Image: Getty Images/Tina Tiller
Image: Getty Images/Tina Tiller

ScienceSeptember 21, 2021

Why improving air quality is key to preventing Covid outbreaks in schools

Image: Getty Images/Tina Tiller
Image: Getty Images/Tina Tiller

New Zealand classrooms are often crowded, damp, mouldy and poorly ventilated – prime conditions for Covid to spread. And with new strains affecting children more than ever, making our schools safer should be an urgent priority, writes public health researcher Julie Bennett. 

Children in most parts of the country have returned to school, but as Covid remains within our borders, it’s critical the classrooms they occupy keep them as protected as possible against its potential spread.

Recently, worldwide attention has turned towards the impact Covid-19 is having on children, with significantly more contracting the virus now than earlier in the pandemic. This increase is probably the result of several factors, including the higher infectiousness of new variants (notably delta); high proportions of children unable to be vaccinated; a return to in-person learning in several countries with high community transmission; and greater numbers of children being tested.

While it was initially thought that children were less likely than adults to experience severe disease, outbreaks have sadly led to large numbers of children being hospitalised, including admissions to intensive care, and there are growing concerns about the long-term effects of Covid-19 illness on children.

Covid-19 outbreaks in schools

Other countries are reporting that most Covid-19 outbreaks are now in schools. Here in New Zealand, children (under 19 years of age) in the most recent outbreak represent the largest proportion of Covid-19 cases (40% of cases to date). There have also been large outbreaks in school communities, with the Marist College cluster generating 96 cases last year. The August 2021 outbreak has seen thousands of students from multiple schools required to self-isolate because they were close contacts of cases and now, two students from a primary school in Waikato’s Mangatangi have tested positive.

A case study reported by the Centers for Disease Control and Prevention (CDC) has described an outbreak at a primary school (ages 5-14 years) in California earlier this year, where an unvaccinated teacher tested Covid-19 positive, two days after experiencing a stuffy nose and fatigue. During those two days, the teacher continued to work, attributing the symptoms to allergies. By the end of the outbreak, 12 of the 22 students in the teacher’s classroom had tested positive. The sick students then infected 14 siblings and parents. The majority of students infected sat in the front rows near the teacher, with a staggering 80% of the students in the two front rows testing positive. While both students and teachers wore masks inside the classroom, the teacher allegedly took off their mask while reading aloud to the class. 

The spread of the delta variant after one teacher got sick (Image: courtesy of the CDC)

The CDC case highlights the importance of using a layered approach to prevent the spread of Covid-19. That is using masks, cohorting, staying home when unwell, and vaccinating staff and students 12 years and above. Another layer that can be added to these prevention strategies is through the improvement of indoor air quality, which can be achieved through ventilation (bringing in as much outdoor air as possible) and air filtration.

Ventilation and Covid-19 transmission

It is well known that poor indoor air quality in schools can fuel community-wide disease transmission of other common viral illnesses such as colds, influenza and gastroenteritis. In New Zealand, classrooms are typically more crowded than houses or offices and they are often damp, mouldy and poorly ventilated. Such environments allow virus-laden particles to accumulate in the air, facilitating transmission.

When spaces are poorly ventilated, the levels of carbon dioxide (CO2) rise as people breathe. These CO2 levels are frequently used as a proxy to estimate the stuffiness in classrooms and measure if a building has adequate fresh air. High CO2 levels have been directly correlated to low productivity and low school attendance, as breathing in high levels can result in headaches, poor concentration, restlessness, nausea and sleepiness. 

Adequate ventilation is critical to creating successful learning environments. In New Zealand, most classrooms normally rely on opening windows for ventilation. However, surveys have shown that windows in schools are rarely opened and ventilation rates are well below what the Ministry of Education recommends. In Auckland and Wellington, CO2 levels in classrooms have been reported to exceed the Ministry of Education recommendations around 40% of the school day, while in Christchurch, students have been trying to learn in very poorly ventilated classrooms, with CO2 levels recorded at double the recommended limit over a week-long period. 

While some ventilation improvements will require structural alterations to school buildings, there are strategies that schools can implement, especially with the arrival of spring. The responsibility for ventilating classrooms should not fall on teachers alone, whose workloads are already overburdened. Some measures schools can take to help improve ventilation are to open windows to get across-room airflow. If it’s too cold to leave the windows open, opening up all available doors and windows for 15 minutes of every hour is recommended for full air exchange (purge ventilation). Child-safe fans can also be used to increase the effectiveness of open windows by safely securing fans to blow potentially contaminated air out and pull in outdoor air, especially on hot days. Schools could also consider having activities, classes and breaks outdoors when circumstances allow.

There are also measures that would be beneficial for the Ministry of Education to implement, with priority given to primary schools, as children under the age of 12 will be the last group to be vaccinated, and not all students are able to wear masks. These include revising the recommended average CO2 level to align with the CDC recommendations, which are considerably lower, and installing CO2 monitors in all classrooms to indicate when to take action, such as opening windows or moving outside. Where natural ventilation isn’t possible or effective and in high-risk areas such as sick bays, portable air cleaners – high-efficiency particulate air (HEPA) filtration units – should be installed. Buildings that consistently have CO2 levels above the Ministry of Education recommendations should be upgraded.

Schools do far more than provide formal education, which is why it is so important that we do everything we can to make sure they stay open safely. Indoor air quality has always been important for children’s health and learning, and optimising ventilation in schools has multiple co-benefits aside from Covid-19 prevention, including prevention of other respiratory infections that circulate in schools and improving children’s learning and concentration. The new Covid-19 variants, like delta, make improving indoor air quality in schools more urgent.

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