Indoor air quality in schools is finally being addressed, and that includes air in school buses. School buses are the most densely populated areas of a school facility and they inherently have poor air quality. Funding is available through CARES Act and the Consolidated Appropriations Act to clean the air in your school buses. Additional funding will also likely be coming through the American Rescue Plan circulating through Congress.  

The options available for cleaning the air on your school buses can be overwhelming. So many terms to consider: CFM, CADR, UVC, etc. How do you determine what is best for your fleet?

First: Whichever solution you choose, you’re going to be questioned by the public that you serve. So, consider if you want to choose a technology that isn’t recommended by the CDC. The CDC recommends improving particulate filtration and using UVC. Any other technologies are considered “emerging technology without an established body of evidence”. 

To quote from a recent release this year from the American Society of Heating Refrigeration and Air Conditioning Engineers, (ASHRAE) on in-room air cleaning, “Technologies such as ionizers, UV-PCO, and many called by other names may claim to remove or destroy multiple types of contaminants but may convert them to other compounds that might be harmful. These technologies are designated by CDC as emerging technologies without an established body of evidence reflecting proven efficacy under as-used conditions. For more information, see the Epidemic Task Force Filtration & Disinfection Guide

If a technology claims to kill or inactivate SARS-CoV-2 cells, which causes COVID-19, make sure you can explain to the public how the system you’re installing is deadly to viruses and bacteria but completely safe for students and drivers to breathe. If it’s not “safe” for virus cells, is it safe for human cells? If you can’t explain it, choose something else.

Second: To help consumers compare the effectiveness of various air cleaning solutions, the CDC references the EPA’s Guide to Air Cleaners in the Home. In that guide, “efficiency” and “effectiveness” are defined in a term called “Clean Air Delivery Rate” or CADR. CADR is the product of efficiency and airflow. Effectiveness takes into account how much of the air you’re cleaning. An air cleaner that is 100% efficient at cleaning 300 cfm of air would have a CADR of 300. An air cleaner that is 90% efficient at cleaning 1,000 cfm of air would have a CADR of 900, and be 3-times as effective. CADR provides you with a metric for comparing various systems. 

A simple CADR calculator can be found on the Lumin-Air website at www.lumin-air.com. Simply enter the airflow and efficiency of the air cleaner and the CADR is calculated. To put the CADR number in terms people can relate to, the output shows the clean air percentage of a bus’s air conditioning system. It also shows how much airflow that is relative to a typical handheld hair dryer. For example, if you calculated a CADR of 350 cfm, would you feel it was adequate to only clean 10% of a 66-person bus’s air conditioning airflow? 350 cfm is the airflow of just over four hair dryers. You’d likely want to have many of these smaller systems located throughout the bus. 

Third: Consider not only first cost, but the ongoing cost of anything you install. While your school system can get funds for the initial installation of air cleaning systems on your buses, this is one-time funding. Calculate the ongoing cost of the system. Ask vendors to include five-year parts warranties, and include five years or more of replacement filter changes in their quotes. 

How do you know when to change filters?  As filters load with dust and contaminants, the pressure drop across them increases, which reduces airflow. Reducing airflow, reduces your CADR, which reduces the effectiveness of your system. The rule of thumb for changing filters is to replace them when the pressure drop across them doubles from their initial clean pressure drop. Pressure drop is related to the dust holding capacity of the filter. Filters with high dust holding capacity last longer than filters with low dust holding capacity. You’ll have to replace dirty filters with clean filters based on how quickly they reach their dust holding capacity. So, make sure vendors provide the dust holding capacity of their filters and their cost of replacement filters for comparison. 

To recap, ask the following questions:

  1. Is the air cleaning technology endorsed by the CDC? 
  2. Is the air cleaning technology safe and effective?
  3. What is the airflow (cfm) of the air cleaning system?
  4. What is the efficiency of the air cleaning system?
  5. What is the dust holding capacity of the air cleaning system?
  6. What is the initial cost?
  7. What is the annual cost, considering consumable items, like replacement filters and UVC bulbs?

While it’s great that school buses are getting the attention and funding they deserve to clean the air in school buses, it’s important to safely achieve the highest level of disinfection for the least amount of money over the life of the system.