Which group was most concerned with improve the dangerous working conditions in early-20th century industry?

The Progressives.

Progressivism aimed to improve society by various reforms, and improving dangerous working conditions was a key concern during the Progressive Era (roughly the first two decades of the 20th century).  Progressives used methods like settlement houses to try to provide better social care for poor urban workers.  The labor union movement was also a part of the Progressive agenda.

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