Training for how to make Corsi-Rosenthal Boxes

How and When to Change the Filters in Your Corsi-Rosenthal Box

En español

Corsi-Rosenthal Boxes provide excellent filtration of indoor air, especially for particles containing the virus that causes COVID-19 and wildfire smoke. After assembling and using your Corsi-Rosenthal Box, you may be wondering when and how to change the filters.

When to change the filters

The filters used in Corsi-Rosenthal Boxes generally have a lifespan between 1,000 to 4,000 hours of actual use time on the mid-flow setting. The range varies based on the size of the filter (one inch or two inches), frequency of use, and differences in the indoor environment in which it is being used. For example, it is best to change the filters nearer to 1,000 hours for a dusty indoor setting with carpeting and/or indoor pets. Changing the filters at approximately 4,000 hours is recommended for a relatively clean indoor environment without carpet.

This filter change recommendation means that you could use a Corsi-Rosenthal Box on the mid-flow setting in an uncarpeted office, for example, for eight hours a day, five days per week and the filters would last for about 25 months. In contrast, a Corsi-Rosenthal Box used for six hours a day, five days a week in a breakroom adjacent to a workshop with people coming and going would need to have the filters changed after about eight months.

Many people underestimate the life of the filter because they compare it to a filter in an HVAC system or assume that it needs to be changed because it looks dirty."

Many people underestimate the life of the filter because they compare it to a filter in an HVAC system or assume that it needs to be changed because it looks dirty. The timeframe for changing the filters in a Corsi-Rosenthal Box is different from changing filters for an HVAC system because the filter width and the air speeds through the filters are different between an HVAC system and a Corsi-Rosenthal Box. In a Corsi-Rosenthal Box, discoloration means that the filter is working and does not necessarily mean that it is time to change the filter.

How to change the filters

  1. Switch the system off for at least six hours before removing the filters to limit exposure to newly deposited viruses.
  2. Take the box outside or to a large open area inside. Removing the filters may release a small amount of particles.
  3. It is recommended to wear a KN95 mask or better while changing the filters.
  4. Working one by one, cut each filter away from the fan and gently place it in a trash bag. Be careful to not shake or drop the filter as sudden movement can disperse dust and particles into the air. Follow this step again for all filters in your Corsi-Rosenthal Box. Change all filters the same day. Never use a combination of new and used filters in your Corsi-Rosenthal Box.
  5. Tie the bag tight and put it in the garbage. Wash your hands thoroughly with soap and water.
  6. Using new filters, you can now reassemble your Corsi-Rosenthal Box.
Note: If the cardboard base is damaged or worn, you will need to replace the cardboard when replacing the filters in order to properly rebuild your Corsi-Rosenthal Box for maximum efficiency.


  • The filters in Corsi-Rosenthal Boxes have a lifespan of 1,000 to 4,000 hours.
  • Many people underestimate the lifespan of a filter.
  • Although risk of exposure from discarding the filters is very low, precautions, such as wearing a KN95 mask or better, are recommended when changing filters.
  • All filters should be changed at the same time.


Watch how to build a Corsi-Rosenthal Box, as demonstrated by Richard Corsi.