Lyddie is full of good examples of the dangerous working conditions that Lyddie and her fellow workers are subjected to. Readers experience right away with Lyddie the ear-splitting loud volumes that come from the factory floor. We learn early on that things like tuberculosis can quickly work its way through the girls on the floor because they all put their mouths on the shuttle in order to suck the weft thread through it. The factory air is filled with microscopic fibers that can be inhaled and cause lung damage, and impact damage to the girls is always a possibility because they are working with heavy machinery that has a lot of moving parts. All of those dangers are physical dangers, and the longer a person is on the floor, the greater the risk. It's a numbers game. The more repetitions a person has, the more likely they are to get injured during one of the motions. Yes, repetition helps build muscle memory to avoid getting hurt, but working incredibly long hours eventually fatigues a person both mentally and physically.
Lyddie and the other girls show signs of both mental and physical fatigue in addition to showing signs of stress from working long, hard hours. Chapter 16 is a good place to find textual evidence for how the long hours both physically and mentally impact Lyddie in unhealthy ways. In general, the long hours mean a lack of adequate rest, and a lack of adequate rest robs the body from being able to better manage stress loads. Sleep and exercise are both good ways to deal with stress, but Lyddie doesn't have time for either activity. Consequently, she is carrying high stress loads, and we see those stress loads start to impact her treatment of Brigid. Lyddie will snap at Brigid for being so slow in chapter 16. This kind of thing is a typical sign and symptom of stress. Later in the chapter, we see Lyddie so tired that she doesn't realize that Mr. Marsden is making a sexual pass at her.
How hot the room seemed. Of course it was always hot and steamy, but somehow...Perhaps if she hadn't been burning up she could have kept her head, but she was so hot, so exhausted that Thursday in May, she wasn't prepared, she had no defenses.
Lyddie does get herself out of the situation and barely drags herself home, only to collapse in a fever. Her immune system is down in function due to lack of sleep and high stress levels. Either of those things can lower immune efficiency, and Lyddie is experiencing both due to the long hours at the factory.