Christopher Herrmann (David Eigenberg) is about as reliable as it gets when it comes to Chicago Fire characters. He can always be counted on to do an exemplary job in the field, even when he's dealing with personal issues like his wife's health.
Unfortunately, Herrmann's health is leading to issues on the job. The character had a brush with death in the season 12 premiere, when he disposed of an explosive device left at the firehouse and ended up caught in the blast.
Herrmann is suffering from hearing loss in season 12
There were hints of hints of a problem at the end of the episode, but Herrmann's health struggles were made crystal clear in episode 2. Herrmann has suffered from a ringing in his ears ever since the explosion, and has had difficulty hearing people around him.
The first person to notice it in episode 2 was, of course, his buddy Mouch (Christian Stolte). He snuck up on Herrmann and the firefighter didn't even hear him. Ritter (Daniel Kyri) eventually put two and two together, and suggested that Herrmann go see a doctor as soon as possible.
The ringing in Herrmann's ears, and the inability to hear clearly, lines up with an ailment called NIHL, or Noise Induced Hearing Loss. According to the National Institute of Health, NIHL is caused by:
""Extremely loud bursts of sound, such as gunshots or explosions, which can rupture the eardrum or damage the bones in the middle ear. Loud noise exposure can also cause tinnitus—a ringing, buzzing, or roaring in the ears or head.""
Herrmann refuses to get help for his ailment
Herrmann was definitely exposed to a loud noise, and the longer he waits, the more he puts himself at risk for prolonged damage. NIHL, if not properly treated, can lead to permanent side effects, which would effectively end Herrmann's career and seriously compromise his personal life.
The problem is, Herrmann doesn't want to go to the hospital. He told Ritter that he doesn't want his family and co-workers to know, but the episode made it clear the problem is going to linger for as long as the character decides to leave it untreated.