Is natural ventilation the key to healthier public spaces?

Researchers at the University of Burgos, Spain, have found that cross-ventilation is the best type of natural ventilation to maintain healthy air quality inside buildings.

However, they found that they could not practically apply it in more than half of the existing public buildings they surveyed.

Since the COVID-19 pandemic, recirculated air in public spaces has become a real cause for concern. People need safe access to public spaces.

Schools, colleges, office spaces, and gyms are essential for a thriving community. But how do we make the air safe?

The researchers developed a replicable model for this. The model assesses the capacity of buildings to maintain good indoor air quality based on real ventilation capacity. Carbon dioxide (CO₂) concentration is a key marker in the model.

This study proposes that ventilation should be a critical design detail in future public buildings.

CO₂ characterizes air quality

CO₂ levels rise predictably with human respiration. Researchers consider it a reliable marker for the dilution of potentially infectious aerosols.

“Good” air quality indoors is when the difference between CO₂ levels indoors and outdoors is less than 500 ppm.

European standards call this IDA2 category. IDAs 1- 4 classify indoor air quality for non-residential buildings. The buildings surveyed in this study were located in Móstoles, Spain.

Keeping CO₂ within an IDA category requires a specific ventilation flow rate. For infection control, the study suggests aiming for about three air changes per hour (ACH).

ACH is a measure of how many times the total volume of air in a room is replaced with fresh air in one hour.

Only CO₂ levels below 800 ppm are safe. More than five ACH can cause discomfort and be counterproductive for disease control.

Most buildings lack natural ventilation

Cross-ventilation is where air can enter and exit through openings on opposite facades or walls. It is the only natural method consistently capable of reaching the recommended ACH.

Researchers emphasized that good indoor air quality is critical for the health and well-being of office workers. It is also imperative to reduce airborne diseases.

Yet about 53% of the 183 analyzed rooms had no possibility for natural ventilation at all! 

Only a few spaces with windows could achieve good air quality with only natural ventilation. For example, designers planned to accommodate 77 people in the city’s Plenary Hall (City Council).

However, the study found that it could accommodate only 20 people while maintaining IDA2 conditions – even with every window fully open.

Because of similar issues, just 11% of the rooms met IDA2 occupancy standards. 

Between 28% and 57% reached the more modest benchmarks of five or three ACH, depending on room type and density of occupation.

How to improve natural ventilation

The findings make it clear that many existing public buildings do not maintain good air quality. Their natural ventilation is inadequate.

The researchers recommended pairing windows with mechanical ventilation to maintain IDA2 conditions. 

They also demonstrated how to transform simple CO₂ monitoring into a daily operational tool. For example, restricting or staggering admission to a space when thresholds of CO₂ are exceeded.

Sensors could trigger window openings and not require manual intervention. This would balance energy efficiency and safety in air-conditioned spaces.

“Natural ventilation is a low-cost, scalable intervention,” the authors noted in the paper. This study did not focus on other air purification methods, like filters or air purifiers.

Designing buildings that breathe

This study provides an integrated approach to designing healthy buildings. They considered specific ventilation conditions, activity types, and occupancy levels. 

The majority of public spaces in this study lacked the architectural design to take advantage of natural ventilation. Therefore, forced or mechanical ventilation needs to be combined with the existing system, making it a hybrid.

The study found that cross-ventilation is the most efficient natural way to dilute indoor contaminants. It requires little capital to operate.

It can be combined with the monitoring of CO₂ levels to protect occupants dynamically. The effects of poor air quality go beyond the spread of infections to other aspects of respiratory health.

Taken together, these findings suggest that the path forward is twofold.

Existing buildings need to be upgraded to include hybrid systems where required. Secondly, architects should design all new public buildings to breathe from the start.

The study proposes that incorporating cross-ventilation into building design is an essential step toward creating healthier public spaces.

The study findings were published in the journal Eng.

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