For the last 15 months, many American offices sat essentially empty. Conference rooms and cubicles went unused, elevators uncalled, files untouched. Whiteboards became time capsules. Succulents had to fend for themselves.

But over the coming weeks, many of these workplaces will creak slowly back to life. By September, roughly half of Manhattan’s one million office workers are likely to return to their desks, at least part time, according to a recent survey by the Partnership for New York City.

Although the risk of contracting Covid-19 has fallen significantly in the United States — especially for those who are fully vaccinated — it has not disappeared entirely, and many workers remain nervous about returning to their desks. (Many others, of course, never had the luxury of working remotely in the first place.)

“If you’re still feeling uncomfortable or anxious, that’s totally understandable,” said Joseph Allen, an expert on healthy buildings who teaches at Harvard’s T.H. Chan School of Public Health. “This pandemic has affected all of us in profound ways, and people are going to be ready to re-enter life again or re-enter interacting with people at different times.”

But scientists have learned a lot about the virus over the past year, and there are some clear, evidence-based steps that employers can take to protect their workers — and that workers can take to protect themselves. Some of these strategies are likely to pay dividends that outlast the current crisis.

“I think it’s important for us as a community, but also individual employers, to think about these questions in relation to not just this week and this month,” said Alex Huffman, an aerosol scientist at the University of Denver. “How do we make decisions now that benefit the safety and health of our work spaces well into the future?”

Address the risks of closures

Although Covid-19 is the headline health concern, long-term building closures can present risks of their own. Plumbing systems that sit unused, for instance, can be colonized by Legionella pneumophila, bacteria that can cause a type of pneumonia known as Legionnaires’ disease.

“Long periods with stagnant, lukewarm water in pipes — the exact conditions in many under-occupied buildings right now — create ideal conditions for growth of Legionella,” Dr. Allen said.

Some schools have already reported finding the bacteria in their water. In buildings with lead pipes or fixtures, high levels of the toxic metal can also accumulate in stagnant water. Employers can reduce both risks by thoroughly flushing their taps, or turning on the water and letting it run, before reopening.

“We know that flushing water during periods of inactivity usually reduces lead levels and also potentially bacteria that may form,” said Jennifer Hoponick Redmon, a senior environmental health scientist at RTI International, a nonprofit research organization based in North Carolina. She added: “A general rule of thumb is 15 minutes to one hour of flushing for long-term closures, such as for Covid-19.”

The Centers for Disease Control and Prevention also recommends that companies check for mold growth and pest infestations before reopening.

Upgrade ventilation and filtration

Because the coronavirus is thought to spread primarily through tiny, airborne droplets, employers should upgrade their ventilation and filtration systems before bringing workers back, experts said.

“One thing you can do before you go back to work is simply ask them what they’ve done,” Dr. Allen said. “And if you hear things like, ‘Yes, we’re meeting code,’ then that’s a flag that something’s not right. They should be going above and beyond the bare minimum ventilation and filtration rates.”

Although the ideal ventilation rate varies, in general, employers should maximize the amount of fresh air coming in from outdoors, he said. In a relatively small space — say, the size of a typical school classroom — employers should aim for four to six air changes per hour, meaning that the air inside the space is being completely refreshed every 10 to 15 minutes. Opening windows can also improve air flow.

High-quality air filters, like those that are rated as MERV 13 or higher, can trap a majority of airborne viral particles. Some commercial buildings are not equipped for these heavy-duty filters; in those offices, portable air purifiers, equipped with HEPA filters, can be effective, experts said.

“These types of portable units can do a great job of taking particles out of the room,” Dr. Huffman said. “And the next level is even a desktop level HEPA filter, where you have a really small unit that provides clean air into your direct breathing zone.”

These personal units may be particularly helpful in poorly ventilated offices, although experts stressed that employers, not employees, should bear the burden of improving indoor air quality.

Be wary of chemical disinfection

While ventilation and filtration are crucial, employers and building managers should stay away from foggers, fumigators, ionizers, ozone generators or other “air cleaning” devices that promise to neutralize the coronavirus by adding chemical disinfectants to the air. “These are all really terrible ideas of things to do to indoor air,” said Delphine Farmer, an atmospheric chemist at Colorado State University.

The compounds that these products emit — which may include hydrogen peroxide, bleach-like solutions or ozone — can be toxic, inflaming the lungs, causing asthma attacks and leading to other kinds of respiratory or cardiovascular problems. And there is not rigorous, real-world evidence that these devices actually reduce disease transmission, Dr. Farmer said.

“A lot of employers are now — and school districts and building managers are now — thinking that they have solved the problem by using those devices,” Dr. Farmer said. “So then they are not increasing ventilation rates or adding other filters. And so that means that people think that they’re safer than they actually are.”

Surfaces pose minimal risk for coronavirus transmission, and disinfectants needlessly applied to them can also wind up in the air and can be toxic when inhaled. So in most ordinary workplaces, wiping down your desk with bleach is likely to do more harm than good, Dr. Farmer said. (Some specific workplaces — such as hospitals, laboratories or industrial kitchens — may still require disinfection, experts noted.)

Nor is there any particular need for special antimicrobial wipes or cleansers, which may fuel the emergence of antibiotic resistant bacteria and wipe out communities of benign or beneficial microbes. “As tempting as it may be to try to sterilize everything, it’s never going to happen, and there may be some real serious consequences,” said Erica Hartmann, an environmental microbiologist at Northwestern University.

Don’t depend on desk shields

In the early months of the pandemic, plastic barriers sprang up in schools, stores, restaurants, offices and other shared spaces. “They can be great to stop the bigger droplets — really they’re big sneeze guards,” Dr. Huffman said.

But the smallest, lightest particles can simply float over and around them. These barriers “may not provide enough benefit to justify their costs,” said Martin Bazant, a chemical engineer at the Massachusetts Institute of Technology. They may even raise the risk of disease transmission, by encouraging riskier behavior or impeding air flow.

There are some environments in which these kinds of barriers may still make sense. “It can be a really good idea for people who would otherwise have very close face-to-face contact, like grocery store workers at cash registers,” Dr. Farmer said. “But past that, in offices where you’re sitting for a lengthy period of time, there is no benefit to putting yourself in a plexiglass cage.”

Carefully consider staffing plans

Social distancing may still have some benefits; if an employee is exhaling infectious virus, people sitting directly in that person’s breathing zone will quite likely be exposed to the highest doses. “If you were sitting at a shared table space, two feet away from someone, then there could be some potential value to moving away a little bit further,” Dr. Huffman said.

But aerosols can stay aloft for hours and travel far beyond six feet, so moving desks farther apart is likely to have diminishing returns. “Strict distancing orders, such as the six-foot rule, do little to protect against long-range airborne transmission,” Dr. Bazant said, “and may provide a false sense of security in poorly ventilated spaces.”

In offices in which most people are vaccinated and local case rates are low, the benefits of distancing are probably minimal, scientists said. Higher-risk workplaces may want to consider de-densification, or reducing the number of people — any one of whom might be infectious — who are present at the same time. “That, to me, has been the biggest benefit of this social distancing indoors,” Dr. Farmer said. “It’s just having fewer potential sources of SARS-CoV-2 in a room.”

Companies could allow a subset of employees to work at home indefinitely or on alternating days or weeks. They could also consider “cohorting,” or creating separate teams of workers that do not have in-person interactions with those who are not on their team.

Creating these kinds of cohorts could also make it easier to respond if someone does contract the virus, allowing the affected team to quarantine without having to shut down an entire workplace. “When we think about reopening, we need to think about what do we do when, inevitably, we see a case?” said Justin Lessler, an infectious disease epidemiologist at Johns Hopkins University. “There are creative ways to lessen the impact.”

Go back to basics

Regular hand-washing, which can reduce the spread of all kinds of pathogens, is always a good idea. “The messaging at the beginning of the pandemic about washing your hands and washing your hands for at least 20 seconds — that is totally valid and still really important,” Dr. Hartmann said.

And when your office itself needs cleaning, a mild detergent will generally do the trick, she added: “Soap and water is great.”

Masks, too, remain effective. “If you’re someone who’s vaccinated and still feeling anxious about going back to work, the best thing to do is continue to wear a mask for the first couple of weeks until you feel more comfortable,” Dr. Allen said.

Scientists recommended that unvaccinated workers continue to wear masks in the office. But for those who are eligible, the most effective risk reduction strategy is obvious, Dr. Allen said: “The No. 1 thing is to get vaccinated.”