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.”

Nearly 3,000 workers of the 1.3 million people represented by the United Food and Commercial Workers International Union have been directly affected by the virus as of Monday — whether through infection, quarantine, hospitalizations and those awaiting test results — and 30 had died, according to the union’s research.

Grocery stores are among the remaining high-risk transmission points for the disease now that many other commercial businesses have been closed, but many workers and customers do not have masks and people can remain in close contact with one another. Workers are 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.