SINGAPORE - With so many different types of face masks now readily available in neighbourhood shops and heartland malls alike, the question becomes: How effective are they against Covid-19 and other respiratory infections?

The Straits Times selected nine types of surgical and reusable masks and put them through two tests.

Working with lab experts from local testing and inspection company Setsco Services, a particle filtration efficiency (PFE) test was conducted to measure how well the mask prevents tiny respiratory droplets - at 0.1 microns or one millionth of a metre - from escaping.

A good mask should have a PFE rate of at least 95 per cent.

A breathability test was also conducted to measure how comfortable it is to breathe through the masks.

This writer tested the breathability of the six reusable masks, while the breathability of the three surgical masks was ascertained through a differential pressure test.

In the latter, the masks were subjected to air pressure to determine their resistance. Generally, the lower the resistance, the better the breathability. A differential pressure reading of less than 40 Pascals per sq cm indicates that the mask is comfortable for the wearer.

To determine whether face masks comply with standards set by ASTM, an international standards organisation, a total of five tests have to be conducted.

They include tests for bacterial filtration efficiency (BFE), fluid resistance and flammability.

The latest DET mask given out by Temasek, which is not among the nine masks tested, has a differential pressure reading equivalent to less than 40 pascals per sq cm, but does not have a PFE reading. Its BFE, however, is at more than 80 per cent.

Out of the nine masks tested, the surgical masks did well at both tests, whereas the reusable masks reflected a trade-off between breathability and filtration efficiency, said experts.

The surgical masks had a PFE range of more than 98 per cent, while most reusable masks had a PFE range of 0 to 20 per cent, with the exception of sample four - the triple-layered mask in black - which had a PFE of 88.51 per cent.

This means that most of the reusable masks were not very efficient at filtering 0.1 micron particles but they may filter particles that are larger in size.

Noting the results, Dr Joel Lee, director of the School of Chemical and Life Sciences at Nanyang Polytechnic, said it was "not surprising" that reusable masks had a lower PFE compared with surgical masks.

But he noted that sample four comprised three layers and had a high-performance filter that kept it dry from external moisture.

It also had a first layer of breathable mesh, which enabled it to balance breathability with good PFE, he added.

Dr Teo Tee Hui, council member at The Institution of Engineers, Singapore and a senior lecturer at the Singapore University of Technology and Design, also noted that the mask had such a high PFE because it was mainly made out of finely woven nylon. The other reusable masks that were tested used coarse cotton for both inner and outer layers.

"Coarse cotton is good for breathability but it is porous. Respiratory droplets are not well-absorbed, explaining the low filtration efficiency," he said.

Most reusable masks are double-layered, with either both layers made of cotton or just the inner layer made of cotton and the outer one from polyester, he added.

Polyester is a good material to use on the outer layer of the mask, as it is water-resistant and can repel droplets from entering. But if the inner layer is made of coarse cotton, it could hamper the mask's overall filtration efficiency.

"Ideally, the best reusable mask should have three layers, with a non-woven fabric as the middle layer to better absorb the water particles. That way, the inner and outer layers can be made of porous materials for better breathability, while not compromising on filtration efficiency, just like surgical masks," he said.

Dr Lee also suggested that masks be two layers or more and made out of a combination of materials such as chiffon weave or flannel for better filtration efficiency.

Out of the six reusable masks tested by the writer, Sample 5, a single-layer mask, likely made out of polyester or polyurethane (a type of plastic material) or a blend of both materials, was the most comfortable, though its PFE was at 0 per cent.

Dr Teo noted that polyurethane, frequently used in lingerie, is known for its stretchiness and comfort, but like polyester, it may not be as effective in trapping droplets.

It could, however, still prevent droplets larger in size from being released into the environment, though it is recommended to choose a mask with better PFE.

Both experts agreed that a good approach would be to wear surgical masks at high-risk and crowded places such as hospitals, and to use reusable masks for low-risk activities.

Wearing a mask should also be done in conjunction with other measures such as safe distancing, to ensure one's safety and hygiene, they added.

Correction note: An earlier version of the article said Dr Teo Tee Hui was a council member at The Institute of Engineers Singapore. This has been corrected. We are sorry for the error.