All About Ventilation and Airflow on MAX and TriMet buses

Here’s some fresh air for transit riders in light of the CDC’s announcement that COVID-19 can spread via airborne transmission: There is excellent ventilation on our buses and trains.

MAX Airflow

While the windows on MAX don’t open, fresh outside air is supplied from two sources: two machines called HVAC units on the roof of every MAX car (four total per MAX train) and the air that flows in every time a train opens its doors at a station.

H to the VAC

The HVAC units suck in the air from each car, combine it with outside air in an 84%/16% blend of recycled and fresh air, pass it through a strong MERV 10 filter, and then return it to the car. While “only” 16% fresh air may not sound like a lot, it means that all the air in a MAX car is changed over every 7.5 minutes from our HVAC units alone. 

Our HVAC units are real workhorses. All the air in the MAX car passes through them (and their MERV 10 filter) about every 74 seconds. 

Finicky About Filters

Filters are rated by how small a particle they can filter out. MERV filters range from MERV 1 to 20. The higher the number, the tighter the weave of the filter, and the smaller the particle it will catch. 

A MERV 1 will trap pollen and dust mites while a MERV 20 will catch tiny particles only a few microns in size. A MERV 17 might be found in a hospital clean room or a facility that deals with radioactive materials. 

So why not just install a MERV 20 filter on transit? 

Well, the higher the MERV rating and the tighter the weave, the harder it is for air to pass. If we were to put a MERV 20 filter on MAX, our HVAC units wouldn’t be able to push air through it. They would literally freeze and break. 

Picking the right MERV filter is about finding the right balance between how much you’re filtering out and how strong an HVAC unit you need to push air through it. 

The Doors of Ventilation

The other source of fresh air coming into MAX cars is when the doors open at a stop. We crunched the numbers and found that the average amount of time between MAX stops is 1 minute and 52 seconds. (In case you’re wondering — because we were — the longest time between stops is 5:35 and the shortest is 0:30. The more you know!). 

We also looked at how long our trains spend in the station (known as “dwell time”). Across all stops, the average dwell time for MAX is 31 seconds. 

What does this all mean?

A lot of fresh air is flowing into a MAX car pretty frequently. And this is in addition to the air being sucked in by the HVAC unit.

But What About the Bus?

Buses are both similar and different than MAX. Both have HVAC units that circulate the air, but buses have windows that open, which provides a constant source of fresh air. So does the frequent opening and closing of the front and rear doors. 

We aim to keep the windows and roof vents open on buses as much as possible in order to let in more fresh air. While this was nice during the summer, it may mean buses will be a bit chilly in the winter. 

We apologize for the inconvenience these drafts may cause. We recommend you wear a warm coat and we’ll be sure to keep the heat blasting. 

Side note: One of our bus models — the 2900s — had windows that didn’t open. We took them off the road until we could replace their windows with ones that did open from our older, retired buses. They’re now back in service, full of fresh air.

There is also an HVAC unit with a MERV 8 filter on top of all our buses — we replace the filters every 6,000 miles. Unlike our MAX HVAC units, the ones on our buses don’t suck in fresh air from the outside. They only filter and recirculate the air from inside the bus. Hence the value of keeping the windows open. 

Keeping Things Fresh

Airflow matters right now. Having good ventilation can be key for preventing the spread of COVID-19 as much as disinfecting surfaces. 

Fortunately, when this pandemic hit, we were well set up to make sure riders were breathing in safe air. We just needed to make a few adjustments — keeping windows open on buses and replacing the windows that don’t open on our 2900 buses — and now we have an environment where it is difficult for COVID to spread. 

This good airflow is one of the reasons why health experts like Multnomah County’s Communicable Disease Director Kim Toevs says she’d feel safe riding transit right now. 

And that is a breath of fresh air to hear. 

Stay safe and stay strong. 

Want to learn more about what science says about the risks of traveling by transit during this pandemic? The American Public Transportation Association just published a comprehensive review of research regarding COVID-19 transmission and public transit [PDF].