[10Minutes]

Sorry, "stuffy" and "no air" is not advisable.

Your hamster needs fresh air and it will suffer without any air source. It may die. My friend's hamster died because it was left in a room just like you described. The hamster was fine for the first few days but just died within the week, on a day when the weather was slightly more hot and humid.

Would humans sit in that kind of room all day long, without any ventilation? If yes, then there should be still be some natural airflow and it is fine. If no, then you should not be doing that.

To clarify, what Souffle said is correct - If it seems OK for you means that if you feel fine sitting in that room all day long, then it's OK. If you would feel uncomfortable, then your hamster would definitely be uncomfortable.

My room is just like that (without natural ventilation), so I leave a USB fan on and a ceiling fan on for my hamster. Just ensure the breeze from the USB fan is very light and gentle, and only aimed at a small corner of the cage so that the hamster has plenty of room to go somewhere else in its cage if it feels cold.

Also, hamsters should be kept at about 65 to 75 degrees Fahrenheit (20 to 22 degrees Celsius), so having a nice chilled environment in the morning is not enough. If the weather is warm, it will heat up in the afternoon, and that's how my friend's hamster died. Please take Souffle's suggestion on the ceramic mug, as it is one of the best tips I have learnt from the amazing people here, and also find more methods to help your hamster cool down.

Really glad that you asked and are concerned about your hamster. Please help your hamster!