Costco will start requiring customers to wear masks Monday

Costco is requiring customers to wear facemasks or coverings in its stores beginning May 4.

The company is the largest US retailer to enact the rule, which is aimed at helping prevent the spread of Covid-19. Costco said the face coverings must be worn "at all times" in the store and will not serve as a "substitute for social distancing."

Certain exemptions are applied, including for customers under 2-years-old and people whocan't wear one because of a medical condition. Costco announced the changes Wednesday on its website.

Costco has enacted a number of changes to its stores as shoppers flock to big box stores for necessities during the pandemic. Other changes include earlier closing times, limiting the number of guests a member can bring and giving first responders and healthcare workers priority admittance into warehouses.

Walmart said in a statement to CNN Business that customer and employee safety is a "top priority" and it's recommending, not requiring, customers wear face coverings. In March, the retailer began taking workers' temperatures at stores and warehouses before their shifts and also made medical masks and gloves available for employees who want to wear them.

Several, but not all, US states are requiring shoppers to wear masks when they visit essential businesses. Earlier in April, the Centers for Disease Control and Prevention recommended people wear face coverings in public, but it's not yet required.