Why Do We Not have HEPA filters or Corsi-Rosenthal Boxes in every US Classroom, COVID ward, home and Business?

SARS CoV-2 virus is spread by aerosols. These aerosols contain many viruses carried in a tiny amount of liquid from a person’s mouth and nose as they breath, talk, sing, or yell. Yelling, singing, coughing, sneezing produce more aerosol than breathing. Risk of transmission in a room depends on duration of exposure (time in the room), amount of ventilation, # of individuals carrying the virus present and their activity . A 5 micron aerosol can stay suspended in air for 30 minutes indoors.

HEPA filters (High Efficiency Powered Air-filters) can dramatically reduce the number of aerosols in a room. This includes not only virus carrying aerosols but also small particulate pollutants, both of which impact the health and safety of children and adults in classrooms, meeting rooms and businesses.

HEPA filters range in price from $150 to $800 or more depending on quality, efficiency and how quiet they run. They have been tested in classrooms and hospitals.

In the hospital setting they have dramatically decreased COVID Virus.

To determine how the filters stand up to real-world conditions, Navapurkar and his co-authors installed them in two fully occupied COVID-19 wards — a general ward and an ICU. The team chose high-efficiency particulate air (HEPA) filters, which blow air through a fine mesh that catches extremely small particles. The researchers collected air samples from the wards during a week when the air filters were switched on and two weeks when they were turned off.

In the general ward, the team found SARS-CoV-2 particles in the air when the filter was off but not when it was on. Surprisingly, the team didn’t find many viral particles in the air of the ICU ward, even when the filter there was off. The authors suggest several possible reasons for this, including slower viral replication at later stages of the disease3. As a result, the team says that measures to remove the virus from the air might be more important in general wards than in ICUs.

You can read the article here.

An Engineering professor and Dean at UC Davis, Richard Corsi, tweeted the design of an inexpensive homemade air filter providing the equivalent aerosol clearing capacity as an expensive manufactured HEPA filter. A colleague in Texas built one with simple components from a hardware store. The result was called the Corsi-Rosenthal box.

Made with four MERV 13 Airfilters, a box fan , duct tape and cardboard.

Dr. Corsi is an expert in the engineering of HVAC systems. He has researched methods to improve indoor air quality and published many scientific studies involving the interaction between pollutants and indoor materials. He estimates that the cost of a home-made Corsi-Rosenthal box is $4.50 per year per student to build and run based on average class size in the US. These are easily made with simple components and instructions available on-line. In fact if you search YouTube you will find many short videos on how to build these in 20 minutes. Their construction and use could easily be a classroom activity.

Professor Corsi was interviewed on the People’s Pharmacy radio show.

Here is a picture of the Corsi-Rosenthal box from Wikipedia:

One box can help to improve the air quality in an average size classroom, offering the equivalent of 7 to 8 air changes an hour.

These boxes would not only benefit health by decreasing circulating virus containing aerosols but also filtering out indoor pollutants that come from latex paint, carpet fibers, cleaning chemicals, air fresheners, fire retardants on furniture and clothing, which all release volatile organic compounds (VOCs) that cause lung damage, increase risk of asthma and autoimmune disease. So why have these not been widely used in our schools, homes, and businesses?

Why has the CDC not recommended this simple and inexpensive highly effective risk reduction approach? If widely implemented soon after Drs. Corsi and Rosenthal invented and advocated it’s use, it would have prevented many infections and allayed some of the fear and anxiety of teachers, students and parents. As the flu season approaches, and with the added risk of a triple threat presented by RSV, Influenza, and Covid, all transmitted by aerosol, now would be a great time to build some for your home, business, office or your chidren’s school classrooms.

In the context of the COVID 19 pandemic I will close with the usual summary.

  1. Avoid alcohol consumption (alcohol wreaks havoc with your immunity)
  2. Get plenty of sleep (without adequate sleep your immune system does not work well )
  3. Follow good sleep habits
  4. Exercise, especially out of doors in a green space, supports the immune system
  5. Get some sunshine and make sure you have adequate Vitamin D levels. Supplement with Vitamin D3 to get your levels above 30 ng/ml, >40ng/ml arguably better.
  6. Eat an anti-inflammatory diet rich in micronutrients.
  7. Practice stress reduction like meditation and yoga which improves the immune system
  8. Eliminate sugar-added foods and beverages from your diet. These increase inflammation, cause metabolic dysfunction, and suppress immunity.
  9. Eliminate refined-inflammatory “vegetable oils” from your diet, instead eat healthy fat.
  10. Clean up your home environment and minimize your family’s exposure to environmental toxins by following recommendations at EWG.org with regards to household products, personal care products, and organic foods. (https://www.ewg.org/)
  11. Drink water filtered through a high quality system that eliminates most environmental toxins.
  12. If you are eligible for vaccination, consider protecting yourself and your neighbor with a few jabs. Age > 50 and/or risk factors (Diabetes, pre-diabetes, insulin resistance, hypertension, obesity, heart disease, COPD, asthma, cancer treatment, immune suppression) suggests benefit from a booster. Risk for complications of boosters in adolescents, especially males, without risk factors, may equal benefit. Previous infection with Covid can be considered as protective as a booster. Discuss risk vs benefits with your doctor.


Eat clean, drink filtered water, love, laugh, exercise outdoors in a greenspace, get some morning sunlight, block the blue light before bed, engage in meaningful work, find a sense of purpose, spend time with those you love, AND sleep well tonight.

Doctor Bob

2 thoughts on “Why Do We Not have HEPA filters or Corsi-Rosenthal Boxes in every US Classroom, COVID ward, home and Business?

  1. Joe Egan

    The number of air exchanges is a hand-in-glove relationship with HEPA filters. More exchanges enhance the effectiveness.


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