Is the air moving on RTD’s buses? Yes – perhaps more than you think

We know our riders are curious about the air quality on our buses. You’re asking us – sometimes several times a week – why you can’t open the windows during a pandemic. You want to feel the air moving around you. You worry that the air you’re breathing is stagnant.

RTD has always been mindful of the air quality on our buses, whether they be the 40-foot-long local vehicles; the longer, articulated buses; or the regional models that carry riders among Denver, Boulder and Denver International Airport. Before COVID-19, operators had been asked to follow specific instructions depending upon vehicle type to ensure that fresh air was cycling through it.

RTD is now giving more thought to airflow on buses than we did before the pandemic. In June, General Superintendent of Transportation Chris Deines issued a bulletin to all bus operators asking that they open the front and rear roof hatches on all vehicles, plus the vent windows on the front half of local and articulated buses, which have windows that can open, weather-permitting. Operators have discretion to take these steps, which provide for significantly more airflow through the vehicles, being mindful of weather conditions that would affect safety or comfort inside the bus.

Deines acknowledged guidance from the Centers for Disease Control and Prevention that COVID-19 can sometimes be spread by airborne transmission, with evidence that, under certain conditions, people with COVID-19 seem to have infected others who were more than 6 feet away within enclosed spaces that lacked adequate ventilation. 

“When that (airborne transmission) was hypothesized or discovered, comments were made that small enclosed areas could be more dangerous than outside areas,” Deines said. He added that he and his colleagues asked, “Are we moving air as best as possible? And what have we learned? Based on that, we had to take into account the best airflow possible while still maintaining comfort inside the buses.” 

According to the CDC, data indicate that it is much more common for the coronavirus to spread through close contact with a person who has COVID-19 than through airborne transmission. And studies in several countries, including Spain, are showing that public transportation is not a main source of contagion and does not present a high risk of infection.

The changes enacted this summer were “the best that we could do, and it seems to be consistent with what the industry is doing as a whole,” Deines said. “Engineering has determined that this is the best way to move air around the coaches and maintain the best cooling.”

When Deines speaks about engineering, he’s referencing the work of RTD equipment engineer Nick Rorres, who studied the airflow on the three types of RTD buses previously mentioned in May, to better understand how airflow and temperature on buses are affected by having windows and roof hatches open to allow for fresh air exchange. Rorres was curious as to whether it would be better to keep windows open when possible, having reviewed studies that said viruses circulate faster when windows were closed. To conduct the testing, Rorres used a digital flow meter to measure airflow and temperature at three different positions on the bus while it traveled at 15, 30 and 50 miles per hour, with windows and hatches opened or closed as was possible, depending upon the vehicle type.

interior windows on a bus
Interior windows on a bus that can be opened for more ventilation

A side note: Rorres did not study MallRide buses, given the frequency of the doors opening and closing on those vehicles. He theorizes from this fact that they provide the greatest airflow and circulation of all RTD bus types.

From his work, Rorres noted, “It was pretty evident that having the hatches and windows open is helpful for airflow.” Hatches alone, he said, make a pretty big difference. And he learned that a lot of air moves through a bus “even at a speed of 30 mph,” he said, which would be the case with regional buses that travel often on the highway. “Sitting at head level, you get a lot of air moving through.”

Rorres concluded that opening the windows and hatches on buses also reduces the risk of virus infection on some level. He wrote: “Various studies have shown that it seems plausible that sufficient airflow can cause viruses to become diluted below the point where they can cause infections. In addition to this, moving air leads to evaporation of moisture, and studies have shown that viruses have trouble surviving in warm dry environments.”

Opening hatches and windows may leave riders uncomfortable, however, especially on Colorado’s hottest and most frigid days. Dave Ober, RTD’s general superintendent of bus maintenance, noted that buses are designed for passenger comfort, with more recirculated than fresh air. Heating, ventilation and air conditioning (HVAC) systems are set up to keep the temperature even, he said. Fresh air enters the passenger compartment when the vehicle doors open. “If we were to design buses around as much fresh air as possible,” he said, “they would be designed differently.”

With the exception of the MallRide vehicles, all of RTD’s buses use air filters with a minimum efficiency reporting value (MERV) rating of 6 or 7, which trap particles down to 3 microns in size and include dust mites, mold and pollen. RTD’s team of diesel mechanics change the air filters on the buses regularly. Both filters and fresh air remove particulates from the passenger areas, Ober said.

RTD has taken one more notable step to guard against the spread of COVID-19: It is installing ion generators on the agency’s newest 80 buses, in the ductwork above the seating area. These are lab-proven to kill viruses and other living particulates. Most of the vehicles getting this feature are going to contractors and will fan out across the agency’s service district. RTD is spending about $175,000 to add the ion generators to these buses, at $2,188 each, and will study the effectiveness of this tool, which will need to be refurbished after four years. Based on the outcome of the study, RTD’s senior leadership team and Board of Directors will need to determine whether it’s in the best interest of the agency to retrofit the remainder of the 40-foot fleet, Ober said.

“The buses are the heart and soul of this company, and RTD is always working to make the best possible decisions from a fiscal and social standpoint,” Ober said. “We’re doing everything we can.”

roof hatch on interior of bus
Hatch on the roof of a bus' interior