Gyms are rarely purpose built facilities. Rather, theyre rented and renovated spaces. In turn, airflow in the structure is planned for light industrial or office space. Almost no thought goes into the fact that gyms are sometimes hundreds of occupants moving the air at two to three times what the OSHA or planning documents indicate. Paints with VoC's, sealants, and even offgassing plastics from gym mats or new equipment can turn the air quality from decent to garbage in a few hours.
I was once contracted to fix an air handler issue at a fitness center. The root cause was a set of 6 un-ventilated panini presses that were placed near the front desk as part of an effort to sell snacks and sandwiches. The added smoke and particulate had decreased the filter life and burned out a blower motor. The solution was either get rid of the electric grills, or start replacing 30 day filters every week.