t remove haze particles from indoor air

But their limited leaf surface area and contact with indoor air mean that they are generally ineffective at removing fine particular matter at a meaningful level.

How are PSI and PM2.5 readings different?

The National Environment Agency provides two indicators of air quality: 24-hour PSI and one-hour PM2.5 readings. Both measures serve different functions.

The PSI, which is Singapore’s official national reporting system for air quality, is a measure of air quality in Singapore, and readings of over 100 reflect unhealthy air quality. It is calculated based on six different pollutants – small particulate matter PM10 and PM2.5, sulphur dioxide, nitrogen dioxide, ozone and carbon monoxide.

PSI readings are averaged out over a 24-hour period, which is why they may not always be reflective of current conditions. For instance, it sometimes appears hazier than what the PSI readings indicate.

Meanwhile, PM2.5 readings refer to concentrations of the dominant pollutant particle during haze episodes. It provides a more reflective indicator of current air quality.

There are four bands on the PM2.5 concentration scale: zero to 55 for normal, 56 to 150 for elevated, 151 to 250 for high, and very high for any higher reading.

When these readings are at elevated levels, vulnerable individuals – including elderly folk, pregnant women and those with chronic lung or heart disease – should avoid all outdoor activities for the next hour.

The public can view a personal guide for planning outdoor activities using these readings at this website.

In a nutshell, people should refer to the PM2.5 values for planning their immediate activities, and to the PSI forecast if they are planning an outdoor activity the next day.

Can I wear a surgical mask instead of an N95 mask to filter out haze pollutants?

N95 masks are designed to filter out at least 95% of the fine airborne particles when worn correctly, said Parkway Shenton acting medical director Sharon Ngoh. For effective protection, the mask must fit snugly around the nose and mouth, without gaps that allow polluted air to enter, she added.

Surgical masks, on the other hand, are mainly designed to prevent the spread of respiratory droplets, she said.

“As they do not fit tightly around the face, fine haze particles can still enter through the gaps,” she added. “While they may offer some protection against larger particles, they are not reliable for protection against PM2.5.”

Ngoh also cautioned that N95 masks can make breathing more difficult, especially during physical activity. “If you experience breathing discomfort, slow down, take a break and move indoors to an area with cleaner air,” she said.

Older adults, pregnant women and individuals with severe heart or lung conditions should also consult their doctors before using N95 masks, she added.

NTU’s Liu said some N95 masks have a small valve that makes breathing out easier and trying another size or brand may help.

Mount Elizabeth Hospital’s Eng added that any form of face cover is better than none.

According to the haze task force, N95 masks are generally not required for short outdoor exposure, like commuting from home to school or work, or when indoors.

A healthy individual who has to be outdoors for several hours may wear an N95 mask to reduce exposure when the air quality forecast is in the “hazardous range”, where PSI exceeds 300.

Vulnerable people such as the elderly, pregnant women and individuals with chronic heart or lung disease may wear an N95 mask to reduce exposure when the air quality forecast is in the “very unhealthy” range where the PSI exceeds 200 and if they have to be outdoors for several hours.

Any person who feels discomfort when using an N95 mask should take a break from using the mask, according to the haze task force.