This news release is adapted from a release by UC Davis. 

A new study by Berkeley Lab and UC Davis finds 85% of California classrooms did not have sufficient ventilation. (Credit: iStock)

Roughly 85% of recently installed HVAC systems in K-12 classrooms investigated in California did not provide adequate ventilation, according to a study from UC Davis and the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

For the study, published in the journal Building and Environment, researchers visited 104 classrooms in 11 schools throughout California that had been retrofitted with new heating, ventilation, and air conditioning (HVAC) units in the past three years. They evaluated the HVAC systems, carbon dioxide (CO2) concentrations, and indoor temperature and humidity through field inspections, monitoring, and a teacher survey.

“Previous research has shown that under-ventilation of classrooms is common and negatively impacts student health and learning,” explained lead author Rengie Chan, a research scientist with Berkeley Lab. “What isn’t known, however, is why this problem is so widespread and persistent.”

Indoor environmental quality matters

Ventilation helps remove indoor air pollutants such as volatile organic compounds, including formaldehyde, which can off-gas from building materials, finishes, and furniture. There is also increasing evidence that CO2, which is exhaled by building occupants, affects our ability to do challenging cognitive tasks. This is particularly important in classrooms, where lots of people gather in a small space.

Mechanical ventilation systems deliver filtered outdoor air to a space to reduce indoor pollutants. (Credit: UC Davis/Berkeley Lab)

Standards for ventilation rates are set to provide acceptable indoor air quality. ASHRAE, a global professional society that sets standards for building performance, specifies a minimum ventilation rate for classrooms of 15 cubic feet per minute per person. In California, the 2016 Building Energy Efficiency Standards, also known as Title 24, have the same ventilation requirement for classrooms.

HVAC problems are widespread

Using measured CO2 concentrations and the number of people in the classroom, the research team found only about 15% of classrooms met the ventilation standard. Researchers characterized each HVAC system by documenting the number of problems due to its hardware, controls, and filter maintenance. Classrooms with one or more HVAC problems tended to have lower ventilation rates and higher CO2 levels.

In addition to ventilation, thermal comfort is known to impact student performance. Prior studies suggest to maintain classroom mean temperature to 20 to 23 degrees Celsius. In this study, about 60% of the classrooms had a mean temperature above 23C (73F). Also, 30% of the teachers surveyed were dissatisfied or very dissatisfied with the temperature in their classroom, and about 10% said the temperature interfered “a lot with the learning environment.”

A call to action

A field researcher from the UC Davis Western Cooling Efficiency Center measures classroom ventilation using a flow hood. (Credit: UC Davis)

Fortunately, the study also pinpointed simple solutions to ventilation issues. “Our study shows that proper installation, operation, and maintenance of HVAC systems is necessary to provide adequate ventilation in classrooms,” said co-author Theresa Pistochini, engineering manager at the UC Davis Western Cooling Efficiency Center.

Pistochini notes that the Division of the State Architect, which provides oversight of construction projects, does not review HVAC replacement projects – yet these projects must still meet Title 24 requirements. This puts the onus of ensuring compliance on school districts.

“There are nearly 1,000 school districts in California,” Pistochini said. “With limited resources, it is unrealistic to expect that school district personnel be adequately trained to ensure compliance. Increased oversight of HVAC replacements, or other ways to address widespread inadequate ventilation in California classrooms, are needed, likely through state intervention.”

In addition to ensuring adequate ventilation, follow-on research by Pistochini and Chan highlighted the importance of air filter choice. In field tests conducted at two additional California schools, the team tested ventilation system filters at two different efficiency rating (MERV) levels: MERV 8 and MERV 13. The research team found that using MERV 13 filters leads to a substantial decrease in PM2.5, which are fine particulate matter that pose serious health risks. “This is important because as you ventilate and bring in outside air, you will also bring in outdoor air pollutants into classrooms,” said Chan.

Researchers recommend the following actions to improve ventilation rates in classrooms:

  • Complete commissioning and acceptance testing of new HVAC systems as required by Title 24.
  • Run HVAC fans when classrooms are occupied to bring in fresh air.
  • Use MERV 13 filters and replace filters two to three times per school year.
  • Monitor classroom CO2 concentrations. Thermostats with CO2 sensors and stand-alone sensors are widely available.
  • Test ventilation rates in existing HVAC systems and make corrections when needed.

Millions of California children spend a large portion of their day indoors at school. Ensuring adequate classroom ventilation will help protect and support the health and well-being of students and teachers.

Additional study authors include Xiwang Li and Brett Singer of Berkeley Lab, and David Vernon, Sarah Outcault, Angela Sanguinetti, and Mark Modera of UC Davis. The study was supported by the Electric Program Investment Charge program, managed by the California Energy Commission.

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Founded in 1931 on the belief that the biggest scientific challenges are best addressed by teams, Lawrence Berkeley National Laboratory and its scientists have been recognized with 13 Nobel Prizes. Today, Berkeley Lab researchers develop sustainable energy and environmental solutions, create useful new materials, advance the frontiers of computing, and probe the mysteries of life, matter, and the universe. Scientists from around the world rely on the Lab’s facilities for their own discovery science. Berkeley Lab is a multiprogram national laboratory, managed by the University of California for the U.S. Department of Energy’s Office of Science.

DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit

Media contact:

Laurel Kellner,  [email protected], 510-590-8034