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.”