With phase two of reopening due to start in two days, a range of restrictions will be eased, and the new default approach is for most activities to be allowed to resume.

But this comes with some risks. The risk of any activity is tied to how it is conducted, experts say.

To gauge whether something is safe, factors to keep in mind include: crowds, close contact and enclosed spaces; communication; cleanliness; and the duration of an activity and diversity of contacts.

Communication - whether people are talking or not - is an important factor, as the virus can be passed through small droplets during speech. If people are dining in noisy environments, for instance, they may be prompted to speak more loudly, increasing the range of viral spread, said Associate Professor Josip Car, director of the Centre for Population Health Sciences at Nanyang Technological University's Lee Kong Chian School of Medicine.

Meanwhile, more time spent on each activity and a greater diversity of contacts - the different types of interactions that take place outside of home and work settings - can also increase the risk of infection.

When people go out for recreational or religious activities, for instance, they expand their network of contacts, increasing the chance of spreading the virus to different parts of the population.

Such factors are a good starting point in assessing risk in the Covid-19 context, Prof Car said.

Looking at crowds and close contact, for instance, takes into account the number of people there are and how close they are to one another. The risk increases when there are more people, packed together.

Enclosed spaces are also key because risks are heightened when ventilation is poor, while outdoor environments allow for the virus to be quickly dispersed.

"Like many other respiratory viruses, Covid-19 is believed to spread through direct contact with an infected person, indirect contact through high-frequency touchpoints and through smaller droplets that can travel large distances in the air," Prof Car explained.

This is why safe distancing has proven effective in reducing transmission, and other measures including increased sanitation and mask wearing can help to dampen transmission opportunities, he added.

Professor Paul Tambyah of the department of medicine at the National University of Singapore's (NUS) Yong Loo Lin School of Medicine, noted that the distance between an individual and someone with Covid-19 who is shedding the virus is important too.

"If you are window shopping right next to someone who is pre-symptomatic or symptomatic, you might be infected at the same rate as someone at a buffet," he told The Straits Times.

Whether an environment is indoors or outdoors might also affect the spread of the virus.

"This is biologically plausible as there is plenty of evidence about the behaviour of the virus at different temperatures and humidity. Cooler temperatures without direct sunlight seem to promote persistence of the virus on surfaces," Prof Tambyah added.

However, as scientists are still learning about the virus, much is still unknown.

For instance, there is no clear evidence to guide shop owners on how they should clean clothes after customers have tried them on, said Associate Professor Hsu Li Yang, who leads the Saw Swee Hock School of Public Health's infectious diseases programme.

Different retailers have come up with different strategies, including closing fitting rooms and keeping clothes that have been tried on by customers for 24 hours before returning them to the shelves, he pointed out.

"How much of this is actually necessary is unfortunately unknown at this point in time," he said.

And whether table shields that separate diners are effective is still up for debate.

"Unless the table shields are very high, it is not clear how effective or necessary they will be. Viral droplets landing on the shield will not infect the next customer unless he or she specifically touches the shield, and then unconsciously touches his or her mouth or eyes," Prof Hsu said.

Prof Tambyah added: "I am sceptical about these shields as they will be hard to clean and I doubt that they will be that effective in preventing people who are intent on sharing food."

Ultimately, it is not just the activity which is important, but also the behaviour of the community and the ease with which viral spread can happen in such a context, said Professor Dale Fisher of the department of medicine at the NUS Yong Loo Lin School of Medicine.

A case in point is a nightclub.

Here, safe distancing is not part of the experience, masks have to be removed to drink and eat, and people tend to laugh and talk loudly.

In this case, the viral spread cannot be mitigated and a person would be at high risk, he said.

"In theory, anything can open, but for some (establishments), it may not be viable or practical with the safety measures required."