Why was factory work dangerous in the 1800s

Because the working conditions were terrible.

In the days of the Industrial Revolution, the workday was abusive and inhuman, as it ranged from 10, 12 or even 18 hours of work. Which meant that employees simply did not have a life outside the factory environment.

Other than that, in a pre-labor time, in the first factories the working conditions were really unhealthy, especially because of the lack of safety care, accidents were common, with mutilations of people, among other things.