Amazon.com Inc.’s move toward one-day free shipping may be good news for Prime subscribers, but not necessarily for those who work in Amazon’s warehouses.

The Retail, Wholesale and Department Store Union, a longtime critic of Amazon’s workplace practices and a backer of unionization at Amazon warehouses, said in a statement Friday that it worries the increased workload could be bad for fulfillment-center employees.

“With two-day Prime shipping, Amazon fulfillment workers currently face speeds of 200-300 orders per hour in 12-hour shifts. They struggle already to maintain that pace,” RWDSU President Stuart Appelbaum said. “If Amazon plans to effectively double the speed, it must also address existing workforce needs and ensure its workers are safe. Increasing fulfillment speeds means they need to hire more workers, under more sustainable speeds that don’t put workers’ lives in jeopardy.”

Amazon dismissed the concerns in a statement of its own Friday, calling them “misguided and self-serving.” Dave Clark, senior vice president of Amazon Worldwide Operations, said the company has used 20 years of business experience to build “a positive, safe environment in our facilities.”

“This enables Amazon to deliver orders faster and more efficiently — not by working harder but by working smarter based on decades of process improvement and innovation,” he said in the statement.

Productivity at Amazon’s fulfillment centers were highlighted in a separate report last week, as The Verge reported Amazon workers are tracked by automated systems that rate each employee’s productivity, and the software reportedly recommends warnings or firings if they don’t handle enough packages. The Verge said between 2017 and 2018, about 300 workers were fired for inefficiency reasons from a Baltimore warehouse that employs about 2,500 people; extrapolating that rate suggests about 10% of Amazon’s 125,000 warehouse workers in the U.S. lose their jobs every year for not being productive enough, according to the report.

Amazon announced the shift toward free one-day shipping last week in its quarterly earnings report, in which is said operating profit doubled, for a fourth straight quarter of record earnings. The company said it expects to spend about $800 million this quarter to shorten the Prime delivery period.