Amazon warehouse workers are suffering physical injuries and mental stress on the job as a result of the company’s extreme focus on speed and pervasive surveillance, according to a new study.

The study, released Wednesday by the University of Illinois Chicago’s Center for Urban Economic Development, includes responses from 1,484 current Amazon workers across 42 states and 451 facilities, in what the authors are calling the largest nationwide survey of Amazon workers to date.

Nearly 70% of Amazon employees who participated in the survey said they’ve had to take unpaid time off due to pain or exhaustion suffered on the job in the past month, while 34% have had to do so three or more times. The most common injury reported by workers was sprains, strains or tears, and nearly half of respondents said they had moderate or severe pain in the leg, knee or foot in the last three months on the job. More than half of workers said they’re burned out from their work at the company, and that response rate intensified the longer the employee had worked at Amazon.

The data adds to a drumbeat of scrutiny around Amazon’s workplace safety and treatment of warehouse employees. Regulators, lawmakers, rights groups and employees have criticized Amazon — which is the second-largest employer in the U.S., behind Walmart — over its labor record. The researchers estimate Amazon is the largest warehouse employer in the country, accounting for an estimated 29% of workers in the industry.

Amazon had roughly 1.46 million employees globally, as of the quarter ended June 30, and the majority are warehouse and delivery workers.

The Occupational Safety and Health Administration and the U.S. Attorney’s Office are investigating conditions at several warehouses, while the U.S. Department of Justice is examining whether Amazon underreports injuries. In June, a Senate committee led by Sen. Bernie Sanders, I-Vt., also launched a probe into Amazon’s warehouse safety.

Amazon has said it has made progress on lowering injury rates and that the company has made adjustments to working environments in order to reduce strain and repetitive movements. It has begun to automate some tasks and is also rolling out more robotic systems in warehouse facilities that the company claims can improve safety, although that prospect has been debated.

About 64% of workers who participated in the survey said they feel the safety of workers is a high priority at Amazon, but that sentiment is lower among those who reported negative impacts to their physical health from the job.

The survey was funded by Oxfam America, and advocacy groups the Ford Foundation and the National Employment Law Project.

Amazon spokesperson Maureen Lynch Vogel disputed the study findings in a statement and said “there’s nothing more important” than employees’ health and safety.

“This is not a ‘study’ – it’s a survey done on social media, by groups with an ulterior motive,” Vogel said. “If anyone actually wants to know the facts, they can read the data that we publish each year and submit to OSHA, 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.”

Amazon said musculoskeletal disorders, or problems like sprains and strains are the most common type of workplace injury across all industries, adding that employees get adequate breaks and that the company provides mental health resources for staffers. Amazon also said it informs managers that productivity or speed shouldn’t be pressed at the expense of worker safety.

Workers have argued that the time off task policy makes working conditions more strenuous and that it’s used as a tool to surveil workers. Amazon in 2021 adjusted its time off task policy so that it averages data over a longer period.

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