Setting on Earth, sometime in the near future (50/100 years).
A large sealed pressurized environment is set up in an even larger industrial complex.
Workers get in and out of the environment for their work shift using protections suit and, if necessary for prolonged (day after day, year after year) exposure, breathing equipment (only if necessary in the atmosphere you are going to provide). Imagine an average 6 hours shift.
Robots are not allowed for plot reasons.
Suitable ports are setup for access to the contained environment to avoid dispersion and provide necessary decontamination (IF necessary by your answer).
The character of the story, let's call him Hans, for plot reason must enter the environment without protection suit or breathing equipment. He will stay inside for a relatively short time from 15 minutes to a bit less than 6 hours. The upper limit is determined by your answer. If the atmosphere you will provide would likely cause dizziness after 30 minutes then he will leave after 30 minutes. He is not forced by others, he enters voluntarily but in a hurry and exits undetected once satisfied.
It is fine if he is going to have some adverse effect. Also some permanent damage is acceptable (poor Hans may differ but he is not the one writing the plot).
But the requirement is that he should be able to survive and retain consciousness throughout. He should also be able to hear other people talking (if close enough).
The environment size is not fixed and you can change it if you find it necessary. Am imagining a large room 30x10x10 meters with several controls along the walls, tubes running through, expansion vases, etc. Could be bigger if necessary.
The environment itself can be adapted as necessary. For instance we can assume there are vents to mix the atmosphere and not allow the heavier components to settle on the lower part of the environment.
Pressure, temperature and humidity can be adjusted as needed to fulfill the requirement.
Hans is just a normal human of average size, fitness and training. He can hold his breath for about a minute if not under exertion.
Hans will not do heavy physical activity while inside. He will mainly walk around, hide and listen.
Hans is determined and will endure suffering. He is no superman though.
Cost for the production of the necessary component gases is not a concern. The owning company is sure will get much more in return.
Ideal side effects:
Ideally the atmosphere will reduce visibility a lot. It is fine to add water vapor if necessary. Workers may navigate the place through special equipment (HUD display or whatever).
Hans should hear sounds and voices. They will be altered by the density of the atmosphere. But words should still be discernable (if Hans is close enough).
Would be a plus if the atmosphere can somehow be connected to an existing industrial process (like Sulfur hexafluoride is in the electrical power industry)
Question: what is the densest breathable atmosphere composition that would allow survival for a limited (from 15 minutes to almost 6 hours) time?
I have found this answer: Dense atmosphere and survival but it is referred to a planet while my question pertains to an artificial, enclosed atmosphere to be survived only for a short time.
Also researched Sulfur hexafluoride and Xenon. On the latter article I found:
"Dense gases such as xenon and sulfur hexafluoride can be breathed safely when mixed with at least 20% oxygen. Xenon at 80% concentration along with 20% oxygen rapidly produces the unconsciousness of general anesthesia".
While they provide a reference it is not clear if it is about the claim about the percentage levels or the purging from the lungs of heavy gasses. I can't access the referenced article in Chest. So I can't accept the 20% claim of the article.
Edit: as by Andrew comment, to allow short decompression times a maximum of 4 atmospheres should be considered. Hans cant stay hanging around in decompression for hours.