Metro is committed to eliminating the spread of COVID-19 and protecting our employees and customers. In recent weeks, we’ve strengthened our cleaning protocols, boarding practices, and social distancing efforts. But there’s one improvement Metro began making two years ago that also helps us respond to the current health crisis: our enhanced air filtration system.
The Heating, Ventilation, and Air Conditioning (HVAC) filters used in Metro’s coaches are what’s known as a “MERV 8” classification. This means the system successfully traps COVID-19, including in droplet form such as from a sneeze or cough, preventing the virus from moving throughout the HVAC system. Previously, Metro had used a “MERV 4” classification of filter, which allowed more particulates to flow through.
The inside air system, when running at its lowest settings, was designed to filter air at a rate of 15 cubic feet per minute per passenger, according to contract build specifications. This means that air is constantly flowing over the filters and trapping particulates. The interior ventilation system’s filter is capable of addressing particulate matter all the way down to 3-10 microns, such as dust or pollen, and includes antimicrobial treatment. Filters are replaced every 6,000 miles, which is typically about once a month.
As a second line of defense, our coil cleaner, which sanitizes our air conditioning units, has additional viricidal protections that deactivate or destroy viruses. Our coil cleaner is approved by the EPA for use against SARS-CoV-2, the cause of COVID-19, and is applied as a part of our regular monthly maintenance process.
With these two elements of protection, Metro’s filtering system and HVAC maintenance eliminate the possibility of COVID-19 being transmitted through the general air and being breathed in on the coach.
It’s important to note that no air filtration system (or even air) can eliminate the chances of coming into close contact with coughs or sneezes. Coughing and sneezing into one’s elbow will go a long way in minimizing droplet spread. Additionally, help keep yourself and others safe by:
- Staying home if you are sick,
- Only using transit for essential trips,
- Not touching your face,
- Frequently washing your hands, and
- Keeping as much space as possible between you and others.
Could Metro swap HEPA filters for the existing HVAC filters?
As referenced above, the Minimum Efficiency Reporting Value (MERV) tells you the efficiency of an air filter. The higher the MERV number, the more restrictive it is, and the less air circulation you have.
High Efficiency Particulate Air (HEPA) filters are a minimum MERV 17 and able to trap particles that are 0.3 microns. It also restricts a lot of air from circulating.
Our current coaches were not designed to accommodate a HEPA filter, but the MERV 8 filters address particulate matter down to 3-10 microns, which includes dust, pollen, and droplets. This rating is the most restrictive our coaches will allow while maintaining the minimum airflow circulation standards required by the manufacturer.
Would Metro consider disabling the HVAC system as an extra precaution?
The HVAC system is an integral and necessary part of the overall vehicle and needs to be functioning as designed to maintain an acceptable interior climate. The system is also part of the cooling system of the Hybrid Drive system on some of our fleets. Disabling the vehicle HVAC systems is not an option, but also would not be required for any reason Metro can ascertain.
How does this differ from the filtration system in a personal vehicle?
The filtration systems within personal vehicles vary by manufacturer. They are also designed to meet air flow requirements for far less people who activate the system far less when compared to a bus. The systems on our buses are industrial and don’t have much similarity to light duty passenger vehicles beyond the basic fundamentals of how they operate.
We continue to look to the expertise and guidance of Public Health – Seattle and King County as we operate during this unprecedented crisis, and nothing is more important to us than your safety.
- King County Metro Transit
- Public Health
- Other King County services
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“With these two elements of protection, Metro’s filtering system and HVAC maintenance eliminate the possibility of COVID-19 being transmitted through the general air and being breathed in on the coach.”
“Eliminate” implies 100%, which is contradicted in the next statement:
“It’s important to note that no air filtration system (or even air) can eliminate the chances of coming into close contact with coughs or sneezes.”
Even if someone coughed or sneezed directly into the filter medium, MERV 8 filters have 90% efficiency on particles that are 3 to 10 micrometers in size – meaning, the larger virus-containing particles. A study of people with influenza found that 39% of people exhaled infectious aerosols. As long as we are sharing an airspace with someone else, breathing in the air that they exhale, airborne transmission is possible.
Aerosols are also more likely to be produced by talking and breathing, which might even constitute a bigger risk than sneezing and coughing. When someone’s coughing, they turn away, and when they’re sneezing, they turn away. That’s not the case when we talk and breathe, which is the far more likely method of infection in an enclosed space in close proximity to others.
While I appreciate that Metro is taking steps to protect passengers, this blog post is misleading, and may even create a false sense of security.
These filters provide a substantial reduction in risk. Zero risk does not, and never has, existed. Taking an absolutist perspective is futile. The 2nd pass through an 80% filter, provides 96% efficiency, the 3rd pass, 99% – sounds pretty good to me – and that doesn’t take into account the dilution from ambient air from window, doors…. The filtration efficiency improves above the below the stated particle size (specs are worst case) – another benefit. This is a pretty good filter system – superior to office building standards and schools. Impressive.
Some bus driver don’t cover their coughs and don’t wear mask
This is good to know and I’m pleased to hear it. Problem is that too many drivers decline to operate the ventilation system, claiming that they can only operate the system in an all-heating mode or an all-cooling mode. Is that true or do some drivers need instruction on how to operate the system? Isn’t there some mode of operation that maintains ventilation while cycling heating or cooling on and off as necessary for comfort? Pet peeve.
Hi Jim, Thank you for letting us know and giving us the opportunity to address this with our team. It’s not true that our systems operate in all heat or all A/C modes. Our HVAC systems are automatic climate control systems designed to maintain interior coach temperatures between 68-72 degrees. Operators have limited control of the them — essentially the ability to switch it on/off and adjust the set point between the 68-72 degree range. Everything else in the way of function is automatic and controlled by the computer. The caveat to that is there are still some legacy coaches (less than 7%) in the process of being phased out that offer a little more manual adjustment capabilities, but the vast majority of our equipment is as described.
*fewer. Not less.
“They are also designed to meet air flow requirements for far less people who activate the system far less when compared to a bus.”
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