Already facing scrutiny from US lawmakers and regulators, Amazon has hit back at new research suggesting the e-commerce giant’s focus on speed and pervasive surveillance is responsible for a significant number of staff injuries and burnout.

In what could be the largest survey of Amazon workers to date, the study, released on Wednesday by the University of Illinois Chicago’s Center for Urban Economic Development, includes responses from 1,484 current Amazon employees in 451 facilities across 42 states nationwide.