Ventilation Measures

Short Description

Through conversation with each site, MOT site walks, and the previous spreadsheet data provided, we've identified any and every classroom that will have students starting on Monday 10/12. This includes the SpEd rooms that are already back. Our focus has been on classrooms of a population of 10-15. Right at school, support spaces, overflow rooms, or specialist rooms are not included. The plan is the every occupied classroom will have the existing ventilation certified or have a portable filter installed by the time students return to that room. Every occupied classroom will also have a sign posted indicating it meets the mechanical ventilation requirements or a portable filter was provided.

Long Description

Here is the steps we've done and criteria we established to ensure the healthiest environment possible:

  1. There are essentially two components of the HVAC system: Fresh air (ventilation) and recirculated, filtered air (filtration). The current guidelines are to increase outside air to dilute the interior air (ventilation) and install new, clean filters (filtration). The guidelines prefer using Merv-13 air filters if the units can handle them but recognize many older units may only be able to use a Merv-8. The guidelines do not require the filter upgrade because many older hvac units cannot function effectively with a merv-13 filter because they create too much of a pressure drop. That was the target we met previously strove for. You can think of it like breathing through your cloth mask is like the units equipped with a merv-8 filter that the units were designed for but breathing through 10 masks stacked up would be the merv-13. That is just an example of the pressure drop and airflow to help understand.
  2. We continued to work to improve the air ventilation and filtration and have set up new classroom guidelines that we are using to certify the classrooms.
    1. The first guideline is filtration. We have done extensive research and have found high performing, low pressure drop merv-13 filters that will work in our older units. Back to the mask example, that is like wearing an N95 mask instead of a cloth mask. We have replaced nearly all filters in the District with Merv-13. This includes custom ordering and modifying some older units to work with the merv 13 filters. Our new guideline is that every room must have merv 13 equivalent filtration as a baseline.
    2. Our second guideline is ventilation. We established a baseline of measuring double the code required fresh air per occupant of 15 CFM (cubic feet per minute) of air per occupant. There are a number of ways the code allows that number to decrease in order to save energy but we are disabling those features to provide a baseline of continuous 30 CFM per occupant. In many cases we will have more than that depending on the age and design of the system but we are setting that as a baseline. Additionally, opening windows will increase that.
    3. We then test the rooms. This is a simple pass/fail test. If it meets those two criteria, Merv-13 and 30 CFM, in gets certified. If it does not meet those criteria then we put a portable filter in the room. All portable filter rooms will also be certified under the portable filter option of our certification.
    4. We also are putting portable filters in rooms that we do not have time to test. We will continue to work through the District and test the units. When we test a room, if it passes, we will pull the portable unit out of the room and re-certify it as passing with mechanical ventilation.
    5. If a room fails, that does not mean the system does not work. Sometimes it can be a 5 minute fix. Other times it could be a 5 hour fix. And yet other times it could require a part that has to be ordered. We are not letting a failure get us bogged down at the expense of serving the District as a whole. We will install a portable filter and then come back to troubleshoot after we have provided for the safety of the rooms.
    6. We have hundreds of different HVAC systems of different ages in this District and the knowledge of the MOT team to keep these diverse units straight is astounding. They have been relentlessly creative in solving the challenges of juggling all these different systems as we try to upgrade them and push them beyond their designed parameters.
  3. We are additionally doing air quality testing to further enhance the systems as a third round of continued efforts to improve the effectiveness of the classrooms ventilation and filtration. This is the bipolar ionization that we have presented briefly in past board meetings. Those results are not known yet but if they yield positive results we will implement the upgrades District wide and we will provide much more detail at that time.

The best way we've heard this explained is to imagine a swimming pool. The ventilation is a garden hose filling the pool. The filtration is the pool cleaning system. Covid is a child peeing in the pool. You would like to filter that water to clean it up but your filter can only do so much so you also need to keep adding fresh water. The more fresh water you have, the more dilute that pee is. So we are trying to both add water and upgrade our filtration.

Frequently Asked Questions

(otherwise known as everything you never thought you'd care to learn about filtration)

Feel free to share as much or as little of this with your staff as you feel necessary. This is meant as a resource and a guide to the myriad of questions around the topic.


  • What filters do I have:
    • Almost every occupied room with mechanical ventilation now has a Merv-13 filter. If they are not upgraded yet, they will be in the next week. Then we will shift to the next wave of openings to make sure those rooms are upgraded.
    • The portable filters have a Merv-8 prefilter and a Hepa filter
  • What is all this Merv business:
    • Merv is the Minimum Efficiency Reporting Value and is a measure of the effectiveness of an air filter. That translates to how small the particulates are that will pass through the filter
    • Merv- 8 blocks particulates at 3 microns
    • Merv-13 blocks particulates at .3 microns
    • Hepa filter is considered Merv-17 or higher and blocks smaller than .3 microns
  • Where are the different filters used:
    • Merv-8 are used in schools, commercial buildings, high end homes, and industrial settings
    • Merv-13 - 16 are used in Hospitals for inpatient care and surgery and high end commercial buildings
    • Merv 17 - 20 (Hepa) are used in cleanrooms, radioactive situations, and pharmaceutical manufacturing
  • Which solution is safer:
    • For a comparison of the mechanical ventilation to the portable filters they are partially doing different things.
    • The mechanical ventilation is both diluting and filtering the air
    • The portable units are only filtering the air but are doing so at an even higher rate that is closer to Hospital grade. Some studies suggest the Hepa level will catch all of the covid aerosols while other studies suggest not 100%.
    • So there is no clear winner between the two directions. They each have their own tradeoffs. As you can tell by the noise of the portable units, the increased filtration requires significantly higher airflow while the built in system allows us to mix in fresh air to dilute the air more without the added noise.
  • What are other Districts doing:
    • Every District is currently asking the same questions we have been.
    • Many are choosing to meet the first step criteria outlined above and stopping there.
    • A few Districts have upgraded to the Merv-13 filters including East Side USD, Las Lomitas ESD, and others.
    • Many Districts bought smaller Hepa filters on Amazon or Home depot. Those are typically sized for a bedroom and do not do nearly enough filtration to be effective.
    • We were the first to commit to the portable commercial grade filters as a backup and were therefore first in line. Two other Bay Area Districts have also just ordered these units.
    • We've talked about the bipolar ionization (step 3 outlined above) that we commissioned air quality tests before making a decision. Other Districts bought them outright and are now testing them. Those Districts include Acalanes, Berkeley, and Bakersfield. Others are considering adding bipolar ionization. Many are waiting for our test results from the vendor.
    • Some Districts are considering installing complicated fan systems with UV light. We researched those and they are less effective and considerably more expensive than the other options we are pursuing.

Portable Filter Locations

  • What is the role of the main office purifier:
    • We added a portable purifier at every main office location. This is because there are various interactions from public, sick students, or faculty pass thru.
  • What about Offices:
    • The theory is smaller specialist rooms or offices do not have a large number of students or can adjust their practice to provide services outside, in larger rooms on campus, or even remotely. This determination is from various conversations with the sites about how these rooms are functioning. If special considerations are needed, please contact me and we can discuss the unique situation.
  • What about larger rooms:
    • Some sites are using larger rooms for programs to increase distancing. Those rooms would require a significant amount of filters to address that large air volume. Therefore, we are not placing the portable units in the large spaces. Additionally, the large spaces are more closely approaching an outdoor environment due to the still small cohorts in the larger volume of air. Back to the pool analogy. It's like someone peeing in a lake vs someone peeing in the above ground pool.
  • Where should the portable filter be placed in the room:
    • We've defaulted to putting it near the front of the room under the assumption that is where the teacher will teach from in this new environment. The filter cleans all the air in the room but it cleans the air around it first. So we've decided that having the clean air around the teacher is the best solution. If we did an air current map, the contaminants tend to stay around their source rather than drift through the room and tend to not linger around the device since they are pulled in and captured. So back to the pool analogy, the kid peeing just keeps peeing and the concentration of pee hovers around him. So we feel the filter near the teacher gives the teacher the most protection.
    • Additionally, putting it near the window or door will help filter the outside air as it enters the room. See more under smoke section.
  • What about the isolation room:
    • We are not putting any additional filtration in those rooms since we hope to not need to use them
    • We have sealed off the return air in the isolation room on every campus. That means the air will not recirculate anywhere in the rest of the system. Those rooms are essentially sealed off from any other systems.
    • When those rooms are used, we will follow a safety protocol to clean and sterilize those rooms.

Operational questions

  • Do I leave my doors and windows open or closed:
    • Either is fine. Since the different ventilation solutions are each doing slightly different things we will leave it up to your comfort level.
    • Each person's comfort level will be different. We leave it to you to adjust the space according to your own safety wishes.
    • The built in systems are providing a lot of outside air. In many cases far more than the minimum baseline we've set. You should be fine with the doors and windows closed if you have mechanical ventilation.
    • The portable filters don't have mechanical ventilation but they trap most of the covid aerosols in the Hepa filter. With the doors/ windows open, you will dilute the air more but also take longer to trap any covid aerosols if you have a sick person in the room.
    • Per standard security protocols, you should close your windows at night.
  • Won't leaving the doors open waste energy:
    • Yes, but that's ok. The more outside air the better though that will waste more energy and the mechanical units will have to work harder. If you are a die hard environmentalist, then close your doors and windows. Otherwise, open or close them as you see fit to adjust the comfort in your room and increase your dilution. We'll make up for the larger energy use by conserving more when we are on the other side of the pandemic. This applies to both the portable and built in systems.
  • Do I leave my unit on every night:
    • Currently the engineers recommend leaving them on at least 2 hours before and after occupancy. Therefore, go ahead and leave them on each day. Again, we'll deal with the added energy used when we get through this. Also, the energy use of the filters is significantly less than air conditioning so we'd rather have the safer room air.
  • Can I turn them off:
    • Yes, but we don't recommend it. There is a switch on the back of the unit. If you want to turn it off, before or after students are in the room, flip the switch but you will then have to remember to turn it back on. We would recommend if you want to turn it off after a room full of students that you let it run at least 30 minutes to help clear out the air.
    • We do not advise turning them off when you have students because they are there specifically for the groups of people.
    • Additionally, if you have a substitute one day, they will not know to turn if on.
  • Can I turn off my Mechanical ventilation:
    • No. At the moment we have overridden room controls to provide continuous ventilation.
    • The current recommendations are to ventilate the rooms before and after they are occupied.
    • Normally, the ventilation only occurs when there are people in the room so we have forced that on.
  • Will custodians turn the portable units on and off:
    • No. Their time is critically spent on the disinfecting they are doing every night.
  • Will I have my portable filter all year:
    • Probably not. Our goal is to continue our pass/ fail testing of the ventilation in the rooms and once a room passes we will remove the portable unit and move it to another area that needs it like the 2/3 grades then the 4/5s and then the middle and high schools. After we've caught up with all the mechanical unit testing, we think the portable ones will remain deployed in rooms that do not have mechanical ventilation.
  • Will the District change the filters on the portable units:
    • Yes, part of the benefit of these are they are standardized and use commercial filters that are easy to acquire. Filters have become one of the hardest things to acquire right now, like toilet paper in March, so having a standard size makes it a lot easier to change and we will do so when we change our built in units.
  • Why are the portable filters so large:
    • They are commercial grade and are sized to meet the needs of a classroom. If you looks at a classroom's HVAC unit, they are this size or larger. To really meet the needs of the space, you need a large unit.
  • Can I just bring in or use my classroom fan:
    • No. Do Not Use Fans.
    • Fans create turbulence which will kick up any covid particles that fall to a surface.
    • The way the filters pull the air does not create the same kind of turbulence.


  • Will my Mechanical ventilation be more noisy:
    • Yes. We've increased the outside air volume and locked the units to continuously provide air. So it will be a little noisier than it used to be. Not a lot, but a little.
  • The portable units are really noisy:
    • Yes. We know this. These are the quietest ones we could get that do what they do.
    • They are 59 decibels which is similar to conversation.
    • It takes a lot of force to push air through a Hepa filter. We weighed this issue and consciously chose to go forward with these units because we felt the benefits outweighed the noise.
    • Back to the item on room placement, if you feel the noise is reduced by putting it in the back of the room, that will work as well.
  • Why is the mechanical ventilation quieter than the portable ventilation:
    • The main reason is the duct size and the Hepa filter.
    • We have large ducts and large registers to supply air from the classroom units to the classroom space. One of the primary jobs of a Mechanical engineer is to design systems with a minimal noise as possible.
    • The Hepa filter is like pushing air through a wall. It takes a lot of force to do so.
  • Can you turn the noise down:
    • Yes, but that also turns down the airflow output. As we've said, we were rapidly deploying these.
    • If you have a portable unit and you are in a standard sized classroom then we shouldn't turn the air down.
    • If you are in a smaller space, let your principal know and they will coordinate with us to adjust the airflow accordingly.
  • Why didn't you buy a quieter model:
    • We did a lot of research. These are commercial grade filters which produce a lot of filtered air to help keep you safe.
    • Some other Districts bought units that cleaned at a quarter the rate but were 70 decibels.
    • The vendor did a test with all their available portable units and found the unit we purchased produced more filtration and was quieter than the other commercial grade ones.
    • This portable unit also has the ability to add bipolar ionization which we believe will be an additional added bonus.

Bad Air Quality / Smoke

  • What do I do now- open my windows for covid or close them for smoke:
    • The best advice we have right now is still to close your doors and windows to keep out the bad air.
    • That advice contradicts with the more airflow goals for covid
  • Mechanical ventilation smoke guidelines:
    • If you have mechanical ventilation, close your doors and windows. There is already sufficient outside air and it will recirculate through the filter.
    • Due to the filter recirculation, the indoor air will always be cleaner than the outside air
    • We are adjusting our controls to more quickly be able to close some of the outside air to more finely tune these reactions.
  • Portable filter guidelines:
    • Your portable filter will do a good job of filtering the smoke. We recommend placing it near a single open window or door so you still have increased outside air but also let the filter capture that air upon entering the room and filter it.
    • Here is a video showing the effectiveness of the filters in a smoke filled room:
  • Is there anything else you can do:
    • The Superintendent's office continues to monitor the air quality daily to determine if the AQI will be over 150 the following day. We are continuing to seek out further solutions and data to provide better recommendations.
    • As part of our bipolar ionization testing, we have a testing company lined up to come the next time we have a smoke day to do specific smoke samples. We are checking weather patterns to be prepared to do those tests.
    • We will test smoke specifically in standard rooms, bipolar ionization rooms, portable filtered rooms, and outside. If we continue to get bad air, the silver lining is we are lined up to test various best practices to improve the air quality when the AQI is not over 150.

Will the District change any of these directions:

Maybe. Probably. It seems that change is the only thing we can count on these days. Let's get back to school and then look at making adjustments. For the moment, we are erring on the side of caution to make our rooms safer.

In the long run, we will continue to make tweaks and continue to learn more about the transmission of the virus. When we make an operational change we will let you know.