What about more esoteric or loosely-defined harm? Is a work environment which is so toxic to its employees that it generates acid reflux, migraine, panic attacks, or heart attacks "unsafe" for its users? What about in cases where employees are covered under the disabilities act? Is an employee who is psychotic or clinically depressed being reasonable in asking for an environment that does not engender psychosis or depressive episodes? Is it something they would have legal protection from? These may seem like edge cases, but what about an environment in which a conflict between fluorescent lights and LCD displays caused epileptic seizures? Surely a line has to be drawn somewhere, and it would be a damn shame if employers got to decide where that line was drawn.
Epilepsy, psychosis, and unipolar depression are all covered under ADA, and of course asbestos and radiation are covered by OSHA (and others). We even have coverage for repetitive stress injury. Epilepsy is of course covered by ADA.
It would seem to me that most employers would respond that they simply don't hire psychotic, bipolar, depressed, schizophrenic &c employees, but the fact of the matter is they're actually required to, because discriminating against candidates for a position on the basis of their mental health or abilities is prevented under EEOC. The most relevant text from ADA seems to be:
Neither the statute nor EEOC regulations list all diseases or conditions that make up "physical or mental impairments," because it would be impossible to provide a comprehensive list, given the variety of possible impairments.
So, your employer might spend a couple thousand dollars on getting you appropriate equipment for your IT job (a proper chair, display, keyboard, mouse, and desk). They would put in a wheelchair-accessible ramp, bathroom facilities, and so on. They'd provide "handicap parking." They'd ensure that the boiler is not exhaling carbon monoxide into staff areas. But would they respond to a request to educate their management on providing an environment for their employees that does not create psychiatric issues (and really, where else could such problems be mitigated if not management)? What if you told them that a year of psychiatric care would cost ten thousand dollars or more, per employee? Mental impairments are not defined (and in fact are explicitly undefined) under ADA as "physical," and are defined instead by the physician/clinician covering the condition.
It's never going to be a pleasant conversation when an employee has to say to an employer, I have a mental impairment and I need you to provide reasonable accommodation, but they are entirely reasonable, and legally covered for, asking.