A year ago, Metro implemented a layered safety approach with an emphasis on improving air circulation and quality for our 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 personal air filters – masks! – aboard. That layered safety approach continues today, and as of March 2021, all coaches have received new, even more stringent air filters (minimum of MERV-9), the highest level possible while still allowing air to circulate.

In addition to the upgraded air circulation systems, 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. (For example, express buses and buses on freeways generally open their doors less than a route that travels on city streets.) Open windows and hatches, on the other hand, provide more consistent intake estimates regardless of route.

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.

Unsurprisingly, 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% or more. Opening the rear hatch towards the front can nearly double the fresh air intake.

To increase the flow of fresh air, 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% of our coaches have fixed windows. Out of 1,476 active coaches in our fleet and those we operate for Sound Transit, 1,278 (86%) have windows that open.

Between newly upgraded HVAC systems, requiring and providing masks, and ensuring fresh air intake through windows and doors, 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 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

As we update and continuously improve our practices, we post updates to this Metro Matters blog and to our Healthier Metro webpage.