Isn’t it against the law for an employer to require workers to wear face masks even when they have a medical exemption?

Maybe they should fire you instead. It's in your employment contract, they can fire you, or you can quit for any reason, without notice.

Do you have a medical exemption that prevents you from getting virus. Do you have a union, if not get one. Another and best alternative is to get vaccinated. At least that way you have better argument. 

the only way that can be decided is if it goes to court.

Under Americans with Disabilities Act, employees are entitled to reasonable accommodations.  Many states are saying that people who can't wear a mask for medical reasons are exempt.  That would indicate that they must be exempt on the job.

various factors need to be considered for the specific circumstance such as how crowded is the work place, the ventilation, are customers/clients exposes, can the environment be modified with plexiglass/other barriers....

the employee would also have to show that they REALLY have a medical issue...most excuses are just whines.  asthma does not prevent a person from wearing a mask 99.9999% of the time for example, acne or minor skin irritation that does not seriously damage the skin would not qualify..

there are legit issues.  with people with developmental disabilities who just can't understand, people with sensory processing disorder that really can't tolerate it.

I myself have legit issues.  i have a neuro issue, breathing in the hot air keeps making me dizzy (I have nearly passed out), but I can be accommodated by having a cool room, briefly removing the mask.  Because of damage to my diaphragm, it hurts to breathe with a mask on sometimes...i find I am breathing differently and my diaphragm can't handle it....resting briefly breathing cool air helps that...If i needed to wear a mask all day, i could possibly  get an oxygen concentrator with cooled oxygen.  I also get unbearable pain from ear loops, but I can tie a mask behind my neck...i have trouble being physically active with it on, i would need to be accommodate by being given tasks that don't involve walking around a lot

but right now, the law is being interpreted to mean that private businesses can insist on masks even if the state allows medical exemptions.  this is an untested area of the law.....so unless it is tested in a state level court or higher...the law is open to interpretation.

Sure but states are very specific on what counts such as mental disability, inability to remove mask, inability to talk to have the mask removed by someone if they can’t remove it themselves. 

Not in red states. they can fire you for any reason they want. 

Not in my state.