Emergency services were called to the Pullman Sydney Hyde Park about 9am on Wednesday following reports gases had spread through the ventilation system on the top floors, a NSW Ambulance spokesman told AAP.
A hotel spokeswoman says the incident occurred as a result of a mistake during pool maintenance.
"The dosage of the pool chemicals was incorrect leading to a gas being emitted and entering the adjacent air intake," the spokeswoman said in a statement.
Acting Superintendent Steve Vaughan says ambulance officers were told the gas was a mixture of chlorine and hydrochloric acid. Severe exposure can be quite dangerous.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"Patients complained of eye irritation and slight airway irritation, one person complained of having difficulty breathing," Supt Vaughan told reporters.
Six patients, including the man who had been mixing the chemicals, were taken to St Vincent's Hospital in a stable condition, NSW Ambulance said.
Supt Vaughan said paramedics treated Wednesday's incident like a mass-casualty event.
"Initially the number of patients outweighed the resources we had," he said.
"Our paramedics have employed our triage sieve and sort process which identifies the emergency urgency. Patients were assessed multiple times and none of them required any physical treatment."
NSW Police confirmed "there doesn't appear to be any criminal element to the incident". They assisted with traffic diversions on Wednesday morning.
