More than two-thirds of Amazon.com Inc. U.S. warehouse workers surveyed by researchers reported that they took unpaid time off to recover from pain or exhaustion sustained on the job.

The new national study, published October 25 by the University of Illinois Chicago’s Center for Urban Economic Development, found that 69% of workers surveyed stayed home without pay to recover, including 34% who did so three or more times.

The data suggest “injury and pain at Amazon are far more widespread” than previously known, said Beth Gutelius, research director at the center and a leading expert on logistics and warehouse work.

The report is based on a 98-question online survey that gathered responses from 1,484 warehouse workers in 451 facilities across 42 states, the researchers said. It was conducted between April and August and measured the percentage of workers who took time off during the previous month. Amazon employs hundreds of thousands of warehouse workers in the U.S.

Researchers found their subjects using ads on Meta Platforms Inc. apps, targeting people who listed Amazon as their employer or lived in areas where the company operates. The project received funding from the Ford Foundation, Oxfam America and the pro-labor nonprofit National Employment Law Project.

Amazon spokesperson Maureen Lynch Vogel said the report was “not a ‘study’ — it’s a survey done on social media, by groups with an ulterior motive.” She recommended that people read the safety data Amazon submits each year to the Occupational Safety and Health Administration, “which shows that rates in our buildings have improved significantly, and we’re slightly above the average in some areas and slightly below the average in others.”

Read more: Amazon Under Fire from OSHA Over High Worker Injury Rates

Lynch Vogel acknowledged there is work to be done but that worker safety is a top priority and that Amazon continues to invest in safety throughout its operations.

The report, the broadest academic survey of Amazon workers to date, adds to the growing scrutiny of the company’s sprawling logistics operation. Amazon is the second-largest private-sector employer behind Walmart Inc., and employs about 29% of the country’s warehousing workers, the researchers estimate. That gives the company outsize influence over the industry’s working conditions and compensation.

Critics say Amazon pushes employees to work too hard and too quickly, leading to avoidable injuries. Workplace safety regulators in Amazon’s home state of Washington allege a direct connection between employee monitoring and discipline and musculoskeletal disorders suffered by its workers. The federal Occupational Safety and Health Administration, meanwhile, has cited Amazon for exposing workers to ergonomic risks at several facilities across the country.

Amazon says the regulators’ allegations are inaccurate and is challenging them, including during weeks of hearings held recently on the Washington state citations. The company says its investments in worker safety, including a push to automate repetitive and arduous tasks, are helping reduce the injury rate.

Read more: The U.S. Retail Sector Has Had the Biggest Rise in Injury and Illness Rates

Gutelius and co-author Sanjay Pinto say 63% of workers acknowledged that Amazon has made safety a high priority. But many suffer injuries anyway, and workers who say they have trouble keeping up are more likely to be hurt on the job, according to Gutelius.

“The harder it is for a worker to maintain the pace of work, the more likely it is that they are injured,” she said.

Employees also suffered work-related mental health issues. More than half of those surveyed reported feeling burned out. The portion of workers reporting burnout increases with job tenure, the researchers say.

The researchers excluded results that didn’t take the questions seriously or appeared to feign employment with Amazon. Managers and drivers were also excluded. Responses were weighted to align demographically with Amazon’s own published statistics on the racial and gender breakdowns of its workforce.

Overall, 41% of workers reported being injured while working at an Amazon warehouse. The share rises to 51% for people who have worked at the company for more than three years.

“They are taking some steps, but tinkering around the margins isn’t going to work,” Pinto said of Amazon’s attempts to reduce injuries within its ranks. “There’s something fundamental about the system that needs to change.”