Newly upgraded air filters and more fresh air now on all King County Metro buses

King County Metro has upgraded all its coaches with new, more stringent air filters that have a minimum of MERV-9 rating, the highest level possible for the bus vehicles while still allowing air to circulate. 

A year ago, King County Metro implemented a layered safety approach with an emphasis on improving air circulation and quality for drivers and riders. Enhancements included upgraded MERV-8 air filters in the HVAC system, a viricidal coil cleaner positioned above the filters that was approved by the EPA for use against novel coronavirus and later requiring and providing masks aboard. 

In addition to the upgraded air circulation systems, King County Metro coaches also have other features that support safe air flow: doors, roof hatches and windows. Every time a coach’s doors are opened to service a bus zone, there is a frequent, substantial fresh air exchange. Each route’s door opening frequency varies, so estimated fresh air intake from doors can vary. Open windows and hatches, on the other hand, provide more consistent intake estimates regardless of route. 

King County Metro’s Vehicle Maintenance Division sought to analyze various open window configurations to better understand which small changes on buses generally had the greatest impact. Using CO2 as a tracer gas, the team established a steady state of air flow, then began making changes by opening windows or hatches or altering settings, such as setting the defrost to “Fresh.” As the concentration of CO2 changed in response to the different scenarios, they recorded the concentration and estimated air movement from graphs of the changes. 

Opening all windows has a marked impact on air flow, more than tripling fresh air intake. However, even opening a single operator window can increase fresh air intake by 10 percent or more. Opening the rear hatch towards the front can nearly double the fresh air intake. 

To increase the flow of fresh air, King County Metro’s policy is for coaches to operate with windows open whenever weather allows. Operators and riders can choose which windows to open or close as conditions change. 

Some customers have noted that not all coaches have other windows that open, which is true – about 14 percent of coaches have fixed windows. Out of 1,476 active coaches in the fleet and those operated for Sound Transit, 1,278 (86 percent) have windows that open. 

Between newly upgraded HVAC systems, requiring and providing masks, and ensuring fresh air intake through windows and doors, King County Metro ensures healthy air circulation and air quality across varying weather conditions, coach types and routes. These steps work in concert with other measures such as implementing reduced passenger limits, supporting physical distancing by closing certain seats and rows, encouraging contact-less payment, installing safety partitions between customers and the driver, and daily disinfecting and enhanced cleaning.