Sublimation can be done anywhere, whether in open or constrained spaces. However, a little airflow is recommended, especially with certain substrates that give off more of an odor and vapors than others. The heat press does not give off anything noticeable other than warmth. However, the transfer papers give off some moisture release and a slight scent of the paper and ink when heated.
Fabrics can often give off a lot of steam, and materials like metals and woods can have a slight scent. Neoprene padding with polyester fabric laminated to it (like mousepads and similar materials) is by far the worst, and without proper air flow like a fan and open window, is hard to bear. An overhead hood and serious ventilation are only needed for high-production environments with low ceilings. Otherwise, just a small desk fan blowing indirectly nearby is usually all that’s needed for even the worst-case scenarios.