Returning to Mines

A tool kit for safely returning to campus

to Mines

A tool kit for safely
returning to campus


Mines Facilities is following guidelines from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) for HVAC operation in a pandemic environment to make sure our buildings are as safe as possible.

Specific actions recommended by ASHRAE and being employed by the Mines COVID HVAC Team and Facilities Management are explained below.

Recommendation: Increase outside air to 100 percent or highest level possible

  • HVAC systems in Mines labs are already supplying 100 percent outdoor air.
  • All newer buildings supply adequate amounts of outdoor air that meets or exceeds ASHRAE Standard 62.1 and have MERV 13 filters.
  • As the Return to Mines Task Force receives each department’s “Return to Campus” plan, the COVID HVAC Team will use this data (which spaces will be used and number of people in each space), along with construction drawings and Building Automation System (BAS) information, to quantify the amount of outdoor air supplied to each room to make sure they have enough outdoor air as recommended by ASHRAE Standard 62.1
  • Despite having reduced building occupancy on campus in the fall (due to continued remote work where possible and remote/online courses), Facilities Management is maintaining the design outdoor air rates as though the buildings are fully occupied. This means that the amount of fresh air per person will be twice the ASHRAE recommendation.
  • Most buildings on campus have smart controls that supply 100 percent outdoor air when conditions are ideal (outdoor air is cooler than indoor) – this is also known as “free cooling.” For the rest of the time, we will supply an amount that meets or exceeds ASHRAE Standard 62.1

Recommendation: Improve filter efficiencies to MERV 13 or higher, as possible

  • All newer buildings supply an adequate amount of outdoor air and have MERV 13 (or better) filters.
  • For older buildings, we are increasing outdoor air and/or installing MERV 13 filters if not already in place, provided the equipment can handle the thicker filters.
  • For rooms with local HVAC systems (like fan coil units), we are installing MERV13 filters or the highest the system can accommodate.
  • For rooms with lower outdoor air but with MERV 13 filters in place, we are increasing the number of times air will go over these filters (recirculation airflow), to make sure they filter out as much contaminants as possible.
  • For the few rooms with inadequate ventilation, we are installing portable HEPA filters if the space cannot be closed or converted into single occupancy.

Recommendation: Operate systems for extended hours to maximize effect of ventilation and air treatment

  • Mines will expand the time when HVAC systems will be operating, from 5 a.m. to 10 p.m. (a typical schedule is 7 a.m. to 8 p.m.). This will make sure buildings are “flushed” of contaminants before any students or staff gets to campus.

Recommendation: Add portable HEPA or high-MERV air filters

  • Mines has purchased several portable HEPA filters and is installing MERV 13 filters around campus.

Recommendation: Add Ultraviolet Germicidal irradiation (UVGI)

  • The Mines COVID HVAC Team analyzed this option but concluded that steps 1-4 are enough to reduce the risk.

Recommendation: Bypass energy recovery ventilation (ERV) units

  • Mines is bypassing these units, which exchange heat from the air leaving the building to the air entering the room. This is done to minimize any risk of cross contamination.

Frequently Asked Questions

General Questions

Q: What is HVAC?

A: HVAC stands for heating, ventilation and air conditioning. These systems process outdoor air so we can be comfortable inside. They consist of fans, pumps, heat exchangers, dampers, filters, smart controls and many other subcomponents that add heat (warm up) or moisture (humidify), remove heat (cool down) and/or moisture (dehumidify), and remove contaminants from the air before the air enters our buildings. In many buildings around campus, most of these subcomponents are inside huge boxes call air handling units (AHU) that you might see on top of the building.

Q: Is my office is safe to work in?

A: While we cannot decrease risk to zero, our goal is to reduce the risk as much as possible by combining social distancing, masks, filtration and ventilation. Specifically, Facilities Management and the COVID HVAC Team are analyzing each space individually as departments submit their Return to Campus plans and will recommend changes as needed. We have already evaluated all classrooms and labs on campus and they all meet ASHRAE guidelines for pandemic environments.

Q: Can buildings help in reducing risk of infection?

A: Yes, however, social distancing and using face masks are still the most important things we all must do reduce the risk of infection. Large saliva droplets have a short range and will settle within 6-8 feet. Small droplets, or aerosols, will remain airborne longer (up to hours) [1], and thus can travel longer than large droplets and have been found in HVAC systems [2]. Building heating, ventilation and air conditioning (HVAC) have a role in aerosol transmission by disturbing transmission pathways inside buildings [3]. HVAC systems can help dilute and filter these aerosols to some extent, but preliminary research shows that the amount of air changes needed can decrease by up to five times if occupants wear masks. Simply said, HVAC is more effective when masks are used [4]!

[1] K.A. Prather et al. “Reducing transmission of SARS-CoV-2,” Science  26 Jun 2020: 368 (6498): 1422-1424.

[2] P.F. Horve et al. “Identification of SARS-CoV-2 RNA in Healthcare Heating, Ventilation, and Air Conditioning    Units,” preprint:

[3] G. Buonanno et al. “Estimation of airborne viral emission: Quanta emission rate of SARS-CoV-2 for infection risk assessment,” Environment International 141, August 2020, 105794.

[4] H. Dai and V. Zhao. “Association of infected probability of COVID-19 with ventilation rates in confined spaces: a Wells-Riley equation based investigation,” pre-print:


Q: What can HVAC systems do to help reduce COVID exposure?

A: What building HVAC systems can do is to reduce the aerosol load (amount of virus in the air), which in turns reduces exposure, thus reducing the overall risk of getting sick. This can be done by using the following methods:

  • Ventilation: Supply cleaner, outdoor air with the main goal to dilute any indoor air contaminants and aerosol load. This can be done by simply opening a window or by the mechanical system, using fans to bring more outdoor air inside.
  • Filtration: Use filters that can be either in rooms or inside HVAC systems (air handling units) to filter out particulates. ASHRAE recommends using filters rated MERV (Minimum Efficiency Reporting Value) 13 or higher or using HEPA filters. HEPA filters can block particulates as small as 0.01 microns. MERV 13 is not as efficient as HEPA but combined with other measures, can be very effective. Most existing HVAC systems cannot be retrofitted with HEPA filters due to size and the resulting restriction of airflow.
  • Ultraviolet Germicidal irradiation (UVGI): This technology can also be in rooms (upper section, above 7 feet) or in ducts. These are lamps that produce UVC irradiation that kills or inactivates viruses and/or bacteria. Please note that these are specific lamps using specific wavelength and require professional installation and monitoring. Some can produce ozone that can harm more than help.

There are other technologies available but are either too new (not enough evidence to prove they are safe and/or reliable) or produce ozone, which irritates the upper respiratory system and can do more harm than good. Thus, ASHRAE only recommends ventilation, filtration and UVGI.

Q: Given that SARS-CoV-2 is new and there are many unknowns, how is Mines keeping up to date with respect to building operation?

A: Facilities Management and the COVID HVAC Team is following the latest guidelines from the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE). ASHRAE has developed a task force consisting of building, HVAC and indoor air experts to keep track of (and also publish) the latest research on SARS-CoV-2. Some of these members have been interviewed as experts on the subject by major news outlets and authored scientific articles about HVAC, indoor air and SARS-CoV-2.

Q: If my office has a window, should I open it?

A: Yes, opening your window will allow outdoor air to get into your office, thus improving ventilation. Some offices and dorms around campus have windows that open, and you can certainly do this. Facilities Management just asks that you close your window when you leave your office/dorm. Leaving a window open unattended can let rain/snow get inside and produce cold drafts or even freeze some pipes in winter, which can cause major problems.

Q: Can I leave my classroom door open?

A: For most buildings, leaving a classroom door open is not recommended and in some cases may be against fire code.  Also, more fresh ventilation air is provided to classrooms than transient spaces like hallways so the effect of leaving the door open might be to actually get less ventilation air in the classroom, not more.

Q: If our buildings and HVAC are safe, can I stop wearing a mask?

A: No. Masks and social distancing is still the No. 1 step we must all follow to reduce the risk of COVID-19 exposure indoors. Wearing face masks reduces the aerosol load, which makes the HVAC system more effective.  Not even the best HVAC system in the world can replace social distancing and masks. In fact, there are some studies that show if people are wearing masks the need for ventilation air can be reduced up to five times to achieve the same effectiveness in reduction of virus transmission.

Q: Will working from home help keep campus buildings safer?

A: In general, working from home is a good idea, but you should first consult with your supervisor or department head to make sure this is OK. Having people work from home means having less people on campus, which translates to:

  • Increased social distance.
  • Increased amount of outdoor air per person inside buildings (HVAC more effective to dilute aerosol load).
  • Helping others who can’t work remotely.
  • Getting to wear slippers while working alongside your pets!

Portable HEPA Filters

Q: Can I bring a portable HEPA filter to my office?

A: Portable HEPA filter units are not a substitute for social distancing or wearing masks while indoors. Please remember that social distancing and wearing masks are by far the best ways to reduce the risk of virus transmission indoors. However, portable filters are useful for spaces where we cannot bring in enough outdoor air (to dilute contaminants/reduce aerosol load) and/or the actual system cannot handle a MERV13 filter and the space cannot be closed or converted to single occupancy.  

As the COVID HVAC Team reviews departmental Return to Campus plans, we will determine current ventilation rates for all required spaces on campus. If we find that a particular common space does not have adequate ventilation and the space must be utilized, Facilities Management will provide HEPA filters for the common spaces and will notify the people who use those spaces.

Keep in mind there are other options that may be more effective, such as:

  • See if you can work from home. If you can, try to work from home as much as possible.
  • If your office has a window, open the window (but remember to close it when you leave).
  • If the space where you work is too crowded, limit the number of people in the space at one time (e.g. ask people to come on different days of the week, or if you are in a reception area, ask visitors to wait outside or come at a later time).

If none of these options work and/or you still want to have your own portable filter, please follow the recommendations below:

  • Remember to replace filter as suggested by the manufacturer (every 4-8 months) and carefully dispose of the old filter (contact EH&S for recommendations).
  • Buy a certified HEPA filter certified by State of California and by AHAM. Make sure they do not produce ozone or they might do more harm than good.
  • See additional filter recommendations from CU Professor Shelly Miller.
  • Buy a HEPA filter with an appropriate flowrate for the space it will located in. Most have a space square foot rating.

Q: If I bring a portable HEPA filter, can I have students in my office?

A: No, having a portable HEPA filter does not waive social distancing requirements. The most important steps to mitigate risk are social distancing and using masks. Current research shows that most COVID-19 transmission occurs via large droplets when people are less than 6 feet away in closed environments. HEPA filters will only help with small droplets that have become airborne.

Q: If I bring a portable HEPA filter to the common space where I work, does this mean we can have more people in this room?

A: No, having a portable HEPA filter does not waive social distancing requirements. The most important steps to mitigate risk are still social distancing and wearing face masks.

Q: How do I know if the HEPA filter device I am buying is a quality unit or something counterfeit?

Make sure to buy a certified HEPA filter certified by the State of California and by AHAM. Specifically, see the filter recommendations from CU Professor Shelly Miller.

Q: How big of a space can I put a portable HEPA filter unit in? How do I determine what size unit to get for a space?

A: It is important that you buy a filter that is rated for the space where it will be used. For example, don’t buy a portable HEPA filter rated for a 100-square-foot room if you are going to install it in a 300-square-foot room. Why? It is important that all the air in your office/room goes to the filter at least twice an hour (or 2 ACH).

Q: The information on the HEPA filter unit I am considering says it has a charcoal filter as well as a HEPA filter. What is the charcoal filter doing? Do I need that for COVID-19 protection?

A: Charcoal filters are typically installed as a prefilter to capture larger particles and gases in the space. It extends the lifetime of your HEPA filter, but the HEPA filter is the system that is filtering small particles. While they are not required, they do extend the life of the HEPA filter.

Q: Will noise from the HEPA filter unit be an issue?

A: Portable HEPA filters do add some background noise, but most portable HEPA filters have 1-3 speeds that you can vary. The added noise should not be that high as to impact your work.

Q: What is CADR and how do I apply that information in making my decision?

A: The AHAM seal/certification provides a clean air delivery rate (CADR) that characterizes the volume of filtered air that an air cleaner can deliver. CADR is calculated and reported by three different particles sizes: smoke (smallest), dust (smaller) and pollen (small). The CADR ratings most relevant for SARS-CoV-2 are smoke and dust. This will also help you size your filter by using AHAM 2/3 rule – size your unit using CADR at least 2/3 your room’s area. So, if your room is 10 feet x 10 feet (with 8 foot ceilings), find a filter with a CADR of 67 or higher.  The AHAM seal should also include recommended room size.

Q: If HEPA filters are best in class, why is Mines not installing them everywhere on campus?

A: While HEPA filters are best in class, it takes a lot of energy (and huge fans) for air to pass through them, as there is a significant pressure drop across them. Most building HVAC fans were not designed to handle this pressure drop, so using HEPA filters will substantially reduce airflow in buildings and the capacity of the HVAC systems to dilute contaminants and increase risk overall.  Also, in a lot of cases, HEPA filters simply won’t fit inside the existing air handling unit.

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