Lisa Harris, a cashier at a Kroger grocery store in Virginia, was surprised last week when a customer offered her a $ 5 bill as a tip. Ms. Harris, a Kroger employee for 13 years, cited store policy in declining the generous offer.

The woman was clearly disappointed that she could not do more.

“She looked at me and said, ‘I just want to show you how grateful I am for what you do,’” Ms. Harris said.

The coronavirus pandemic has cultivated a growing appreciation for people who work in grocery stores, pharmacies and meat plants. Laborers who were once considered unskilled are now “essential employees,” even heroes to some, because they are providing the nation with food and other crucial supplies at a time when the risk of infection is acute.

How employers and public health officials protect these workers has become a critical issue during the outbreak. Some workers’ rights advocates fear that safety standards are eroding at a time when they should be strengthened.

Last week, the Centers for Disease Control and Prevention issued new guidelines saying it is OK for essential employees to keep working after potential exposure to Covid-19 if certain conditions are met. Previously, those workers were told to quarantine at home for two weeks. Some say the new recommendations will put already vulnerable workers at even greater risk.

“I think the C.D.C. guidelines that they put out the other day are soft,” said Marc Perrone, the president of the United Food and Commercial Workers International Union, which represents 1.3 million workers. “I think they are trying to keep the workers inside the plant.”

According to the union’s research, nearly 3,000 U.F.C.W. workers had been directly affected by the virus as of Monday — whether through infection, quarantine, hospitalizations and those awaiting test results — and 30 had died. Workers at other companies like Whole Foods, Amazon and Instacart have protested what they consider dangerous conditions and insufficient pay.

In March, Walmart announced that two employees of one of its stores in Illinois had died from the coronavirus. The family of one of them, Wando Evans, has filed a wrongful-death lawsuit, claiming the store was negligent by not providing employees with adequate protection.

A representative for the C.D.C. did not respond to a request for comment.

The new C.D.C. guidelines state that essential workers who may have been exposed to the virus may continue to work provided they are asymptomatic, wear a mask at all times for 14 days after their last exposure and have their temperature taken before entering the workplace.

Workers who may have been exposed to the virus must follow C.D.C. guidance on social distancing, remaining at least six feet from co-workers and potential customers. If they show symptoms, they should be sent home immediately and all surfaces at the workplace should be cleaned and disinfected, according to the guidelines. In addition, anyone who came within six feet of an employee with potential exposure should be notified and considered to have also been exposed.

Labor advocates like Marcy Goldstein-Gelb, the co-executive director of the National Council for Occupational Safety and Health, say the new guidelines may encourage employers to pressure workers to return to their jobs too soon, often without adequate protection or pay.

“It’s a complete reversal of the policy that the C.D.C. has for the public,” Ms. Goldstein-Gelb said. “It disregards the fact that, right now, workers are dying every day needlessly in unconscionable numbers.”

Grocery stores are among the remaining high-risk transmission points for the disease now that many other commercial businesses have been closed. The employees are in regular contact with customers at checkout, behind counters and as they stock shelves along the aisles. Larger stores can accommodate from 800 to 1,200 people a day, and served as many as 10,000 a day just before the nation shut down in March, when panicky customers feared shortages.

Mr. Perrone said he visited eight stores in Northern Virginia last week and was “appalled” by the hazards he saw, including many workers and customers without masks and people in close contact with one another. He said, at the very least, anyone entering a store should wear a mask. The C.D.C. recommended in early April that everyone wear cloth face coverings in stores, but the policy is voluntary.

Workers are also imploring customers to take more care while in stores. They say many have been throwing used gloves and wipes in carts and on floors for employees to pick up. Many customers are still browsing with their hands and not their eyes and blaming workers for lack of goods on shelves.

“The fear that we feel is absolutely real,” said Gregg Finch, 44, a Stop & Shop produce clerk in New York, who added that customers must help in the effort to “keep everyone in all of our stores safe.”

New Jersey recently mandated that all customers must wear masks to enter a store and Gov. Andrew M. Cuomo of New York ordered employers to provide masks for all of their workers.

Ms. Harris, the Kroger cashier and a U.F.C.W. member, said her store is trying to maintain safety by making modifications. Like many stores across the country, it put up plastic barriers to separate customers from cashiers and placed markings on floors to urge six-foot distancing. Her Kroger is also closing two hours early every day to conduct comprehensive cleaning.

She said she interacts with roughly 300 customers per day, but neither she nor any of her co-workers have been provided with masks yet. A local charity has promised to give them homemade versions soon. No one at Ms. Harris’s store has become ill, she said, but two employees at nearby stores tested positive. She added that her boyfriend delivers food from local restaurants, and they both feel a sense of inevitability.

“Chances are high that I will get this thing,” Ms. Harris, 32, said in a telephone interview from her home. “It’s getting closer, and it’s terrifying.”

In an email, a spokesperson for Kroger said that the company had taken steps to promote safety, including a limit on the number of shoppers allowed in stores at one time, the installation of plexiglass partitions at checkout lanes and enhanced daily sanitation practices, among other measures.

“We continue to take additional actions across our family of companies to protect our associates and customers,” the statement said.

The U.F.C.W., in addition to about 900,000 grocery store workers, represents roughly 250,000 meat plant workers, including those at a JBS plant in Greeley, Colo., where last week 30 workers tested positive for the virus. In Sioux Falls, S.D., a Smithfield pork processing plant that supplies tens of millions of servings of food per week closed indefinitely on Sunday after dozens of workers were found to have Covid-19, the disease caused by the virus.

Mr. Perrone is especially worried about the meat packing plants under the new C.D.C. guidelines. Many are in rural areas, some with 2,000 to 4,000 employees working side-by-side in chilly conditions. Because of the remote locations, a major outbreak at one of the plants could overwhelm the nearest hospitals.

Just 62 miles to the east of the Sioux Falls plant, Smithfield operates another plant in Worthington, Minn., a state with several meat packing plants. Dr. Kris Ehresmann, the director of infectious disease epidemiology and prevention for the Minnesota Department of Health, said she and her staff are closely monitoring all of the state’s meat processing plants.

She said that when the new C.D.C. guidelines were issued last week, her office issued stricter recommendations about when workers should return following exposure to the virus — in particular, only after a 14-day isolation. To return before that, she said, the worker must be considered so essential to a critically important job that his or her absence would create a “crisis situation.”

In those cases, she said, employers should contact the health department to determine the safest possible path.

“We look to C.D.C. as a lead and we have decided in Minnesota that we want to take a bit more of a conservative approach to this,” Dr. Ehresmann said, “and we have amended those guidelines in the ways that we think are better for our state.”

Ms. Goldstein-Gelb said it was time to re-examine the term “essential employee,” as well as to improve safety and pay for workers who meet the definition. Ms. Harris said her Kroger gave all employees a $ 300 bonus and added $ 2 per hour in “hero pay,” which she said was insufficient.

“We were heroes before this, and we will be after it,” she said. “Our customers are starting to realize that.”