How is the air cleaned in Union Station’s bus concourse? You’ll be amazed

In February 2016, it finally happened for diehard Denver Broncos fans: The team defeated the Carolina Panthers to win Super Bowl 50, after many close attempts to get there since the prior win 17 years before. Two days later, an estimated 1 million people converged in downtown Denver for a victory parade, an epic orange party in Broncos Country.

Many fans took public transit to and from the celebration, which had them wading through the relatively new bus concourse at Union Station. Never before had so many people squeezed into the nearly 1,000-foot-long facility – to that point, thousands of RTD customers moved through the space each day to catch or exit buses, and many more dashed through to reach rail platforms. But there had not been such volume at once.

Union Station

Robin McIntosh, RTD’s senior manager of facilities, was there to see it. “It was amazing,” he said. “I walked through the crowd, and I was like, I can’t believe this.”

That was also the thought of facilities maintenance supervisor Clarence Pauls, who joined RTD when Union Station was under construction and knows the air quality and infrastructure of the facility better than anyone. That winter day, he observed a line of about 200 people snaking through the concourse for a single bus, with another 1,000 people on the move.

Clarence Pauls, Facilities Maintenance Supervisor

A concentration of people exhaling carbon dioxide (CO2) is not a big issue for those occupying a space for a short time, but a high level of CO2 – greater than 5,000 parts per million (ppm) – can lead to side effects such as a higher heart rate, headaches and drowsiness. It’s not clear what the CO2 level was the day of the Super Bowl celebration, but no such symptoms were reported or noted during that time. A typical level of CO2 in an occupied space with good air exchange ranges from 350 to 1,000 ppm. Inside the Union Station bus concourse, it was about 300 ppm during a pre-COVID-19 morning or afternoon rush. When Pauls checked the CO2 level on a recent Friday with little foot traffic, it was 56 ppm.

During the pandemic, some of you have wondered about the quality of the air moving through the bus concourse because the facility is underground. Pauls notes that RTD has received no complaints about air quality in the space since it opened in 2014. Is it safe to be there? “I haven’t missed a day yet,” Pauls joked, “so evidently there’s something working.”

A patron of this space might not give too much thought to how it functions, Pauls acknowledges. But if you could look behind the locked doors of this massive facility to glimpse it as he does, you would be astounded by the scale, the size, the sound. More than 80 vents pull air into a wind-tunnel-like plenum that runs the length of the facility, extending underground from the entrance to the Crawford Hotel to the light rail tracks. From there, three huge blue fans – each 200-horsepower, pushing 150,000 cubic feet per minute (cfm) of air – pull bus exhaust up and out of an air exhaust vent tube near Chestnut Pavilion, one of five entrances to the concourse. A typical bathroom exhaust fan, for comparison, runs at 75 cfm.

We’ll bet you’ve noticed the trio of vent tubes near the light rail platform. The largest is for air exhaust from the fans, the smallest is for mechanical exhaust from plumbing and bathroom ventilation, and the third is an intake tube to bring fresh air inside. Several studies were conducted to determine the correct placement for these tubes, with safety, security and wind patterns factoring into the finished design. They are spaced apart and lean away from one another, Pauls said, “so the function of one doesn’t affect the other – you don’t want the exhaust coming out and going right back in.”

Union Station ventilation tubes

The air pumped into the passenger concourse is all fresh, brought in through the screened-in intake vent. Inside a deafening mechanical room, the outside air is conditioned with three increasingly finer levels of filtration before RTD patrons breathe it. Air ultimately passes through pleated filters carrying a minimum efficiency reporting value (MERV) rating of 13, trapping particles between .3 and 1 micron in size. Such material includes various types of dust, bacteria, tobacco smoke, auto fumes and sneeze nuclei.

Why is the air filtered that much? “To guarantee that there’s nothing coming in here,” Pauls said.

The facility’s heating, ventilation and air conditioning elements are controlled by a building management system that monitors for pollutants and levels of positive and negative pressure, the latter keeping air moving outside or inside, respectively. And the system doesn’t just provide ventilation for the concourse – it also manages smoke. In the event of a vehicle fire, for example, heat detection would lead the smoke to be pulled away from the area, through vents toward the plenum, reaching the large fans. Each fan was stress-tested at the manufacturing facility in Tennessee to run at more than 500 degrees for over an hour.

Before the advent of COVID-19, a bus pulled away from the 22-bay facility every 48 seconds during morning and afternoon rush periods. Now, there’s significantly less vehicle and foot traffic as the pandemic continues. Regardless, RTD’s approach to airflow in this space remains the same, with the only change made to infrastructure being the operating speed of the fans, to provide a higher exchange of the outside air coming in and increase the level of air moving out.

Day in, day out, Pauls will continue to keep an eye on the function of this space. Don’t be surprised if you see him walking through the concourse; he’s here all the time.

“Union Station is one of the main facilities I have to pay attention to,” Pauls said. He’s not exaggerating much when he added, “I basically live here.”

Union Station's bus concourse