SINGAPORE - With the United States no longer enforcing a mask mandate on public transport, the relevance of mask wearing has been thrown into the spotlight.

As Covid-19 cases continue on a downward trend in Singapore, The Straits Times looks at how useful mask wearing is now.

Q: When should I mask up?

A: It is down to common sense, an individual's own personal responsibility and safety judgment on when and how to mask up.

For instance, if you have compromised immunity or live and interact with someone who does, it will be a good idea to wear a mask and maintain safe distance.

Masks are also important in hospitals and settings with vulnerable people, such as the elderly.

But if you are healthy and have had your booster shots, the risk of getting seriously ill with Covid-19 is very small.

Professor Dale Fisher, a senior infectious diseases consultant at the National University Hospital, sums it up: "The degree of ventilation and air movement, as well as the density of people present should be considered. Open spaces with few people have a lower risk, compared to large numbers of people crowded in a closed space."

But it is a spectrum, and describing each setting and mask requirements will not be realistic and will be difficult to enforce, he noted.

It will be easier to make more generic guidelines, such as saying that masks are not required indoors after the Covid-19 situation here has settled further, Prof Fisher said.

"But if the situation worsens to a point where many people are being admitted to hospital, then the community may have to accept that indoor mask mandates can return for a period of time."

Q: Should I mask up outdoors?

A: There is little scientific evidence to prove that face coverings offer much added protection in many outdoor spaces such as sidewalks or parks.

For instance, if you can feel the wind on your face, it would mean that outdoor ventilation is adequate and masks are not necessary.

However, if you're in a crowded space standing less than an arm's length from the next person, then it could be seen as a case for outdoor mask wearing.

There is even less worry if the venue requires people to show proof of a negative Covid-19 test.