How indoor air quality in schools affects student learning and health

This collection of research on indoor air quality in U.S. schools, originally published in December 2022, was updated April 12 with a new report that raises concerns about the number of school districts that have not followed the federal government’s guidance on strategies to increase indoor airflow and ventilation to prevent the spread of COVID-19.

The COVID-19 pandemic has forced a spotlight on indoor air quality in schools and the importance of proper ventilation.

It also has brought renewed attention to a longstanding problem in many U.S. classrooms: air pollution, ranging from pet dander and paint fumes to mold, trace metals and formaldehyde.

For many years, schools facing funding shortfalls put off costly projects that would improve indoor air quality and ventilation — replacing roofs, for example, and updating their heating and air conditioning systems. When the federal government announced in 2021 that it would distribute $123 billion in emergency relief funds to help schools prevent the spread of COVID-19 and recover from its impact, school districts had backlogs of deferred renovation and maintenance.

Education officials have a lot of flexibility in how they use that money, the bulk of which remained unspent in late 2022, according to a Washington Post analysis. School districts have until September 2024 to spend the money.

Public health leaders, researchers and advocacy organizations have urged them to put some of it toward improving indoor air quality. Meanwhile, the CDC has stressed that improved air flow and ventilation are crucial in helping stop COVID-19 from spreading indoors.

On April 7, the CDC released a report indicating many school districts have not adopted any of the four key strategies it recommended for increasing air flow and ventilation, including inexpensive changes such as opening windows and using fans to circulate air.

To better understand schools’ efforts to improve indoor air quality amid the pandemic, the private firm MCH Strategic Data surveyed a total of 8,410 school districts nationwide late last year.

Epidemiologist Miguella Mark-Carew, the lead author of the report, said journalists should ask school district leaders about barriers that might be preventing them from improving indoor air quality. She also recommended asking officials about their experiences implementing changes.

“For school districts that have implemented ventilation improvements, understanding lessons learned from their ventilation improvements may help others school districts at the beginning stages of making such improvements,” Mark-Carew wrote in an email interview. “Learning about how improving indoor air quality has impacted students, teachers, and staff (i.e., reduced staff and student absenteeism) would also provide information about the benefits of proper ventilation.”

As news outlets scrutinize schools’ use of emergency relief funds and cover the progress of these projects, it is important journalists know what the research says about how indoor air quality can affect student health and learning.

Children are especially susceptible to health problems linked to poor air quality.

“Exposure to various air pollutants in school buildings risks severe damage to pupils’ health since they inhale a larger volume of air corresponding to their body weights than do adults,” a group of 19 researchers write in an October 2022 paper, “Indoor Air Quality and Health in Schools: A Critical Review for Developing the Roadmap for the Future School Environment.”

Below, we’ve gathered and summarized several academic studies and government reports that examine this issue in the U.S. and abroad. Together, they suggest:

Tens of thousands of U.S. schools have been operating with faulty or outdated HVAC systems, and many schools lack air conditioning.
Poor indoor air quality and ventilation are common in schools across the planet.
Certain pollutants often are found in higher levels inside school buildings than in homes and commercial buildings.
Poor indoor air quality is linked to a variety of health problems, ranging from coughing and wheezing to more serious conditions such as asthma and cancer.
Breathing polluted air harms students’ academic performance. If students get sick, they miss school. Studies also show that poor classroom air quality reduces cognitive ability.
Remediation efforts are hampered by the fact there are “no generally accepted criteria for distinguishing a problematic level of dampness and mold, which adversely affects health, from a non-problematic level of dampness and mold,” one researcher writes.
Improvements in outdoor air quality over the years has helped boost student test scores, raising the possibility that cleaner air could help reduce historic differences in test scores between Black and white children.
Climate change has created new challenges for schools, considering some airborne pollutants are more prevalent in warmer climates. As temperatures rise, some schools in cooler regions of the country have installed air conditioning.

Read more about these findings below. Toward the bottom of the page, we’ve included links to other useful resources, including investigative journalism projects focusing on indoor air quality in schools.

The condition of school facilities

Ventilation Improvements Among K-12 Public School Districts — United States, August-December 2022
Miguella Mark-Carew; et al. Morbidity and Mortality Weekly Report, April 2023.

When 8,410 U.S. school districts were asked about indoor air quality since the COVID-19 pandemic began, many reported using at least one of these strategies the CDC had recommended for improving airflow and ventilation inside classrooms and other buildings:

Replacing or upgrading HVAC systems.
Maintaining continuous airflow in school buildings during active hours.
Installing or using in-room air cleaners with high-efficiency particulate air (HEPA) filters.
Installing ultraviolet germicidal irradiation (UVGI) devices.

The majority of districts, however, reported they did not use any of the four strategies — or did not know whether any had been implemented in 50% or more of their public schools, according to this report from the U.S. Centers for Disease Control and Prevention. For example, 27.9% of schools surveyed in the Midwest indicated they had not replaced or upgraded their HVAC systems. Another 39.8% reported it was “unknown” whether most schools had done that.

“That none of the four ventilation strategies was reported by more than approximately one half of school districts underscores the ongoing opportunity to improve indoor air quality among K-12 school buildings in the United States,” the authors write.

Between Aug. 8 and Dec. 29, MCH Strategic Data surveyed school district office staff members. It conducted weekly telephone surveys of districts serving 2,500 students or more, and biweekly surveys of smaller districts.

The resulting report provides a broad look at some of the ways education officials have tried to prevent the spread of COVID-19 among students, school employees and the many people who visit or do business with local schools. It also indicates some schools could not or would not make inexpensive changes such as opening windows and using fans to keep indoor air moving.

Among the districts that did make changes, 50.7% reported maintaining continuous airflow through school buildings during active hours at most of their schools and 33.9% reported replacing or upgrading HVAC systems at most schools. Meanwhile, 28% reported installing or using in-room air cleaners equipped with HEPA filters and 8.2% reported installing UVGI devices.

The authors of the report point out that rural districts reported replacing or upgrading HVAC systems and using in-room air cleaners less frequently than districts in other areas.

“This difference might be due to limitations in resource availability and difficulty finding contractors available and willing to complete capital improvements,” they write. “In addition, rural schools might be more likely to use natural ventilation, such as opening windows, than are suburban and city schools because of less exposure to noise and air pollution in rural areas or simply having windows that can be opened.”

School Districts Frequently Identified Multiple Building Systems Needing Updates or Replacement
U.S. Government Accountability Office report, June 2020.

An estimated 41% of U.S. school districts need to update or replace HVAC systems in at least half their schools, which means at least 36,000 public schools are operating with faulty or outdated HVAC systems, according to this report the U.S. Government Accountability Office prepared for Congress.

The report spotlights a range of problems common in public school buildings nationwide that can affect student learning, health and safety. The findings are based on a nationally representative survey of school districts conducted in late 2019 as well as in-person visits to 55 schools in six states — California, Florida, Maryland, Michigan, New Mexico and Rhode Island — from June to September 2019.

Some of the other big takeaways:

27.7% of districts reported needing to repair or replace roofs in at least half their schools. Moisture from leaky roofs “can lead to the growth of mold, fungi, and bacteria; the release of volatile organic compounds; and the breakdown of building materials,” according to the National Institute for Occupational Safety and Health. “Moisture also provides a favorable environment for cockroaches, rodents, and dust mites.”
8.9% need to address “environmental conditions” such as mold, lead or asbestos in at least half their schools.
20.6% of districts need to update or replace systems for monitoring indoor air quality in at least half their schools.

Site visits revealed that schools in cooler areas of the country have had to alter class schedules or install air conditioning systems as climates have warmed.

“Officials in a Michigan [school] district said about 60 percent of their schools do not have air conditioning, and in 2019, some temporarily adjusted schedules due to extreme heat,” according to the report. “Officials in a Maryland district said the district retrofitted some schools with air conditioning, but did not update pipes and insulation serving the HVAC systems, which has caused moisture and condensation problems in these buildings. Officials were concerned the moisture and condensation could lead to air quality and mold problems, but said that to remedy these issues could cost over $1 million for each building.”

The Ventilation Problem in Schools: Literature Review
William J. Fisk. Indoor Air, November 2017.

Ventilation rates in classrooms across the globe often do not meet minimum standards, concludes this paper from the Berkeley Lab , a U.S. Department of Energy laboratory managed by the University of California.

Ventilation rates refer to the amount of outdoor air allowed into a building. In the U.S., a commonly used ventilation standard for classrooms calls for a minimum ventilation rate of approximately 7 liters per second or 15 cubic feet per minute per occupant, explains senior scientist William J. Fisk.

In this paper, Fisk reviews the scholarly literature published from 1995 to 2016 on school ventilation and carbon dioxide levels in classrooms across various countries, including the U.S., China, Greece and Sweden. He asserts that the cost of boosting classroom ventilation rates is small compared to the benefits in terms of student health and academic performance.

“The evidence of an association of increased student performance with increased ventilation rates is compelling,” he writes. “There is evidence of associations of reduced respiratory health effects and reduced student absence with increased ventilation rates.”

Fisk points out that schools’ energy costs would rise if they improved the ventilation in their classrooms. Many schools also would need to upgrade or repair their HVAC systems.

“The economic value of increases in student performance and health and of reductions in absence are not easily quantified,” Fisk writes. “However, the annual incremental energy and capital costs of increasing ventilation rates as needed to meet or exceed current standards, range from a few dollars to about ten dollars per person.”

How common is mold and other types of indoor air pollution?

Ventilation Improvements Among K–12 Public School Districts — United States, August–December 2022
Miguella Mark-Carew; et al. Morbidity and Mortality Weekly Report, April 2023.

Replacing or upgrading HVAC systems.
Maintaining continuous airflow in school buildings during active hours.
Installing or using in-room air cleaners with high-efficiency particulate air (HEPA) filters.
Installing ultraviolet germicidal irradiation (UVGI) devices.

“That none of the four ventilation strategies was reported by more than approximately one half of school districts underscores the ongoing opportunity to improve indoor air quality among K–12 school buildings in the United States,” the authors write.

The resulting report provides a broad look at some of the ways education officials have tried to limit the spread of COVID-19 among students, school employes and the many people who visit or do business with local schools. It also confirms that some school districts could not or would not make inexpensive changes such as opening windows and turning on fans to keep indoor air moving.

The authors of the report point out that rural districts reported replacing or upgrading HVAC systems and using in-room air cleaners less frequently than districts in other areas.

Asthma Prevalence and Mold Levels in US Northeastern Schools
Evin J. Howard, et al. Journal of Allergy and Clinical Immunology: In Practice, March 2021.

Researchers analyzed dust samples collected from 32 schools in densely populated areas in the northeastern U.S. and found they contained several types of mold. They also discovered mold levels in classroom dust often exceeded levels of mold found in dust collected from local homes.

Researchers tested samples taken from a total of 114 classrooms and the bedrooms and living rooms of 33 homes from 2014 through 2018. They tested for 30 types of mold, 26 of which are associated with water damage and 10 of which are associated with outdoor sources.

Classrooms had higher levels of these molds, associated with water damage: aureobasidium pullulans, eurotium amstelodami and penicillium crustosum.

Classrooms also had higher levels of these molds, associated with outdoor sources: alternaria alternata, cladosporium cladosporioides 1, cladosporium cladosporioides 2, cladosporium herbarum, epicoccum nigrum and penicillium chrysogenum.

For most remaining mold types, differences were not statistically significant.

When the researchers looked for links between mold levels and rates of asthma among schoolchildren living in the area, they found asthma was most common in schools with higher levels of outdoor mold — the kind found in soil and on leaf surfaces, for example.

“Therefore, the differences in the prevalence of asthma was not indicative of any significant differences in mold growth resulting from water-damage indoors but rather factors associated with the increased levels of molds from outside entering and accumulating in the schools,” the researchers write, adding that 30 of the 32 schools studied did not have air conditioning.

“In addition to AC, the frequency of window and door opening could also affect the levels of [outdoor] molds,” they write. “Cleaning frequency and thoroughness could also affect the build-up of [outdoor] molds inside schools.”

Association Between Allergen Exposure in Inner-City Schools and Asthma Morbidity Among Students
William J. Sheehan, et al. JAMA Pediatrics, January 2017.

This study, which also focuses on the northeastern U.S., found high levels of mouse allergen in 37 elementary schools. It concludes that children with asthma exposed to high levels of mouse allergen, which is present in the urine, hair and dander of mice, were more likely to experience increased asthma symptoms and poorer lung function.

Researchers analyzed samples of dust taken from the classrooms and bedrooms of 284 students, aged 4 to 13 years, who had been diagnosed with asthma and attended one of the 37 elementary schools from March 1, 2008, to Aug. 31, 2013. Researchers tested for allergens associated with mice, rats, dogs, cats, cockroaches and dust mites.

Mouse allergen was most prevalent. It was detected in 99.5% of school samples and 96% of home samples. However, mouse allergen levels were higher in schools than in homes.

Cat, dog and dust mite allergens also were common in schools and homes, although at much lower levels. For example, researchers found cat allergen in 94.8% of school samples and 79.4% of home samples. Allergens from cockroaches and rats “were mostly undetectable” in both locations, the researchers write.

They note that allergen levels and the types of allergens present in schools differ according to the climate of the area.

“In the inner-city schools in our study, mouse allergen was the predominant exposure, whereas levels of cockroach, pet, and dust mite allergens were undetectable or low,” they write. “In contrast, other cities with warmer climates and different building conditions have demonstrated high levels of school cockroach allergen. The low levels of dust mites and cockroach in our study are likely owing to the long, dry, and very cold winters in the studied region, as these pests require humidity and warmth to survive.”

Classroom Indoor PM2.5 Sources and Exposures in Inner-City Schools
Aleshka Carrion-Mattaa, et al.Environment International, October 2019.

For this study, researchers went into 32 schools in densely populated areas in the Northeast to measure airborne levels of fine particulate matter with diameters of 2.5 micrometers or smaller (PM2.5). Prior research has found this type of air pollution is associated with cognitive impairment and exacerbated asthma.

Researchers analyzed PM2.5 levels inside schools each fall, winter and spring from 2009 to 2013 and compared them to outdoor levels. They learned that indoor and outdoor concentrations of PM2.5 were comparable, “demonstrating penetration of outdoor pollution to indoors.” They also learned PM2.5 levels inside schools varied by season and contained a range of pollutants from automobiles, roads, soil, burning vegetation and other sources.

Pollution from motor vehicles was greatest during the fall and winter, the researchers write.

“Activities in the school environment during winter such as using motor equipment/vehicles to clean the snow and spreading salt, and during the fall such as cleaning the leaves may have driven these differences to the other sources,” they explain. “In addition, higher contribution of biomass burning in winter is more likely associated with wood burning at the neighborhood of schools.”

Of the 32 schools studied, four had air conditioning systems, 15 had radiant heat “natural ventilation” and the rest had either classroom-based vents or a combination of ventilation types.

Indoor air quality and child health, student performance

Indoor Air Quality and Health in Schools: A Critical Review For Developing the Roadmap for the Future School Environment
Sasan Sadrizadeh, et al. Journal of Building Engineering, October 2022.

This paper summarizes 50 years of classroom air quality research conducted in 40 countries across six continents, concluding that various pollutants pose severe risks to student health.

Indoor air quality in schools “is characterized by a complex of contaminants,” including molds, bacteria, volatile organic compounds, particulate matter, and trace metals from road traffic, the researchers write. They note that several studies have found that levels of certain airborne pollutants were higher inside schools than in homes and commercial buildings.

“Inhalation exposure to air pollution has increased children’s mortality rate, acute respiratory disease, and asthma,” they write. “Due to different responses of the children’s immune systems to indoor air exposures, various chronic diseases and symptoms have been reported and characterized as ‘sick building syndrome.’”

This paper is based on a review of 304 reports and research papers published between 1970 and 2022. It offers a broad overview of research examining types of pollutants. It finds, for example, that volatile organic compounds — a group of chemicals used in finishing and furnishing — are one of the most dangerous pollutants found in classroom air.

“Construction materials, furnishings such as desks and shelves, resins of wood products, adhesives, glues, paints, cleaning chemicals, and carpets are primary [volatile organic compounds] emission sources in schools,” the researchers write. “The VOC concentrations in newly built or recently renovated school buildings may be significantly higher than ordinary ambient levels.”

The paper also discusses factors that influence levels of pollution inside schools such as ventilation, temperature, outdoor wind speeds and classroom cleaning protocols.

The researchers point out that research demonstrates a link between indoor air quality and student achievement. Studies “confirm that poor air quality affects both typical schoolwork of pupils, i.e. performance in simple learning tasks such as math and language exercises and pupils’ examination grades and end-of-the-year results,” they write.

Does Dampness and Mold in Schools Affect Health? Results of a Meta-Analysis
William J. Fisk, Wanyu R. Chan and Alexandra L. Johnson. Indoor Air, November 2019.

While research suggests students are at an increased risk of developing respiratory issues when dampness and mold are present in schools, renovations aimed at eliminating dampness and mold do not always alleviate such health problems, according to this Berkley Lab analysis.

To better understand the health conditions associated with mold and dampness in schools, researchers combined and analyzed data they collected from 11 studies of the issue published between 1995 and 2016. What they learned: The evidence most strongly suggests that coughing, wheezing and nasal symptoms are associated with mold and dampness in schools.

When the Berkley Lab researchers reviewed four other studies on renovations to correct dampness and mold in schools, they found mixed results. One of those studies reported improvements in some — but not all — student health symptoms after a thorough renovation was completed. For example, there was not a statistically significant change in the number of student complaints about headaches. That study, which focuses on four schools in Finland, also found that “a partial renovation did not significantly improve health.”

Berkley Lab researchers note that efforts to reduce dampness and mold might be more successful is there were generally accepted criteria for distinguishing problematic levels of dampness and mold.

“Ideally, school districts seeking to reduce dampness and mold in their buildings would have clear criteria defining the dampness and mold conditions that trigger remedial actions,” the researchers write. “However, the various studies cited have employed a variety of definitions for dampness and mold and there are no generally accepted criteria for distinguishing a problematic level of dampness and mold, which adversely affects health, from a non-problematic level of dampness and mold.”

Association between Traffic-Related Air Pollution in Schools and Cognitive Development in Primary School Children: A Prospective Cohort Study
Jordi Sunyer, et al. PLOS Medicine, March 2015.

Children who attended schools in parts of Barcelona, Spain where there were high levels of automobile traffic pollution showed less progress on cognitive exams than did children attending schools where traffic pollution was low, this study finds.

For example, students in more polluted areas demonstrated a 7.4% increase in working memory over the course of a year, on average. Meanwhile, students in less polluted areas showed an average increase of 11.5%.

Researchers note there also were gender-based differences.

“Boys appeared more susceptible to air pollution, although both boys and girls showed an adverse association of school air pollution with cognitive development,” they write. “Although results could be due to chance, in animals, males were more susceptible to airborne metals than females, which may be because of sex-specific altered dopamine function.”

Thirty-nine schools participated in the study. A combined 2,715 students aged 7 to 10 years took computerized tests every three months for a year so researchers could measure their inattentiveness and development of working memory.

The researchers also collected data on levels of three types of pollutants — carbon, nitrogen dioxide and ultrafine particles — present inside classrooms and outside in school courtyards.

The researchers point out that air pollution was highest in a wealthy area of Barcelona and that most of the schools in high-traffic areas that participated in the study are located there. Children attending schools where traffic pollution was lowest tended to have better-educated mothers, more siblings and fewer behavior problems than children attending schools in more polluted regions.

The researchers suggest the harms of air pollution could stay with children years into the future.

“Impairment of high cognitive functions has severe consequences for school achievement,” they write. “Thus, reduced cognitive development in children attending the most polluted schools might result in a disadvantage in mental capital, which may have a long-lasting life course effect.”

Outdoor air pollution

Air Pollution and Student Performance in the U.S.
Michael Gilraine and Angela Zheng. National Bureau of Economic Research working paper, May 2022.

Improvements in outdoor air quality over the past two decades have raised student test scores, according to this analysis, which also concludes cleaner air would help reduce longstanding differences in test scores between Black and white children.

To investigate the relationship between air quality and student achievement, the authors analyzed satellite-based measurements of fine particulate matter, or PM2.5, and other pollution-related data from 2002-03 to 2018-19. They then linked air quality data to math and reading test scores for more than 11,000 U.S. school districts for the academic years 2008-09 through 2017-18.

The authors also factored in schools’ proximity to power plants and year-to-year variations in production at individual plants.

They determined that concentrations of fine particulate matter dropped nationwide over the period studied, driven largely by reductions in coal use. For students, on average, PM2.5 concentrations dropped by 3 micrograms per cubic meter.

The authors point out that test scores, measured in standard deviations, rose, although not as much as they could have.

“Substantial improvements to student performance and equity through cleaner air are still possible, however,” they write.

Other resources for journalists

The U.S. Environment Protection Agency provides reports and resources on indoor air quality in schools, including national air-quality standards for particulate matter.
The Indoor Air Quality Scientific Findings Resource Bank, located at the Berkeley Lab, gathers research on how indoor air quality affects people’s health or work performance.
ASHRAE, formerly known as the American Society of Heating, Refrigerating, and Air-Conditioning Engineers, is a professional organization with a mission of “serv[ing] humanity by advancing the arts and sciences of heating, ventilation, air conditioning, refrigeration and their allied fields.”
The Indoor Air Program at the University of Tulsa conducts research on indoor air quality in schools, homes and workplaces.

News investigations of indoor air quality in schools

Toxic PCBs Festered at This Public School for Eight Years as Students and Teachers Grew Sicker, The Seattle Times and ProPublica, 2022.

Mold Notebook: Which SC Colleges Had the Most Mold Reports, The Daily Gamecock and The Post and Courier, 2022.
Toxic City: Sick Schools, The Philadelphia Inquirer, 2018.
Toxic Schools: Mold, Air Quality Spark Thousands of Complaints in Central Florida, Orlando Sentinel, 2010.